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Volume 30 No 6 September 2003

Journal of the Austra lian Water Association

Editorial Board F R B ish o p , C h airm an

.B N Anderson, W J Dulfer, G Finke, G Finlayson, G A Holder, B Labza, M Muntisov, P Nadebaum,J D Parker, F R o ddick , G R yan, S Gray

\ Water is a refereed journal. This symbol indicates that a paper has been re fereed .

Submissions Instructi ons for auth ors can be found on page 12 of chis journal. Submissions accepted at: www.awa.asn.a u / publicat ions/

Managing Editor

OPINION 2

Growth, Change and Youth; How the AWA Works for Members; My Point of View, The Challenge of Good Water Management, J Kea r y

ASSOCIATION ACTIVITIES 6

Including IWA Australia Report

Peter Stirling

PROFESSIONAL DEVELOPMENT

Technical Editor

10 Details of courses, classes and other upcoming water events

E A (Bob) Swinton 4 Pleasant View Cres, Wheelers Hill Vic 3150 Tel/Fax (03) 9560 4752 Email: bswinton@bigpond.net.au

NEWS BYTES 12 Featuring selected highlights from the AWA email News

Water Production

CROSSC URRE NT

Hallmark Editions

16 Gold Coast City Council - Pimpomo Coomero Water Futures now on Public Display

PO Box 84, Hampton, Vic 3188 Level I , 99 Bay Street, Brighto n , Vic 3 186 Tel (03) 9530 8900 Fax (03) 953089 11 Email: hallmark@halledic.com.au

CONFERENCE REPORT 18 Community Consultation in the Australian Water Industry, S Love

Graphic design: Mitzi Mann

INDUSTRIAL WATER AND WASTEWATER FEATURE

Water Advertising

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Natio n a l Sales Manage r : Brian R a u lt Tel (03) 9530 8900 Fax (03) 9530 8911 Mobile 04 11 354 050 Email: brault@h alledit.corn .au

Water (ISSN 0310 · 0367) is published eight times a year in the mon ths of February, March , May, J une, August, September, N ovember and Decem ber.

Australian Water Association PO Box 388, Artarmon , NSW 1570 Tel +61 294 13 1288 Fax: (02) 9413 1047 Email: info@awa.asn.au A.BN 78 096 035 773

AWA

Federal President Rod Lehmann

Executive Director

ifj

AUSTRALIAN WATER

" ANAEROBIC TREATMENT OF JUICE AND FRUIT-PROCESSING EFFLUENTS Laboratory trials demonstrate potential for gos production L Palmowsk i

26

" DESALINATION OF THE BURRUP PENINSULA: OPENING UP FRONTIERS Q: Which desolinotor is best? A: Horses for courses G Cri sp

32 WATER CYCLE WITH ZERO DISCHARGE AT VISY PULP AND PAPER, TUMUT, NSW An award-winning system 0 Szolosi

39 WATER MANAGEMENT INITIATIVES: THE MANILDRA GROUP Attention to detail in on industrial water /wastewater cycle saves S D Fergusson

WATER 44

•, FLOCCULATION: A NEW DESIGN PROCEDURE Theoretical analysis and practical applications for impellers

ASSOC IATION C hris Davis Australian Water Association (A WA) assumes no responsibility for opinions or statements of faces expressed by contributors or advertisers. Editorials d o not necessarily represent official AWA po licy. Advertisements are included as an information service to reade~ and are reviewed before publication co ensure relevance to the water environment and objectives of AWA. All material in Water is copyright and should not be reproduced wh olly or in part without the written permission of the Managing Editor.

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61

Water is sent to all AW A membm eight times a year. It is also available via subscription.

S Griffiths

ENVIRONMENT SS

•, STORMWATER RUNOFF TREATMENT USING CONSTRUCTED WETLANDS

Surface and sub-surface constructed wetlands for stormwoter management P Geary, M Sa un d ers, D Waters

~ THE USE OF SEWER PRESSURE SYSTEMS FOR MOUNTAINOUS REGIONS Pressure systems suit hilly areas, and con be linked to conventional gravity systems K Logan, M Cavan ey

Visit the Australian Water HOME PAGE Association and access news, calendars, bookshop and over 100 pages of lnfonnatlon at I

OUR COVER: Pressure sewerage systems seem to offer the potential for far less environmental disruption than traditional sewerage systems, particularly in enviro11111entally se11sitive areas such as temperate rainforests. See article on page 61. WATER SEPTEM BER 20 0 3

1


FROM

THE

PRESIDENT

GROWTH, CHANGE AND YOUTH Victoria

I attended the Victorian Branch ACM Dinner in August and what an event this turned o ut to be. It was attended by close to 500 people and networking was the th eme of the night. The speaker at the Dinner was Deputy Prem ier J ohn Thwaites, who was enthusiastic, articu late and informed - Victoria is lucky to have a water leader of such calibre and suc h a high profile within Cabinet. At the dinner, Branch President Peter Robi nson ended his term and handed over the Pres idency to Dr M elita Stevens, a deserving and popular successor. Peter now takes on the role as an Executive Member of th e Board and we look fo rward to hi s contributions. Women in water

It is especially pleasing to see Dr Melita Stevens taking over the helm of one of our strongest branches. In Queensland we also have Dr H elen Stratton, as the current Branch Vice President and a member of AWA's Board. The infusion of women into our Association leadership is most pleasing and follows previous good work done by Margaret Domurad from the Western Australian Branch, who was both B ranc h President and a Federal Councillor, and Therese Flapper ¡who served as NSW Branch President a few years back. These women provide excellent role 1nodels for others in the water industry. Illness

In NSW, Roger Pettitt, who has been serving on AW A's Board for some years now, has had to give notice of his resignation, so he can focus on a course of chemotherapy. Roger has been a valued contributor to AW A affairs, in NSW and nationally, and his insightful comments and discussion will be missed. I wish Roger every success in his treatment regime and we hope to see him back in the fray soon. It is also sad to note that long-term AWA stalwart, Leon De Witt H emy AM, has been hospitalised with a heart problem after a heart atta ck. Apart from his long and industrious career in the water industry and his long standing and extensive support and inputs to AW A activities, Leon was known as the person who always had the tricky question at our annual general meetings. We wish Leon a speedy recovety. 2

WATER SEPTEMBER 2003

Rod Lehmann Inquiry into the future of urban water supplies

I was pleased to be able to participate, with our CEO, Chris Davis, in a presentation to the Parliamentary Committee inquiring into the future of rural water supplies in Australia . We had made a substantial written submission a year ago, so we foc used on a few key points and then were engaged in a lively debate with the committee. I was particularly keen to make the point that, when I atte nded the World Water Forum in Kyoto, it was clear that many countries are paying serious attention to the possible impacts of climate change and that we should be taking serious steps to strengthen our systems. I also commented o n recycling and the need to facili tate the practice, rather than having barriers and conflictin g requirements arou nd the countty. Chris raised the very valid point that the appointment of one Minister to look after water federally would be a big step fo rward. We also raised the issue of small rural communities and the need to provide ways of improving standards and viability. The wired world

AW A's web site has been operating for several years now, through two incarnations. We've realised that, much as it has a lot of information (5,000 files) it is actually not vety user-friendly ; parts are out of date; and there's a great deal of scope to enrich the content. As the web site (http:/ /www.awa.asn.au, in case you aren't aware) is our most widely accessible product, and as it has potential to enhance member services, especially for those members not conveniently located to participate in capital city or major regional activities, we are preparing for a radical revamp and improvement.

Although we are engaging professional help to advise us and to do the design for the new web site, I would be very glad to hear from individual members abou t their particular expectations and ambitions for a relevant, useful web site. W hat would you like to find w hen you visit the site? Librari es of documents; the Bookshop; interactive calculation pages fo r common water problems; or anything else' Please just drop me a line on president@awa.asn .au and give me your w ish list and your comments - we hope to translate the most sought-after services into reality. Generation gap

There seems to be a general view that the younger generation (presumably any person younger than yourself) are not interested in membership of industry associations such as AWA and community clubs such as Rotary or Lions . Whether this is right or wrong is obviously debatable but there has been a general decline in membership in all of these sorts of clubs and associations over the last ten years brought about by an aging of the current membership and a failure to induct new younger members . This has been a trend in AWA and one which we have addressed through the engagement of a Membership Sales R epresentative and through our current initiative to upgrade the web site to provide greater electronic access to information (on the basis that younger people use the web as their main source of information). This topic was also one which was addressed by one of our keynote speakers, Ron Mcfarlane, Executive Intelligence, at the recent Executive Briefing in Melbourne (it preceded the Melbourne Branch AGM and D inner). Ron made the point that there is a new culture emerging internationally which is suggesting that the smart water companies and water businesses are engaging the community proactively in community improvement of environmental, social and other problems. Where problems emerge these companies/businesses are taking a lead role in sorting out problems, doing new things and resolving issu es. It is my belief that the community is looking for the emergence of a social conscience and that we should be working to promote that. Rod Lehmann


ANAEROBIC TREATMENT OF JUICE AND FRUITPROCESSING EFFLUENTS L Palmowski Abstract

Eilluents fro m the j uice and frui tprocessing industries have high organic matter content. Disc harge of these eilluents without appropriate treatment would therefore have a negative impact on t he environment. Hi gh orga nic co nten ts and low contamination levels make such eilluents suitable for biological treatment, especially anaerobic digestion. In the latter process, significant amou nts of digester gas can be produced, turning a waste stream into a source of renewable energy that can be used for electricity and h eat production, leading to financial benefi ts. Th is paper investigates the feasib ility of anaerobic digestion and the gas generation potential of five different eilluents from th e carrot-j uice, orange-juice and sultana pro cessing industries. Benefits are assessed in terms of digester gas production and organic matter reduction. T he results show tha t the specific gas production ranges between 665 and 860 1113 per tonne of effl uent treated (as organic dry matter). Furthermore, nearly 100% of the organi c matter is converted into gas in the case of the carrot- and orange-juice processing residu es, wh ile a 84.5% reduction of the organic matter was found to be achievable in the case of the sultana wastes. Whil e these results are promising, fu rther testing will be required to validate them in a larger scale.

Introduction Liquid and sol.id eilluents from the food and beverage indu stry contain a high proportion of orga n i c matt e r (Austermann- H aun et al 1999) . T he discharge of such untreated liquid effl uent in to the sewer often causes signifi cant problems for local water au thorities, while the dispo sal of solid residues in landfills is a major source of odours, leaching and groundwater contamination. Eillu ent treatment p rior to discharge is therefore a necessity. The food, beverages and tobacco manufactu ring sector has by far the

20

WATER SEPTEMBER 2003

(/)

-s0.

E

Anaerobic treatment

Digester gas

!

(/)

-s0. s

0

Electrical Energy, Thermal Energy,

CO2

Aquaculture, Algae Production, Crop Irrigation

Figure 1. Inputs and outputs of an anaerobic treatment system.

highest water intake of all manufacturing industry sectors in Australia with 242 GL for the 2000-2001 fi nancial year, half of wh ich is self-extracted and half is mains water (Australian Bureau of Statistics, 2001). T his sector also has the highest expenditure for environment protection of all manufacturing sectors in Australia: $164111 for 2000-2001 ($68111 for solid was t e and $ 57111 for liquid waste m anagem ent). In addition, the capital expenditure amounts to $101111 for the same year. Strategies are needed to transform waste streams from the food, beverages and tobacco sector into valuable products, shifting a fi nancial burden into beneficial outputs. The high organic matter content of j ui ce and fruit-processing industry effluents as well as their low contaminant levels make them suitable for biological treatment such as anaerobic digestion or aerobic composting. In th e case of food industry effluent, an anaerobic process has many advantages over composting due to:

• production of digester gas that can be L1Sed to generate heat and/ or electricity as a renewable energy so urce. This energy prod uction can lead to a net finan cial benefits for the industry ; • low sludge produ ction; • better handling of suspensions and waste streams with high moistu re content; and • absence of required aeration, w hich is particularly energy consuming in the case of food-processing eilluents. The aim of this study was to evaluate the suitability of anaerobic digestion for the treatment of a variety of liquid and solid eilluen ts fr om the fruit- processing and j uice industry. For th is purpose, the digestion efficiency has been m easured through: • digester gas production; and • degradation degree, representing the organic matter reduction. T he following section gives some background information on the mi crobial process involved in t h e ana erobi c digestion process. lt also presents the range


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o f inputs suitable for this process together with th e num e r o u s b e n e fi c ial o utpu ts that res ul t fr om digesting organ ic matter anaerobically .

se t-up consisted of a lL batch reactor, in w hic h the sample was introdu ce d w i th see din g m a t e ri al, a nd a g as collection column. Each test was ca rri ed out in The anaerobic dupli cate a nd r es u lts digestion process were averaged fro m the two parallel tests. The Anaerobic digestio n is reac tors were kept at a the biological degradation A B constant tempe rature o f of complex o rganic m atter 35°C in a water bath and 1 11 an oxygen free stirred manually once a environm ent. It consists of day . T o e nsure a good a four -st e p bi o logical b alan ce be twee n th e process invol vin g various various bacteria populam icro-organism populations and a successful ti ons. In the fi rst two start o f the experime nts, step s, c om pl e x the sa mples we re seeded com pounds are broken at the beginning of the d own b y fe r m e nta tive D e xperiment with sludge bacteria to o rganic ac ids, Figure 2. Examples of effluents tested. A: Liquid effluent f rom ca rrotfrom a local anaerobic alco hols, carbo n dioxide ju ice processing; B: Solid residue from carrot-juice processing; C: Solid di gester treating piggery a nd h y d ro ge n. Th ese residues from sultana processing; D: Solid residue from orange-juice waste . T w o r eactors interm ed iate products are processing. co ntaining only seeding then tran sform ed int o sludge were used in acetic acid and fin ally in to orde r to quantify the gas me than e and CO 2 . Th e production resulti ng from the slu dge deco mpositio n itself. T he digester gas produ ced contains betwee n 50 and 75% methane, experime ntal set- u p is ill ustrated in Fi gure 3 . depending o n the co m positio n of th e organic matter used. This represen ts a valuable source of re newabl e energy that ca n be Th e experime nts were carried out fo r a period o f 22 days. used to gen era te electricity and heat. Alongside with digester Th is degradatio n period was c hosen to provide results useful gas, the remaining solids fo rm a stabilised humus that ca n be fo r most large-scale p rocesses, w hich typi cally operate with used as a ferti liser, generally after a short aerobic post-treatm e nt rete ntion time o f up to 20 days. Th e digester gas generated from ai ming at re mo ving residual levels of volatile acids. T he liquid ea ch reactor was co ll ec ted in a co lum n imm ersed in water and content of the remaining so li ds can be increased by a dewatering the volum e of gas produced was recorded daily. Both th e total pro cess, making its transpo rt and use as a fert iliser easier. T he solid and volatile solid content of the samples and of the seeding sludge were meas ured at the start and end of the expe ri ments. liquid phase ca n be recovered and used for irrigatio n or as nutrient solution due to its high nutrie nt conte nt. Figure 1 Definition of the parameters used outl in es possible inputs into an anaerobic treatm en t syste m as well as th e vario us products fro m the process. Gas production data were used to quanti fy the specific digester gas production , defi n ed as the quantity of di gester gas Liquid and solid effluents from the juice and produced per mass of dty organic matter co ntained in the sample fruit-processing industry (Equation 1) (Palm owski and Mi.iller, 2000) R epresentative sa mples were taken from a ca rrot-juice v s - v s.ss Specifi c gas production = -~--~- -J000 [L/kg] p rodu ctio n site, an orange-juice productio n site and a sultana Ill in ¡ VS Substmtc processing plant. Photographs of some o f the samples are shown Equ atio n in Figure 2, w hile total solid and volatile solid conte nts are presented in T able 1. wh ere Vg [L] Am ount of d igester gas produ ced

Experimental set-up To assess the digestio n efficie ncy in terms of gas prod uction and o rganic ma tter reductio n , a series of anaerobic di gestion experimen ts was condu cted. T he basic unit of th e experim ental

vg,ss

ILi

m in

[kg]

Am ount of digeste r gas produced by the corresponding am ou nt o f sa mple containing seeding sludge only Mass o f substrate added in the reacto r

Table 1. Total solid and vol atile solid content of th e investigated effluent streams. Effluent type

Total solid content [g/ kg)

Volatile solid content [g/ kg)

Orange1uice liquid effluent

0.89

0.83

Orange-juice solid effluent

102.8

95.8

Carrot-juice liquid effluent

2.45

2.33

Carrot-ju ice sol id effluent

48.14

45.78

Sultanas waste

8 0 5 .5

7 08 .5

22

WATER SEPTEMBER 2003

VSsubsmre [g/kg] V o l a t i I e solid co nte nt of th e substrate Note : Th e gas volum es Vg and V g,ss are prese nted in the follo wing in no rmal conditions (T 11 = 0°C, p 11 = 101.325 Pa) to elimina te any influ ence of varying tempe rature and pressure conditio ns. T o d esc r ib e th e o rga ni c m a tt e r reductio n during the degradation process, th e degree o f degradatio n was calculated using a maximum sp ecific gas production ,


INDUSTRIAL

WATER

AND

WASTEWATER

established through theoreti cal consideratio ns. For an y organ ic co mpound, a che mi ca l reaction can be written to summarise the overall digestion pro cess and the production of methane, ca rbon di oxide and hydrogen sulphide (Equati on 2) (Roediger et 11/, 1990): h

o

7n

s

7p

c

h

o

C H ON SP + (c-- - - + - + - + -) H 2 O ' " " "'" 42424 c

h

o

3n

s

Sp

(2 + 8 _4 _8 _4 + 8) CH 4

Sn

~

s

3p

+ (2-8 + 4-8 + 4 + 8) CO 2

+ nNH ; + (n - p)HCO ; +sH 2 S + pH 2 PO ~ Eq uation 2 For a known substrate com position (i.e. the content of each elem en t (c, h , o, n , s and p) is known), the amou nt of d igester gas ca n be calculated using this model reaction . T he maximum spec ific gas produ ction can then be determ ined usin g Equation

Figure 3. Experimental set-up conta ining 12 digestion units

(left: the reactors, con nected to gas collection co lumns, here on the right).

3. (c - n

~Ji T I

+ s + p) - - '

Maximum specific p = - -- -- - - ~~ - - - 10 6 [Ukg] . c + h + 16 . + 14 . gas production 11 + 32 . s 0 12 Equation 3 11

IKl Temperature in no1111al conditions T 11 = 273, J5 K = 0°C P" f Pa I Pressure in normal cond itions p 11 = 101.325 Pa 9\ LJ l(K• mol )I U niversal Gas constant The degradation degree can then be calculated as the ratio o f the specifi c gas producti on to the maximum specific gas production. For the purpose of these investigations, th e composition of the substrate was taken from t he li terature (Souci et 11/, 1994) usi ng an average C, H , 0, N , Sand P concentratio n for proteins, lipids and ca rbohydra tes . w here T "

cases is to segregate liquid strea ms as close as possible to th eir production so urce and to send o nly the organic rich streams to the digester, while diluted stream s suc h as those resulting from the last rinsing step of a clean ing process are excl uded. Specific gas production and degradation degree

Th e gas production alone does not describe the degradation efficiency, as the amount of gas produ ced does not on ly vary with the e fficiency of the process but also with the concen tration of organic matter presen t in the sample. To overcom e this fa ct , the spec ific gas prod uction, defined as the gas production per mass of organic dry matter in th e sam ple, is used. End resu lts

Gas produced per volume of liquid residue treated

o

2.00 - , - - - - - - - - - - - - - - - - - - - - - - ,

4)

Results and Discussion Gas production per volume or mass of wet residue added

T he cumu lated amount of gas produced is represented 111 Figure 4 ove r the degradation time. Lt is presented per volume of liquid residue treated in the case of orange- and carrot-j u ice residues, and per mass of wet solids for the soli d residues treated (oran ge and carrot pulp, and su ltanas) . For all sa mples, it can be see n that the most important production rate is occurring du ring the first 12 to 14 days of degradation. A small to almost no nexistent gas producti on was recorded aft er that. This can be explain ed by the presence of large amo unts of readily biodegradable matter in the sa mple , which is tra nsfo rmed to gas within the first two weeks of degradation . After exha ustion of this matter, micro-organisms utilise m ore com pl ex compounds following a different kinetic model w ith lower digestion rate (Palmowski and MijlJer, 2002). Figure 4 highlights the benefits of highly concentrated residues as the ratio of gas produced per volume of reactor occupied becom es fa r more advantageous the higher the o rganic matter concentration of the treated sample . This would not only affect the capital investment for the reactor but also the operational costs due to the lower heating requirements, which represent a key cost factor in anaerobic processes. P re-treatments such as sedimentatio n , flo tation or centrifugation can be applied to concentrate the organic matter in the residue before subjecting the eilluen t to anaerobic digestion. H owever, in the case of dissolved organic matter, wh ich is dominant in the case of liquid residues from the juice industry, cost-effective methods of organic matter concentration do not exist. An approach to adopt in such

E ..-, 1.75 -t-- - - - - - - - - - - ---,..,..a-===-=-----l .2 E

~e

g 8_ ";; § :§

·-

ti

1.50

1.25 +-- - - - - - - ~- - - - - - - - - ---! 1.00

1/)

~ 0.75

::, "O

+--- - - - - - ~,C......---- - - - - - - - - - i

-1--- - - - ~- =-

'8.. ·s o.5o -+-- ---=-=-1r?-=-------1 er ~ = 0.25 -!---.._...~~- - - - - - - ! ~

0.00

...._ Carrot juice

~~~-~---~--=;:::::;:::::==::::::;==:::'......---l 5

0

10

15

25

20

Degradation time [d)

Gas produced per mass of wet solid residue treated

5

10

15

20

25

Degradation time [di Figure 4. Top: Gas production per volu me of liquid resid ue treated (orange- and carrot-juice residues) . Bottom: Gas production per mass of solid (wet) residue treated (orange, carrot and sultana residues).

WATER SEPTEMBER 2003

23


INDUSTRIAL

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for the five samples investiConclusions and Gas production per kg of organic dry matter treated gated are presented in Figure further work 1000 5 . The results are relatively T he small-scale exper900 consistent for all samples, iments carried out in this C: 800 with a specifi c gas .5! ';:' study demonstrated the u 700 :, ~ production of 665 and 860 "C .; feasibility of ana erobic 0 E 600 111 3 per tonne of eHJuent a. ~ digestion for the treated (as organic dry 500 ~ "C treatment of liquid and 0) ~ matter). 400 " 0 solid streams from th e ~ Dividing the previous 300 C. ÂŁ juice and fruit-processi ng en results by the maximum 200 industry. Very good gas specific gas production that 100 productions (66 5 and would be achieved if 100% 0 860111 3 per tonne of Orange liquid Orange solid Carrot liquid Carrot solid Sultanas of the organ ic matter within o r ganic dry m atter the sample was transformed treated) and high degraFigure 5. Specific gas prod uction for the different investigated into gas, gives the degradation degrees (84.5% sam ples after 22 days of anaerobic digestion. dation deg r ee or and above) were obtained d egra d a ti on efficiency for all i nvestigated (Figure 6). It can be seen samples. Throughout the Based on the chemical reaction for that a degradation degree of 100% (or digestion proc ess, no in hibition or slightly more) is obtained after 22 days anaerobic d igestion presented above nutrient lim itation was observed as the for the liquid and solid residues from (Equation 2) and on th e composition of gas p roduction shows a smooth growth orange- and carrot- processing. These the samples from literature data (Souci et over the entire degradation period, until high degradation degrees reflect the fact al, 1994), the methane content of the expi ration of the availab le organic that the organic matter presen t in the digester gas produced was determined matter. samples was fu lly biodegradable under (Table 2). Based on data obtained fr om these anaerobic conditions and that no nutrient U sing an efficiency of 25% fo r an preliminary studies, it was found that a limitation (although no external nutrients electricity generator, a typical orangestandard ora nge-juice factory has the were added to the sample) and no juice factory with an annual solid waste potential of generating 1,260,000 kWhelec inhibition occurred during the process. per year by b urning the digester gas stream of 10,000 t/y and annual liquid The values exceedi ng 100% can be waste volume of 250 ML/y, has the produced in an anaerobic digester from explained by some error introdu ced in its solid (10,000 t/ y) and liqu id wastes potential of generating 1,260,000 kWhelcc the calculation of the maximu m specific (250 ML/y). Additional benefits could be per year. To this significant generation gas production. As descri bed in section deri ved from the use of th ermal energy of green elec tri city comes therma l 5 above, the maximum specific gas as well as liquid and solid residual energy, a by-product of the electricity production was determined using th e strea111s . generatio n that ca n be used to heat composition of the samples as fou nd in surrounding bu ildings, offices, greenThese encouraging results will need to Food composition and Nutrition tables be validated through further investigations hou ses, etc .. As shown in Figu re 1, a (Souci et al, 1994) The samples used can in continuous pilot-scale experiments liquid and a solid stream are also differ slightly from those referenced before be ing ap pli ed to large-scale produced. Due to the microbiology of average values, thus leading to small processes. Furthermore, investigations on the anaerobic process itself, nitrogen and errors in the maximum specifi c gas the feasibility of anaerobic technology phosphorus are fo u nd in these liquid and production. should be extended to other foodsolids streams, wh ich make them h ighly The degradation degree of the sultanas processing sectors. Most food-processing suitable as fertilisers. is 84.5%. This can be considered as a very industries have liqu id and solid eHJuents satisfactory result compared to degradation with characteristics similar degrees obtained by oth er to those of the juice and degradation processes and to fruit-processing industry: Degradation degree based on the maximum specific gas other substrates treated by production high o rganic co n ce n anaerobic diges ti on. tration, high fractio n of H owever, it indi cates that so luble and r ead il y parts of the samples were ~ 100% ava ilable substrate, and not degraded in the process. low levels of contaminants 80% Woody particles present in and inhibi tory substances. the original sample, as can 60% This should open the path be seen in Figure 2 (c), have for anaerobi c technology 40% no t been su bj ect to and green-power generanaerobic digestion. T his 20% ation from digester gas to conforms to literature data numerous indust1y sectors. (Tong et al, 1990; Young Acknowledgments and Frazer, 1987) and was visible by observation of the Figure 6. Degradat ion degree after 22 days of anaerobic digestion, T he author thanks Jock reactor conten t at the end Charles from C harl es IFE based on the maximum specific gas production, established using and Peter McNarn from of the experiment. the co mposition of the samples. M~

24

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Eastview systems for their support in the project and the companies that provided the samples but whose names have been kept confidential in th is report.

The Author Dr Laurence Palmowski joined the School of Engineering and Technology, Deakin University, Geelong, Vic 3217, in 2001 after completion of her PhD on th e anaero bi c d igestion of organic materials. Email: lpalm@deakin.edu. au

References Austermann-Haun U , Meyer H , Seyfried C F, R osenwinkel K H (1999) : Full scale experiments with anaerobic/aerobic treatment plants in the food and beverage industry. Wm. Sci. Tec/i., 40, l, 305-312 Australian Burea u of Statistics (2 0 02) : En vironme nt Protecti on, Minin g and M anufacturing Industries, Australia 20002001. Al3S - Commonwealth of Aust ralia Palmowsk i L M, MLiller J A (2000): Influence of the size reduction of organic waste on their

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Table 2. Methane content and lower heating value of the digester gas produced. Effluent type

Methane content [%]

Lower heating value of the digester gas [kJ/ m3 ]

Orange-j uice liquid effluent

50.0

Orange-j uice solid effluent

50.7 50.6 51.8 49.2

17,925 18,176

Carrot-ju ice liquid effluent Carrot-ju ice sol id effluent Sultanas waste

anaerobic digestion. Wat . Sci. Tec/1., 41 , 3, 155-162 Palmowski L M , Muller J A (2002): Anaerobic degradation of organic materials - Significance of the substrate surface area. ENVIRO 2002 - Ill IWA World Water Congress, 7-12 April 2002, Melbourne R oediger H , Roediger M, Kapp H (1990): A 11aerobe a/kalisclie Sclila11111ifa11/1111g. 4th ed. , R.. Oldenbourg Verlag, Mi.inchen Souci SW, Fachmann W, Kraut H (1994): Food Composition and Nutrition T ables - Die Z u sa mm cnse t z un g d c r Lcbcnsmittel,

18,140 18,570 17,638

Nahrwertcn-T abellen - La C omposition des Aliments, Tableaux des valeurs nutritivcs. 5th ed., Medphann Scientific Publishers, CRC Press, Stuttgart Tong X, Smith L H , M cCarty P L ( 1990) : M ethane Fermentation of Selected Lignoccllulosic Materials. Bio111ass 2 1, 239255 Yo ung LY , Frazer A C (1987): The Face of Lignin and Lignin-Dcrivcd Compounds in Anaerobic Environments. Ceo111icrobiolo& )'

)011r11al 5, 3/4, 261-293

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DESALINATION ON THE BURRUP PENINSULA: OPENING UP FRONTIERS G Crisp Abstract

• A wastewater disposal system to service industry in the Kin g Bay - H earson Cove Industrial Estate. • A desalination insta llation inside ch e Burrup Fertiliser site.

Desalination of seawater can enable econom ic development in remote water-scarce coastal locations where th e development of conventional water resources is n ot poss ible, li1nited or Seawater Supply trm• i d environmentally constrained. The Seawater will be abstracted Burrup Peninsula, situated in the ound from King Bay by means of a Pilbara Region of Western common-use r intake pump Australia, is a location where the ,,.._,.. facility. The seawater will be potential for indu strial develfiltered to a nominal 80 opment is enormous due to an 11CM.£ I : 121000 Ill M microm.ecers and chlorinated abundant su pply of natural gas Nickol then suppli ed to all of the offshore, but co nstrained by Hay industrial planes th rough a l imi t ed water suppl i es. common-user seawa ter supply Desalination can provide the pipeline of 280 ML/d capacity solution. to service two seawater cooling Future exp an sion o f th e towers and as a feedstock for current water supply would cost desa li nat ion. T h e seawa ter in excess of$100 million, as new intake pum.ps will be controlled sources are remote and as yet, to maintain the water level in a unproven . The Water se r v ice tank from which Corporation has been requested seawater will gravitate to the to provide a seawater supply individua l b uffer tanks on system, a wastewater disposal and demand , controlled by ta nk the first of a number of potential inlet control valves at each of the desalination installations. individual sites. T hese valves will This paper is an overview of close, slowly, under failure t h ose aspects t he Water Location of the industrial area on the Burrup Peninsula. conditio ns either of the seawater Corporation has considered when user or of the seawater supply choosing the optimum desaliKing Bay - Hearson Cove and Which nell system. The service tank is sized co contain nation process to satisfy projected water Ease Industrial Areas, they include: sufficien t seawater to prevent air enterrequirements of current and future Burrup tainment into the seawater supply pipeline Burrup Fertilisers Pty Ltd (ammonia); Peninsula industries. Dampier Nitrogen (ammonia/urea); Japan on shut down of the seawater intake Introduction pumps. DME (methanol and dimethyl ether); Methanex (methanol); International DME The Burrup Peninsula, located in the Wastewater Disposal (DME (m.echanol and dimethyl ether). Pilbara Region of Western Australia, is Only wastewater (comprising brine Potable water will be supplied from the being developed as a major industrial area from cooling circuits and desalination current scheme on a commercial basis, but for downstream processing. The region facili ti es, industrial wastewate r and future potable sources are remote, as yet may realise up to $10 billion of Gas- tounproven, and are estimated to cost over domestic wastewater) that co mplies with Liquids (GTL) development over the next relevant DEP/EPA licence criteria will be $ 1 00M: even the groundwater 10 years. Several projects are currently component of the current source is being accepted into the Water Corporation planned or u nder development in the re-assessed . wastewater disposal system (Environment The Water Corporation of Western Protection Agency [2002]). Wastewater This paper is an edited and updated version of Australia has been requested to provide: will be discharged into King Bay through the paper presented at the 21st AW A Convention together with a paper to be presented to the a sub-sea pipeline and diffuser array. The • A seawater supply system to service l nccrnational Desalination Association in the wastewater discharge will mix w ith the industry in the King Bay - H earson Cove Bahamas in early October 2003 . seawater sufficiently co meet: Industrial Estate.

...

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• Tr igger v alu es for toxicants fo r protection of 99% of species, as defined in the ANZECC Gu ideli nes, at the edge of the outfall diffuser mixing zone. • Trigger values fo r physical and chemical st r esso rs, exceptin g nitrogen a nd phosphorus, as defined in the ANZECC, Gu idelines at the end of the outfall p ipeline . The ou tfall diffuser array is designed to achieve rapid dispersion of the wastewater into K ing Bay. To achi eve this, th e velocity of wastewater discharge from the diffuser ports m ust b e maintained at approximately 4 .5 m /s. Each syste m user will pu m p their wastewate r in to a co mm o n u se r wastewate r pipeline . T he wastewater will be discharged to the ocean such that the exit veloci ty from the diffuser ports rema ins relatively constant, irrespective of instantaneous system use. Th e diffu ser ports will be designed to mini mise the pote ntial fo r marine growth or sedim ent i ngress into the ou tfall pipe.

System Availability/Reliability T he seawater suppl y and wastewater disposal systems are essential fo r the

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Table 1. Costs of the considered desalination processes. Investment ($/ kl/ d)

Process

Capital ($/ kl)

MED ME-TCD

1400 · 2800 1380 - 2 780

MVC

1390 - 3880

0.4 - 0.8 0.4 - 0 .8 0.4 - 1.2

SWRO

1240 · 2480

0.3 · 0.7

Note:

Operation ($/ kl)

Total ($/ kl)

Energy

Other

0.6 · 1.7 0.5 - 1.6 0.9 - 3.2 0.5 - 2.0

0.3 · 0.8

1 .3 · 3.3

0.3 · 0.7 0.5 · 1.5 0.4 · 1.1

1.2 · 3.1 1.8 · 5.9 1.2 - 3.8

Costs for the desalination component only. Additional costs apply for feedwater supply and brine disposal. Power cost based on an electricity cost of $0.10/k Whr. The larger the plant, the lower the unit cost of water produced.

operati o n of the indu strial developments and must achieve practically 100% ava ilabili ty. Maintenance activities w ill need to be well planned, coordinated with all of the system users and executed such that system downtim e is minimised. In the event of a failure, the system will requ ire rapid temporary reinstate ment co m inimise impac t on the users. Where practicabl e, equipm ent will be installed with redundant un its that can be rapidly brought on-line. T he seawater supply an d wastewater disposal pipelines, however, will only be single pipes. Similarl y, on ly

a single power trans missio n lin e will be installed. The capi ta l cost estimate fo r the provision of a seawater supply system, wastewater (brin e, blow-down and treated wastewater) disposal system and power transmiss ion system for the first t hree stages is in the order of AU$70 m ill ion.

Desalinated Water Supply D es alination of seawater will b e cond ucted at eac h of the individual sites, either by the develop er or by the Water Corporation, as required, the selection of

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the most appropriate pro cess being the subject of this paper. The seawater supply w ill be typi cally adequate for fe ed to a thermal desa lin ation plant, but will require significant further treatment to be suitable for feed to a seawater reve rse osmosis plant, should that be co nsidered. There is a potential fo r a central desalination facility (up to 20 ML/d), servicing the requirements of multiple ind ustrial users, but th is may not o ccur in the short term. Th e facility may utilise either seawater or wastewater as a feed source.

Desalination Technology Opt ions Several options exist for the desalination of seawater. Selection of the desalination technology may differ from project to proj ect dependent upon, primarily, the type and cost of energy available. Th e gas process plants are exp ected to operate continu ously, 24 hours per day, 365 days per year. Commercial imperatives imply that the Water Corporation provides a reliable desalinated water s uppl y without inte rruption. Consequently, multiple desalination units are likely to be necessary to meet th e very high availability required. Those that offer the highest reliability and availability, commensurate with cost would be the obvious choice. Where possible, desalination equipment will be based upon standard design 'package' systems to reduce the capital cost. Potential desalination tec hnolo gies include : • Low Temperature T h ermal Plants

Mechanical Vapour Compression (1v!VC) These systems use an electric motor driven compressor as the driving force fo r distillation . MVC units typically utilise between 8 and 14 kWh to produce 1 kL of distillate. Energy consumption of less than 10 kWh / kL is typical for multi-cell

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Table 2 . Energy use of the considered desalination processes. Process

Gain Output Rat io (GOR) *

Electrical Energy Consumption (kWh/ kl)

MED

6-12 6 -14

ME-TCD MVC SWRO

N/A N/A

Thermal Energy Consumption (kWh/ kl)

Total Energy Consumption (kWh/ kl)

2.5 - 2.9

6.5 · 4.5

2 · 2 .5 8 -17 4.5- 8.5

12 · 6.5

9 · 7.4 14-9 8 - 17

N/A N/A

4.5 - 8.5

* GOR: Gain Output Ratio - the ratio of fresh water output (distillate) to steam.

plants. The advantage is that the system has a relatively high recovety in the region of 30 to 45% and produces a distillate at a temperature onJy slightly higher than the ambient condition. It can be supplied in modular planes and is better suited to smaller production units of less than 2ML/d capac ity. Of the thermal desalinati o n processes, MVC is inhere ntly the most t herm o d y nam ica ll y e ffi c i e nt [lnzelberg and Kronenberg, (2000))

M11/tiple Effect Distillatio11 with Thermo Compression (ME- T CD) This system uses medium pressure steam (approx imately 4 bar a or greater) as the motive energy and produces a warm disti llate and warm brine stream. Th e recovety of the system will be lower than that of a MVC system , resulting in increased seawater demand . These plants are likely to have a lower capital cost than MVC but require an external steam source if a dedicated boiler is not provided. Such a boiler would be an additional cost, but viable plants do operate in the Gu lf States. Multiple Effect Distillation (MED) This system is potentially the most efficient but does require considerabl e integration into the process design of the gas processing plan t. It shou ld be located very close to the steam turbines (or other steam so urce) and would accep t lowpressure (LP) pass out steam (at least 0 .3

bar a) as the motive energy. UnJike MVC, this unit requires a separate condenser and co nsequently needs a larger flow of sea water to the unit to m eet both cooling and feedwater demands.

M~tlti Stage Flash (MSF) This system has not been considered as these plants are used in large co generatio n facilities with water production in excess of25ML/ d, eliminating the possibility of a staged approach. T hese plants also require high quantities of LP steam < 4 bar a and would not be cost effective. • Membrane Separation Plants

Seawater R e11erse Os111osis (SWRO) D esalination is performed using a se mi-p enneable membrane operated at high press ure. No th ermal energy is required. The produ ct and reject streams will be at ambient temperature. R ecovety is typically around 40-50% but the product water will be higher in TDS than from thermal plants, requiring additional treatment, by means of second reverse osm osis pass and io n exchange desalination , redu cing th e recovery by up to 15%. Capital and consumable costs are likely to be lower than for thermal desalination systems. SWRO is likely to be problematic in this region due to the high turbidity of th e seawater and m ay not achieve the req uired availability and reliability, especially since th e region is

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prone to cyclones which will affect feedwater qual ity. Ship movements and tides may also impair feedwater quality.

Process Selection Criteria Cost of Desalination

Indicative costs of the desalination processes to be considered are presented in Table 1 (Crisp el 11/ (2001)]. These costs are for typical plant sizes between SML/d and 20ML/ d. For smaller plants the capital costs can be considerably higher. R edu ced costs for larger plant arise purely because of economies of scale . From the above it is evident that large scale ME-TCD could be the most cost-effective. However, since the initial demand is relatively small the economies of scale related to large plant, in excess of l 0ML/ d , do not apply. !f a cen tral desalination facility is adopted, it is likely that chis would be staged, once agai n eliminating the advan tages gained through economies of scale. Due to the fact that energy avai lability and type is intrinsically linked to the GTL process, desalination plant selecti on will be influenced by the energy available, either as steam or electricity. Consequently the capital cost of the plant to be supplied on the Burrup Peninsula, will not have as marked an affect on the selection process as other factors such as product water quality and availability.

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Energy requirements

Th e type of energy available for desalination on th e Burrup Peninsula is dependent on the GTL process to be adopted . The now discontinued Syntroleum project, fo r instance, included processes w hich resu lt in excess medium pressure (MP) steam at 5 bar a being avai lab le at no or ma rginal cost: this would be a strong driver for the METCD process to be adopted. The Burrup Fertiliser process requi res large quantities of elec tricity for start-up and therefore has excess installed capacity available at no or marginal cost. H ere SWRO or MVC have been considered. Table 2 presents the energy requirem ents required fo r the processes in co ntention [Crisp el 11/ (2001)]. The thermal energy req uirement has been presented in kWh/kL instead of steam co nsumption. This is based on the equ ivalent amount of electricity that could be produced from the steam, making comparison easier. From Table 2 it is evid ent that if the cost of energy is high, SWRO has the advantage. However, in the Burrup Peninsula this is not the case and fo r this reason, SWRO is not necessarily the forerunner from an energy cost viewpoint. Feedwater Salinity and Quality

Mermaid Marine harbour offers the best location for the seawater intake for a number of reasons. However, feedwater quality on the Burrup Peninsula would be highly variable. T he TDS of the feedwater is around 40,000 mg/L and the temperature range is 19°C to 32°C. Feedwater quality variability is further aggravated by the fact that vessel movements do occur at Mermaid Marine and high tidal ranges (up to 4m) prevail. The region is also prone to cyclones. A desalination process that is not h ighly sensitive to the prevailing feedwater quality conditions would be attractive. Plant Availability, Reliability and Useful Life

The process water systems are essential for the operation of the industrial developments. Loss of process water supply will have almost immediate impact on a process plant. Hence,

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the supply of process water n1ust ach ieve practically l 00% availability. At this stage it is envisaged that desa lination of seawater will be conducted at each of the individual sites, although a central desalination facility co uld be a viable option. In te rm s of reliab i l it y Low Temperature Thermal options score highly. T hese plants are rugged and proven, and combined with extremely low corrosion and scaling rates m inimal maintenance is requi red. Experience from installations worldwide shows that these units can run for 12 months before shutdown fo r cleaning. Significant changes have occurred in th e m aterial selection specified for these plants in the last decade. The development and use of stainless steel continues as an understanding of corrosion mechanisms and the associated kinetics is gain ed and this has resul ted in a wide range of alloys u nder the umbrella title of "stainless steels" being readily available. T his has led to the increased reliability of disti llation plan ts (Sommariva, (2001]). Further, m any of these plants utilise sta ndard equipmen t (ie . mechanical vapour compressors) which facilitates maintenance and enhances reliabili ty. Suppliers of the newest generation of distillation plants normally guarantee 6 months continuo us operation betwee n cleaning and guarantee pla nt ava ilabili ty nomin ally at 95 +%. Fu rthermore, with recommended maintenance, these plants can have a life of 30 years. Many fully functioni ng plants around the world are over 20 years old. With regard to SWRO, experience shows availab ility is of the order of 85 to 90% at best, especially in a region with variable fee dwater quality and temperature changes. SWRO plants can require fou r cleans p er train per year. Obviously

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th ese plants are designed w ith a standby train for the 1st pass of the RO plant. Th e provision of a spare train provides more flexibility for the design and operation of the plant rather than adding additional spare capacity to eac h train to make up for capacity losses during cleani ng. Membranes do degenerate in time with lives realistically between 3 to 5 years, with a slight increase in salt passage and required driving pressure occurring in tin1e. Des igni ng redundancy in to the SWRO plan t d oes not necessa rily increase the availabili ty if adverse conditions influencing feedwa ter qu ality p revail. Cyclon es, unseasonal algal blooms, ship movements and tides can bring about adverse conditions. Product Water Quality

A high quality of water (less than 10 m g/L TDS) is required for process water p urposes. All thermal options will produce high quality distillate Qess than 10 mg/L TDS), irrespective of variations in seawater t em pe rat ure, salinity and seasonal bacterial bl ooms which occur. A product-staged 2 pass SWR.O will at best produce permeate between 35 to 40 mg/ L T DS unless ve1y low recoveries are ex pected. At recoveries of around 25% a 2 pass SWRO at the Burrup Pen insula w ill produce permeate at less than 10 mg/L TDS. Additionally variations in seawater temperatu re w ill infl uence the product water quality. The h igher the temperature the higher the TDS of the produced permeate. If high quality product water is consistentl y requi red then th ermal technology is the obvious choice.

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Operation and Maintenance

High staff proficiency is required for the operation ofSWRO plants. SWR.O plants requ ire constant monitoring and supervision especially when feedwater quali ty is highly variable . In biologically active seawater, m en1brane cleaning can be required as frequent ly as every ten to twelve weeks. Generally all equipmen t in contact w ith raw water is constructed of corrosion resistant materia ls and maintenance of pipework and pressure vessels will be minimal. T he usual maintenance associated with pumps applies. T he most vulnerable components are the membran es themselves. Th ese usually have a life of between 3 to 5 years under normal conditions. Normally som.e membranes are stored on site, with an arrangement with the supplier to be ab le to provide membran es in su ffic ient quantities at short notice if requ ired. Low Temperature Thermal plants are more forgiving and a continued presence by operators, though preferential, is not mandatory. Further, operations personnel do not have to be as high ly trained and monitoring does not have to be comprehensive as with SWR O. Low Temperature T hermal plants are robust and apart from prescribed passivated acid cleans up to twice a year which take a few days, maintenance requirem ents are minimal. Th e only rotating parts, apart from the m echanical vapour compressor in the case of MVC plants, are associated with the pumps. Generally all equipment in contact with raw water is constructed of corrosion resistant materials. Scaling risk associated with these plants is low, due to the low concentration rates and th e low temperature of operation, less than 73°C. T he injection of a scale inhibitor is advisabl e.

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Installation Footprint

Although the available space for desalination facilities could be li mited, it is not expected that this will have a m ajor infl uence on th e selectio n of the desalination process or processes. Essentially there is not m u ch difference in the footpr int size required for the differen t processes wh en the output is less than 10ML/d. Normally SWRO has the advantage when large plants are considered , but with th e pretreatmen t component included , a SML/ d SWRO plant requires a similar fo otprint to that of a thermal process. The thermal processes have th e advan tage that they do not require housing which can also limit the footprint size. Should a boiler be proved for a ME-TCD option , footprint size wi ll be larger, but n ot substan tially so. Environmental Considerations

In general th e Environmental Protection Age ncy (E PA) supports the concept of industry desalinating seawater for process requirem ents on the Burrup Peninsula. It is the EPA's op inion that the proponents should address the following relevant factors: • m arine flora and fa u na; and • visual amenity. In relation co mari ne fauna and flora, handling of the reject brin e strea m to receiving waters has been the main aspect of environmental interest for the proj ect. SWRO may resul t in brine with higher traces of chem.icals such as antiscalents, whilst thermal processes m ay result in bri ne at a higher temperature. It has been concluded that w hi chever desalination meth od is ch osen, th ese factors can be overcome especially considering that the bulk o f wastewater is being produ ced by the actual GTL process.

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WASTEWATER

Sommariva C, (200'1), 1ncrease in Water Production in Middle East, In ternational D esalinatio n Association, Singapore C onference, March 2001 . Water Corporatio n o f Western Australia, (2002), Prelimi nary D esign Report - Burrup Peninsula Industrial Water Supply Stage 1, Western Australia.

Acknowledgment The author acknowledges the lace Alan Linstrum fo r bringing the Water Corporation and W estern Australia , in particular the Burrup Pen insula, into the serious age of desalinati on .

The Author Gary Crisp is a Busin ess D evelopment Manager at the Water Corporation. He has been primarily involved with Water Corpo ration desa lination projects for the last 4 years. During this time he spent 7 months in the UK and M iddl e East with a maj o r desalina tion contractor. Gary has been a member of all teams involved in all Water Corporation desalination endeavo urs (thermal and m embrane separation), including th e Perth Desalination Project (87 ML/day), Bu rrup Fertilisers Proj ect (3.6 ML/day), the Kwinana Water R euse Plant (16.7 kL/ d) and numerous small RO plants. Gary is a m ember of the International D esalination Association and has represented the W ater Corporation at the world forum on a nu mber of occasio ns. Email: gary. crisp@ watercorporation.com.au

Conclusion The Burrup Fertilisers proj ect will be th e firs t GTL installation constructed o n the Burrup Peninsula. It is expected that fi rst product delivery will occ ur in mid 2005. Since this installation w ill produce excess electricity at a low cost the logical desalination process choi ce is eith er MVC or SWRO. After assessm ent of all aspects relating co feedwa ter qual ity, produ ct water quality, desalination plant availability and access to electricity have been taken into account, it was recommended that MVC dis~illation be th e process of choice . The advantages of the MVC process clearly ou tweigh the disad vantages as outlined in previous sections. T his does not imply that MVC will be the process of choice for all installations and ultimately there may be a range of desalination processes on the Burrup Peninsula, even if a central desalination facili ty is the fin al solution. Future desalination installations to support the other GTL processes will be decided on a case by case basis, with en ergy type and availability most likely to be the prime determinants. A total capaci ty of 20 ML/ d is expected to be real.ised w hen all installa tions are in place.

References C,isp G, H ands P, Linstrum A, (2001), Desalination In Western A ustralia - An Overview, Water Corporation, Leedervilie, Australia. Environmental Protection Autho rity, (2002), Upgrade of multi-user seawater supply and introduction of wastewater to ocean outfall, Burrup Peninsula, Change to Enviromnental Conditions, Water Corporation, Bulletin 1044, Perth , Western Australia. Inzelberg H , Kronenberg G . (2000), R eview of Low Temperature Distillation Processes and R ecent D evelopm ents in Multi Effect Distillatio n (MED) and Multi Effect Mechan ical Vapor Compression (MEMVC) D esalination P roj ects, IDE T echnologies LTD. R a'anana, Israel.

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WATER S EPTEM BER 2003

31


INDUSTRIAL WATER AND WASTEWATER

WATER CYCLE WITH ZERO DISCHARGE AT VISY PULP AND PAPER, TUMUT, NSW 0 Szolosi Abstract The Tumut Pulp and Pap e r Mill produ ces 2 40,00 0 t o nn es of unbleached pulp and paper board per annum using ad v an ce d c l ea n e r p roduction technology and achieving significant reuse. Trea ted effiuent is irrigated o n 110 ha fa rm, producing feed for animals. Sludge and other waste products form a combined fertiliser fo r the fa rm. Vi sy Industries have developed a new Pulp and Paper Mill in Turnut, NSW, w hi ch showcased innovative environmental and sustainable energy technologies, including the biggest continuou s bio mass energy facility in Australia .

General Description of System The Tumut Mill use d extensive industry exp erience and many w ell known companies in the paper industry to engineer an advanced mill concept with zero levels of effJu ent leaving the site. C onstruction of the 1niU began in 1999 and was completed by 200 1. Total investme nt in the project was $435 million.

The Tumut Mill is producing 240,000 tonnes of unbleached pulp and paper board per annum from around 800,000 tones of pine plantation pulp logs and fo rest waste plus recycled paper and cardboard and sawm ill residu es To ensure the long term viability of the mill an addi tional 30,000 hectares of plantation is being established, which also will have a positive impact on the dry land salinity problem and serve to protect water catchment areas.

Water Cycle The Tumut Visy Pulp and Paper Mill is the cleanest mill in the world, reflecting

I ndu s tri es ' Vi s y commjtment to sustainable manufactu ring. Ca r e ful proj ec t planning included steps to mjrumjse raw water intake, maximise reuse opportunities and reduce effJu ent. B y comparison to other pulp and paper m ills, th e mill uses very little raw w at e r. Its ra w w a t e r co nsumption (5.5 m 3 / tonn e o f pap e r) and eflluent discharge (1.5 111 3/ to nn e o f pap e r) a r e believed to be the low est in the w orld. Other pulp and paper mills use up to fi ve times as much w ater. Raw Water Supply

Fresh water supplied to th e mill is sourced from the Tumut River throu gh a 375 m m DICL (ductile iron cem ent lined), bitumen coated, 14. 6km long pipeline. The water is pumped fro m th e river and boosted up at the mill site into th e R aw Water Dam, located 63 m above the mill. The holding capacity of the dam is 190 ML of water which is enough for 1.5 months of fresh water supply. During the off peak energy time, the w ater is pumped into the mill and into the R aw W ater Darn at the sam e time.

EFFLUENT FLOWS

TOTAL FRESH WATER DISTRIBUTION

RO

..

Reject

,,

Boller Blow Down 2%

~'V'.i'ii'~~-

TOTAL

WSAC

"" 32

WATER SEPTEMBER 2003

Clean Conden sa te 48%

Cooling

Tower 31 %


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INDUSTRIAL

WATER

AND

WASTEWATER

• RO plant (reverse osmosis plant) : filtering further down to 0.2 micron, feeding th e recovery and power boiler. • cooling towers • paper machine Effluent

In peak energy time, the water is supplied to the mill using the same pipeline, through gravity (only) from the dam. There is only one fresh water intake pip eline connection to th e mill. Filtration System

T here are two filtration units in the water distribution system. One at the river pumps and the second at the mill.

At the river, there is only one ABF brush filter (800 micron) and at the mill there is the pre-filtration unit w ith two screen filters (> 3.5mm) and a fine filtration system with three EBS screen filters (< 25 micron). Raw Water Distribution

After filtration, the water is distributed towards:

During the process and operation there are the following eilluent sources: a) Mill effluent discharged fro m th e mill: • excess clean co ndensate • cooling water bleed • wet surface air condenser bleed • boiler blow down • recrystallisation bleed • micro filtration rejects; b) Domestic sewage from the administratio n building and control room_(this is the only nutrient addition source to the wastewater treatment plant); c) Run-off water from th e wood-yard area. Most of the efiluent is reused. For exa mpl e, white water from the paper machine is used in the fibre line for pulp washing. After that, it go es to the evaporation plant to ensu re that any eilluent from the mill is free fro m salt. The remaining eilluent (clean condensate) is treated and it goes to onsite irrigation.

Waste Water Treatment Plant (WWTP) The WWTP of the Mill is a sequencing batch reactor (SBR) with biological nutri ent re moval (BNR) activated sludge process.

COD IN & OUT Average COD IN 261 ppm ; Av erage COD OUT 65 ppm 600 ~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~

550 1--- - - - - - - - - - - -- - - -~ - -- - +-- - - - - - - - - - l 1--- - - - - - - - - - - - --1-- ----- -- -+-- - - - - - - - - - l "' '--~ - - - - - - - - - ----- -1-- - ---- - - ----- -+- - - -----l

SBR during treatment

lOO

e

----------1

HO -

:: 300

C 250 0 c.., 200

-ff-- - --½--c~ - - ~-

LIO

l

ll

2l Jt ,1

31 61

71

81 91 l!ll l:.l l2l lll

:u

!Sl Ml 111 J.81 ~l 21)1 2ll 221 231 241 Hl Hl 211 2~! 291 301 lll 321 331 lU Bl 361

Datt

l• coo IN • coo our I SBR after treatment

36

WATER SEPTE MBER 2003


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WATER

INDUSTRIAL

This operatio n completes all unit process steps within the same reactor, eliminating the need for both secondary clarifie rs and sludge recycle system. Also, the aerator mixer system used for the diffusion of oxygen can both aerate and mix without the need for additional equipment. The required efiluent quality, as per the EP A license, is shown in Table 1. After the treatment, the treated efiluent is delivered th rough a 225 mm uPVC pipeline, through gravity to the farm into the Winter Storage Dam (480 ML capacity).

AND

WASTEWATER

Table 1. Required effluent quality, as per the EPA license. Parameter

50%ile

Effluent Quality 90%ile

100%ile

Units

BOD

mg/ L

15

20

40

ss

mg/L

20

30

45

TP

mg/L

2

TN

mg/L

20

O&G

mg/L

5

PH

6. 5-8.5

Irrigation T he treated effiuent is irrigated from the w inter storage through five centre pivot irrigators on 110 hectares of land, irrigating oats, sorgh um and pasture fo r silage and hay production. The farm also incorporates around 2000 head of cattle.

Waste Management/Monitoring The sludge from the WWTP, the lime m ud from the lime kiln and the ash from the boiler is used as a combined ferti liser for the soil. T he whole operation is under a strict monitoring system: • soil monitoring • ground water monitoring • surface water monitori ng • plant tissue monitoring • air monitoring The Tumut Mill has received several awards, including the Rabobank E nergy and Env iron m ent Awar d 2002 , sponsored by the Australian Greenhouse Offi ce; fin alist in t h e B an k si a E nviro nment Awa rds 2002; and NSW Engineering Excellence Award 2002. W hile the Tumut Mill is continuously imp rov ing i ts enviro n mental and manufacturing pe1fonnance it has already been hailed as one of the world's leading examples of sustainable manufacturing in an agricultural system.

Conclusion The T um ut Mill is an example of the Best Available Technology applied in a cost effective manner in an agricultural environment and lessening the impa'ct on water resources usually associated with paper mills.

The Author Otto Szolosi is Projects Engineer, Irrigation/Waste Wate r & Water M anagement , with Pratt Water Pty Ltd (D ep t . o f V isy I ndustries). Email otto. szolosi@visy.com.au 38

WATER SEPTEMBER 2003

Before irrigation

After irrigation

5.5-9,6


INDUSTRIAL WATER AND WASTEWATER

WATER MANAGEMENT INITIATIVES: THE MANILDRA GROUP D Fergusson Abstract T he M an ildra Group operates a water-intensive wheat bioprodu cts manu facturing fac ility in Altona, Victoria. Successive improvem ents to the water and wastewater cycles have resulted in reduction of trade waste charges fro m $SM to $2M pa. are rep orted in this paper.

Introduction M anildra Group com menced operatio ns in 1952 with a flo urmill located at Manildra, New South Wales. Progressively, the company has expanded th rough construction and acqu isitions to now include three flourmills, two starch and glu cose plants, an ethanol plant, a joint venture sugar refine1y and a pasta plant in Australia as well as opera tions in the United States of America. Man ildra Group is the largest user of wheat for industrial purposes in Austra lja, and also refines the entire N ew South Wales suga r crop. It is a leadi ng supplier of raw ingredients to th e food, beverage, confectionery and paper producing industries. In 2002 M anildra Grou p purc hased th e Bioproducts manufacturing fac ilities of George Weston Foods which include manufacturi ng sites in Q ueensland , Victoria and Western Australia, whi ch produce starch, gluten and glucose from wheat flour. The Victorian site located in Altona North is the largest of these sites and will be the focus of this article. Manildra Group, Altona North, co nsists of a starch plant, glucose plant and small corrugating Adhesive manufacturing fac ility.

The Manufacturing Processes T he manu factu ring process is essentially a water separation process, followed by further fractiona tion and purification. The firs t stage in the process is to separate the starch from the gluten; the gluten is then washed and d ried. Th e second stage of the separation process is to size classify the starch and then remove any residual impuri ties. The starch is then su ppli ed as a sluny, processed into glucose or dried. The Altona North site has two starch plan ts, Plan t One is based on the Martin Process described below, while Plant T wo is an advanced three phase decanter (centrifuge) called a Tricanter which separates the starch and gluten internally. The Tricanter generates three process streams, wh ich are: A Starch; Gluten and B Starch; and Lightphase, w hich contains the C Starch and soluble starch. The separation of the Gluten from the B Starch is readily achi eved through rotary sieves and the product streams are then com bined with the Plant One Streams. The formatio n of the dough is critical to the separation of the starch from the gluten. The work performed to create the dou gh causes the protein to form large elastic networks. The development of these networks aids in the separation of the starch from the gluten. The dough is then pum ped to a separation tank where it is mixed with large amounts of water to wash the starch

from the gluten by recycling any settl ed dough to break it apart. Once the starc h is wash ed from the gl uten its density is lower than the starch slurry and it overflows from the agglomeration tank and is pumped with the starch slurry to a Contrashear screen which is a contra rotating cy]jndrical wire screen. As the overflow from the agglomerati on tank is pumped o nto the screen th e al ready form ed pieces of gluten are rolled together while the starch slu rry passes through th e screen fo r fu rther processing. T he glu ten is th en pumped to the gluten washers where it is worked to remove starchy water from the gluten to increase the protein level. The gluten is washed, to achi eve th e protein content required and it is pumped to the Gluten drying area for drying. A dewateri ng screw is used to remove as much water as possible fro m the gluten before drying. The gluten is then fed

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WATER SEPTEMBER 2003

39


INDUSTRIAL

into a ring dryer. Ring dryers consist of a drying ring and an internal mill that breaks apart any large particles. Product is removed from the mill through a classifier and is collected using a combination of cyclones and a baghouse. The produ ct is then sifted and oversize material reintroduced to the dryer. The starc h slu rry contains thre e different size fractions of starch (being A, B, C, of which A starch is the coarsest) bran and pentosan, in addition there is a soluble component of approximately 10%. O nly the A and B starch are suitable for use in fi nal products. The bran and pentosan are recovered as byproducts and sold as stock feed and the soluble and very fine colloidal material traditionally sent to drain. The fractionation or separation of the size fractions is performed either through the use of hydrocyclones or centrifuges. Once the starch is separated it needs to be screened to rem.ove any bran or pentosan . After fractionati ng and washing the starch it is then concentrated and stored in slurry form for use. The A starch as well as been supplied as a slurry is also dried either unmodified (as native starch) or chemically modified. The B starch is normally used in the production of different types of Brewing Syrups and the colloidal and soluble starch has traditionally been treated as wastewater and sent to drain. It is possible to produce glucose for the food industry by two different methods these being, acid/ enzyme or enzyme/ enzyme conversion. In the acid/ enzyme conversion process, used by Manildra Group, Altona North, the starch is acidified and then cooked to break the starch down into simpler carbohydrate followed by the use of enzymes to convert the starch to glucose as required. O nce the desired conversion is achieved the material is then filtered and then purified using activated carbon and ion exchange as required. Finally, the glucose syrup is then concentrated to form a final product.

WATER

WASTEWATER

AND

Flour

Dough Formation

Modification Slarch Slurry To Glucose

Brcv. ing

Or Custome rs

S}TUp;

Packing & Di, tribution

Packi ng & Obtribution

Figure 1. Starch manufacturing process flow.

this time it was decided to install a biological wastewater treatment plant (WWTP) to process the plant wastewater. The technology chosen was Upflow Anae robic Sludge Blanket (UASB) Reactors with a pre acidification tank and it was installed and conunissioned in 1985. When installed the WWTP had a design capacity of approximately 40 t COD/d. However due to plant expansions to increase production capacity the actual loading on the WWTP was approximately 55 t COD/d in a wastewater stream of approximately 2.7 ML per day. Due to the aging of the WWTP and th e subsequent deterioration of the reactor internals affecting the perfom1ance, as well as the increased loading, the

Starch Phml Effiu<'nl (C

& Soluble Starch)

The Water Cycle The starch separation process is very water intensive with 10 to 15 tons of water required for each ton of flour processed. In addition the historical plant yield for the Altona North site has been in the order of 83% dry solids basis. T his resulted in approximately 17% of the flour feed going to drain and ultimately to sewer as a very dilute wastewater stream of 1-2% solids. D uring the late 1970s and early 1980s the cost of discharging untreated starch plant wastewater to sewer started to increase significantly. At

40

WATER SEPTEMBER 2003

\ Valer

I----

Prostock To Sto<:kfeed

Enzyme

Ethanol Production

Figure 2. MVR evaporation and feed system process flow.

conversion of the UASB R eactors declined from 85% to approximately 75% COD co nversion. This resulted in an additional increase in the trade waste disposal fee up to approximately $2.50/kL, with an annual operating bill for the WWTP in the order of $S M. On purchasing the Altona North site, Manildra Group identified that one of the major costs in operating the plant was the treatment of the wastewater from the process . To improve the viability of the business it was identified that it was essential to reduce the water treatment and disposal costs to as low as reasonably practical. In addition, there was also a need co increase the production rate by 25%. In order to meet the objective of reducing the costs of wastewater disposal, there were two key objectives. These were: 1. To reduce the total volume of water discharged from the site, and 2. To reduced the organic loading in the wastewater. To achieve these objectives, as well as to facilitate the increase in production rate, a capital improvement program was implemented. T his program included a total reconfiguration of the starch production process including extensive water recycling through the process, the installation of a third Gluten dryer (to facilitate the increase in production) and the installation of a Mechanical Vapour R ecompression Evaporator and associated equipment.


INDUSTRIAL

The reco nfi guration of the starch p la n t was aime d at in c reas in g th e p rodu ction rate, red ucing th e plan t fro m t w o pro duc ti o n pla nts to o ne and int egra tin g wate r recycl ing into the sta rc h separatio n process. T he ultimate goal o f the reco nfiguratio n is to reduce wa ter discharge fro m the plant to approxin,ately 3 tonnes per to nne o f flo ur pro cessed. T h is reco nfiguration has been und er ta k e n in sta ges du e to t h e requirement to maintain productio n at the site w hil e the cap ital w orks have been un d ertaken. P hase one of th e starch plant reco nfi guratio n fo cussed on introducing water recycling and simplifying the Plant One starc h size fractionation process from using w a t er intensive hydrocyclo nes to the use of high speed no zzle cen trifu ges. T his ph ase also co mbine d the Plant O ne and P lant T w o Starc h Strea ms for comm o n processing . P hase two o f th e starch p lant reco nfi g u rati o n will fo cu s on th e g luten sep a ration from the starch. It w iJI increase the level of water recycling as w ell as consolidate th e tw o starch plants into one plant w ith a h ighe r produ ction rate. In additio n p hase two will also see an

WATER

AND

WASTEWATER

Table 1. Summary of Ke y Performance Indicators fo r site wastewat er. KPI

Historical

Yield

Current

Projected

83%

>95%

>98%

Wastewater Flow

1 15 m3 / h

80 m3 / h

45 m3 / h

Daily Wastewater Flow

2. 7 6 ML/ d

1 .92 ML/ d

1.08 ML/d

WWTP CO D Loading

55 t/ d

20 t/d

Trade Waste Charge

$2 .50 / kl $5 M p. a.

$0 .97/ kl

6 t/ d $0.60/ kl

$2 M p.a.

< $1 M p.a.

WWTP Operating Cost

increased level of redundancy, w hich w ill result in increased plant availability, and red uce un planned shutdowns due to equ ipme nt breakdowns. The purpose of the starch plant reconfi guration is to reduce the ove rall water usage. H owever, a process e fflu ent stream is sti ll generated w hich contains C starc h and so luble material, this acco unts fo r almost 20% of the starch produced. In order to reco ve r t his mate ri al as a useful b yprod u c t th e M ec ha nica l Va po ur R eco mpressio n (MVR) Evapo rator w as i nstall ed. Prior to th e waste stream being fed to the evapo rator it is passed t hrough a liquefaction process, w hich co nsists of a jet coo ker chat breaks the coLloidal starch down and de natures th e protein. Th e denature d prote in is then separated fr om

th e liquor to produce a protein ri ch material, w hich is supplied as a stock feed. T he liquor is th en trea ted w ith e nzymes to reduce th e viscosity of the liqu or. T h e liquor is the n fed to the evapora tor , w hi c h prod u ces a syrup a t approx imate ly 45% solids. T hi s syrup is th e n u sed as a feed stoc k fo r the production of ethano l. At present th e capital works program is still unde rw ay, w ith P hase On e of the Starch Plant reconfiguration complete, the MVR Evapo rator complete and Phase two in progress. T he curre nt plant confi gu ratio n has sign ifi cantl y i mproved the qua lity and volume o f waste wate r disc harged to sewer. T he redu ction in the volume has been mostl y achieved du e to the intro-

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41


INDUSTRIAL

WATER

duction of water recycling in the plant and this has reduced the water discharge from approximately 115 m 3 /h to 80 m 3 /h. In addition 40 1113/h of this wastewater is fed to the MVR evaporator and converted into ethanol feedstock, and the condensate diverted directly to sewer. The remaining wastewater is pumped to the WWTP where it is treated using the UASB R eactors. One of the major issues associated with recycling water in a food processing plant is microbial growth , which affects the product quality. Th e control of microbial activity is a difficult task, which requires good engineering design as well as the addition of an anti-microbial agent to control m.icrobial growth. During th e reconfiguration of the plant a great deal of effort has been placed on removing dead areas in the pipework as well as the installation of a biocide dosing system . The completion of th e MVR Evaporator and Phase One of the Starch Plant reconfiguration have a significant effect upon the recovered yield increasing it from approximately 83% to 95%. This has consequently reduced the loading on the WWTP by 75%, which in turn has improved the quali ty of the wastewater discharged to sewer significan tly. As expected by reducing the loading on the WWTP the quality of the wastewater produced has been improved as well as a significant reduction in th e total opera ting costs. O n the completion of the capital improvement program it is expected that the product yield w ill be improved further to the goal of 98+% recovery, this will furth er reduce the WWTP loading. The improvement in the quality of the w astewater has been achieved by not on ly the reduction in the CO D loading on

NIJHUIS WATER TECHNOLOGY 16 Ling Place Palm Beach, QLD, 4221 PH: 07-5520-0587 AH: 0405-176-472

WASTEWATER

the WWTP but by also reducing the hydraulic loading. Th e largest reduction has been achieved by diverting the MVR Evaporator condensate directly to sewer. The COD of the condensate is approximately 200 mg/L, wh ich is much lower than the discharge from the UASB reactors, therefore in addition to reducing the h ydraulic loading ofche WWTP it also has the added advantage of diluting the final wastewater. However, due to the high q uality of the wastewater Manildra Group has been investigating the potential to reuse this water either interna lly or by supplying thi s water to external users. T he cost saving at the WWTP has been achieved due to several factors, which have resulted from the reduced hydrau li c and COD loadings. T he benefits of these reductions have been a lower chemica l consumptio n , including a two-thi rds reduction in alkali usage, a reduction in the production of scum, which is reused as a soil conditioner, as well as a reduction in the trade waste discharge fee to $0. 97 / kL. To date, the annual operating costs of the WWTP have been reduced from approxima tely $SM to $2M. In addition to the reduced treatment charges there is also the added benefit of greater product recovery. Ultimately, it is expected th at it will be possible to furth er reduce the operating coses of the WWTP . Manildra Group is currentl y investigating the future requi rements for wastewater treatment at the Altona North site. This investigation is exam.ining the future composition of the wastewater to determ.ine the most efficient and cost effective treatment process. Based on the data coll ected at this point it is expected that there will be a requirement to retain one UASB reactor in the wastewater treatmen t process. The capital improvem en t program has provided the foundation to reduce water consumption and to reduce the WWTP operating costs; however, it is not the whole story. In addition, Manildra group is looking at ways to further reduce wastewater generation and to reuse some of the high quality wastewater produced onsice.

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In late 2002 the Victorian Seate Government in conjunction w ith the major Melbourne water retailers initiated the Smart Water Fund to fund water reuse proj ects. Discussions with City West Water Limited identified the condensate from a triple effect evaporator that is bypassed around the effiuent treatment plant as a potential water source for a reuse project. The proposed project was to supply the 9 m 3 / h of condensate to a local wool scourer at approximately 40°C to replace the use of potable water. The supply of the condensate at an elevated temperature has the added benefit of reducing greenhouse gas em issions. This project was approved early this year and commenced on 1 July. In addition to the above project, Manildra Group has been investigating the feasibility of supplying the M VR evaporator condensate to possible users in the local area for in-process use, wash down or dust abatement. The suitability of the condensate for use in wool scouring is still being assessed; ho wever, the in itial results are promising. Apart from the process eilluent, the other major uses of water at Manildra Group, Altona North includes truck washing and production area washdown. In order to reduce the amou nt of water used trigger nozzles have been fitted to all suitable wash down hoses around the site; the on ly excl usion is steam water mixers due to the poten tial for li ve steam to pass through the hose. A new type of steam water mixer which is designed to prevent the possibility of live steam passing through the hose is currently under evalu ation, however these are very costly an d wi ll be slowly introduced to site.


INDUSTRIAL

In addition, a piston pump is under triaJ to was h settled starc h from road tankers. It h as been d emonstrated at other Manildra Group Sites that this not only reduces the amount of water required to wash out the truck but also reduces the unloading time for the tankers. The other major user of water for truck washing is glu cose outl oading. Due to the different grades of glucose and good fo od hygie ne deli vered to c ustom ers it is necessary to clean the road tanke rs between each delivery . C urre ntly this is performed on a ti m ed was hing basis, however investiga ti on into the duration of the wash ing cycle is required to determine the minimum time required. Other areas under exam ination to r e du ce water con sumpt ion in th e ma n ufacturing areas include the use of pump gland f] ush water, greate r recovery of steam condensate fo r return to the boilers and reduction of spillage from plant and equipment due to in co1Tect operation. One of th e best reso urces for ide ntifying where wa ter is wasted is through

AGAL Australlan Government Analytical Laboratories

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talking to the p rocess operators and asking fo r ideas on how water can be saved. It is important to rem ember that w hen implem enting an impro vement that it is functional and will not increase the complexity of performing a job.

Conclusion The cost of water and the disposal of wastewater are major cost items in processing wheat for gluten an d starch , du e to the wate r intensive nature of the process. In order to co n tinue to be co mmercia ll y co mpeti tive it is essential these costs be minimised. Manildra Group is worki ng towards this on th e Alcona N orth site by maximising product recove ry, redu cin g water usage th rough c hanges in equipment, using recyc led wate r and c hanges in work pra ctices. In addition Manildra Group, Altona North , is produ cing a higher quality wastewate r , segregatin g wastewate r strea ms co reduce hydrau]jc loading on the WWTP and identifying wastewater streams that may be suitabl e fo r reuse either interna lly o r exte rnally.

Although a great deal of money has been invested by Manildra Group into the Altona North site to im prove the manu facturi ng process, there is¡ still opportunity to reduce water consumption through minor changes to plant and equipment and procedures. T his is an ongo ing process that will never be comp lete, however, it is important co criti ca lly analyse each improve ment to ensure that the cost of implementation does not outweigh the benefit.

The Author David Fergusson , a c h e mi ca l engineer, graduated from the University of Melbo urne with Honours in 1998. Afte r working in the explosives industry fo r nearly fou r years he j o ined Man ildra G roup in 2002 as a process enginee r. Da vid's key areas of responsibility are th e managem ent of the wastewater treatm ent plant and Environmental Compliance for t h e Altona Nor th plant. Email: dfergusso n@bigpond. co m


~

WATER

FLOCCULATION: A NEW DESIGN PROCEDURE S Griffiths reduced ca p ital cost . However, these types of Although exte n sive l y impell er need not be run at researched, flocc ulation in the similar tip speeds to a picket water treatment industry lacks a fence, and can in general nm design procedure to accurately faster, w hil e producing access the performance of th e similar shear distributions many different types of impeUers within the basin. The reason on the market today. New high 1.0!E>OZ for this is that the far lower efficiency, hydrofoil type impeUers t.01[•02 pitch of the blades creates 9.5(1(•01 offer great potential for flocc ua.m,01 less localised shear. However t.2!£•01 lation duties in many areas, but 7.60E+ol the disadvantage of flow only w hen applied correctly. This 6.91E+o l producing impellers is that 6.ll[•0t review has been w ritten to 5.70[•01 if the speed is decreased to 5.01Et01 demonstrate the importance of not 4.4l£t01 reduce s h ear rat es , only the velocity gradi ent but also eventually they will lose the distribution of shear rates 2.5!£•01 New Chot.1,,.,.,.. Floc:cul..tor lb- 25 2002 cont ro l of the m ixing within the flocc ulation system. fl,...,t 4.50 : ::::: Av.,-oge· Veloel~y <Inc Turbl (Ft/Mini process creating dead areas The objective of the paper is to 6.llE•OO Stoulotlcn ~ By lefl°"_ 4.SO Ccpyrti,t 0-J,-fl,..,.t Inc.__ .OOE,+o·-L---....:.._..:.._ _-__:_:......:. _lcl _ _ _ Inc _ 2000 _..,__ __ in the tank, and therefore propose a new design procedure being of little practi cal use. Fig 1 - Velocity Distribution (Image courtesy of Chemineer) to ensure minimum agitation and With this type of equ ipment help engineers correctly evaluate it is therefore imperative that different types of proprietary (GMax) and the shear distribution may be the impeller is correctly sized and specified impeUer designs. dramatically different. In practice this has at the design stage. meant that the industry has tended to In this paper we will attempt to go Introduction sp ecify a particular type of impeller back to the basic stru cture of floe The flocculation process follows the system, usually the picket fence , with not particles, to analyse w hy floes break up , rapid mixing of coagulants into a water only the velocity gradient, but also a and how, by applying basic agitation stream . It involves gentle agitation of the maximum tip speed. The specification of theory, we can estimate the requ ired process water to allow floe aggregates to the velocity gradient and the maximum running speeds w hen using various be formed from particulates within the tip speed defines both the maximum local different types and sizes of impellers. We water. These aggregates are then removed and the vessel average shear rates . shall also attempt to provide a method of in a downstream process 1 typically by However even with the simple vertical impeller evaluation , and a sp ecification either flotation or sedimentation. impeller blades of the picket fence, shear method to prevent the under-design of To help impeller design specification, distribution can vary quite dramatically flocculation impellers, w hich would Camp and Stein (1943) introduced the with the number and width of the blades. result in flocculators of little or no practical concept of velocity gradient, or vessel use. The final obj ective is to prevent Flow producing impellers such as average shear _rate: problems with loss of performance of the pitched blade turbines and hydrofoils (see soli ds removal sys tem that can be Figure 2) offer an attractive alternative G= ~ p attributed to the flocculation process. with higher pumping efficiencies and µ-V (1) Equation 1 above has been widely used by the water indust1y and is still regarded as the easiest method for specifying a flocculation system. Unfortunately the major drawback of this procedure is that it assumes that the shear is applied evenly throughout the tank or mixing basin, which is not realistic to real world situations. Any agitation process w ill provide areas of high shear near to the impellers, and low shear away from the impellers (see Figure 1). So although for a particular agitation system, the vessel average shear Fig 2 - Hydrofoil Impeller (Image courtesy of Chemineer) (the velocity gradi ent) may be the same as for another, the local maximum shear

Abstract

l.911Et02 l.111Et02 l.'17E•02 1.71[•02 USE•02 l.51E•02 l.52Et02 l.'6(>02 l .3'Et02 l.l3£•02 1.21{•02 t.2Q(t02 1.1<£'°2

~::::..------------------.---i

44

WATER SEPTEMBER 2003


WATER

Flocculation Flocculation equations have been developed by researchers such as Argaman and Kaufi11an (1965, 1970) to expand on Camp and Stein and try to take the distribution of shear into account. Th ey have produced equations such as: 3

NL1 =4- K 5 - Rf· n 1 -nf -u

2

(2)

Equation 2 relates the number of contacts of primaty particles, w ith various factors such as radius of floe particles, m11nbe r concentratio ns etc. To th is they introduced the break- up factor:

dn 1 = B · R: · n f · u dt ,{

..

Primary Particle

Flocculi Collection of primary particles

, Figure 3: Floe Structure

An aggregate of flocculi , bound by intermolecular forces and including a substantial proportion of fluid.

2

(3)

Equations 2 and 3 have now bee1; sirn.pli fied in recent textbooks to:

dn1 = B · R: · nf. u dt ,/

2

(4)

E quation 4 relates the number concentration of particles at the start, and any subsequent point in the flo cculation process. It uses well-known constants such as G facto rs and tank reside nce time, but introduces KA th e aggregation constant, w h ich can be calculated, and K 8 , whic h inclu des a paddle petfo rmance coefficient. Unfortunately both these fac tors are likely to vary in practice. KA would vary w ith th e incom ing water quality, and K 8 not o nl y with the type and m an u facturer of an impeller, bu t also with diameter and sp eed .

Floe Structure E xtensive research work has been done on th e structure of floe particles. Mi chaels and Bolger( 1962) derived a mul tilayer flo e mod el from th eir obse rved data. Other eviden ce fo r mul ti-level flo e stru c ture is found in the floe density - floe diame ter relatio nships such as Lagva nkar

and Gemmell's (1968) studi es of an iron sa lt floe, and Tambo and Watanabe's (1970) studies of al uminium hydroxide floes. T here seems to b e som e confusion between the researchers as to the exact definition of som e of the levels of th e structu re. Van de Ve n and Hunter (1977) defi ned a fou r level structure as fo llows: a. Primary particles. b. Flocculi fo rmed from a collecti on of primary particles. c. Floes fo rmed fr om close packe d flo cc uli. d . Floe aggregates. H owever, other researchers do not differentiate floes and floe aggregates, thus co m bini ng c and d toge the r. For simplicity this is the model we wi ll use. Each flo e we wi ll define as an aggregate of flocculi bo und together by interm olecular forces and encompassing a substa ntial proportio n of fl uid w ith in its fram ewo rk. See Figure 3.

Floe Break-Up Floe break-up has been investigated by Firth and H unter (1976) . They used th e analogy of a Bingham mod el to describ e

the flow of an electrically charged sol. T hree parameters are of importance, as shown in Figure 4. T he Bingham yield value, 1: 8 , occurs w hen the flow curve becomes linea r at a valu e of C 0 . ft was fou nd that this theoty fitted the elastic floe model rather than a single particle or a hard non-deformable type flo e model. This would m ean that the floe particle can be deformed witho ut breaking, but once a certain shear rate has bee n surpassed it will fracture. T he significance of this to mix ing will be discussed_in the m ixing theory section following. It is ge nerally accepted by resea rch ers (S p ielman , (1 978), Arga m an (1970) Tambo and Watanabe (1970) that flo e break- up occurs by one, or a combinatio n of the following two mechanisms: a. Surface erosion of prima1y particles . b. Fracture of flo e to form sm alle r aggregates. The simultaneo us processes of floe aggregation and break- up by either of the above processes will fi nally result in a stabl e floe size where th e rate of aggregation equals the rate of break-u p. This can be expressed using the fo!Jowing equati on:

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WATER

To help w ith th e visualisation of floe break-up , the yield stress limit ('C 13) should be remembered. As discussed earl ier, the bonds ho lding the fl oes together are elastic, and as such w ill break when the yie ld values are exceeded .

Figure 4. Shear Stress vs Shear Rate for a Coagulated Sol

Shear rate (s-1)

(5) Where d5 is the stabl e fl oe size, C 5 is the coefficient related co fl oe strength and L is the expone nt de pendent o n the flo e break-up mode and size regim e of eddies that can cause disruption. However this

equation like Camp & Ste ins' velo city gradient assumes perfect disttibution of the vel o city gradients (G) in the mi xing systen1. In practice this eq uation cou ld only be related to flo ccu lation system using exac tly th e sam e impeller type and size .

Francois a nd Van H a nce (1983) rewo rked data from Tomi and Bayster (1975) to show a relatio nship between floe diameter and largest stress within the bonds, as shown in Figu re 5. This shows us that the larger the flo e, the higher the stress in the bonds all o ther conditions being equal. It therefor e fo ll ows that to reach the yie ld valu e for the bond, larger floes will require less shear input than smaller floes, w hi ch is consistent with expectations. T his is also co nsistent w ith the theory behind tapered fl occu latio n , w here the growth of large floes is a particular advantage, such as in the sedimentation process. Higher shear rates are important for rapid floe build up in their early stages of d evelopm ent. These shea r rates w ill ca use collisi ons of small scale f.locculi, but as is sh own by fig 3, are not likely to be e ffected too mu ch by hig he r shea r rates. Th e effect of slowly

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simon

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Engineering

tap ering down the shear rate as th e fl oes grow, w ill res ult in a larger fi nal floe size, w hjcl, is more easily settled . This theory of t apered flocc ulati o n can be simplified into the form o f an equation: 1' yield

= 1:bond + 1:.1·hear

(6)

A s the yield value (-ryicld) is consta nt and th e shear stress in the bo nd ('C1,ond) increases w ith larger floe sizes, t h e shear stress as a resu lt of the mi xing effect ('C,1,c.ir) must be re du ced.

Mixing Theory Th e agitator m an u fa cturer must conside r at least three dimensio nless groups, w he n sizin g mixin g impe lle rs. Th ese are :

v2. N ·P

- -- = l~cynolds Number µ N = p " p-N J ,Ds NQ

Power Number

(7)

(8)

_ _ Q_ _ N . D J - Flow or Pu mpin g Number

-

(9)

Th e R.eyno lds number (equation 7) must be co nside red to evaluate the fl ow regime of th e impeller, i.e. laminar, transitio nal o r turbulent. ]fit is turbulent, which it w ill be for almost all wa ter and wastewater appli cati o ns, bo th N,, and NQ are co nstants fo r a g iven imp ell e r in a give n tank. Bearing thi s in mind, N ,, and NQ ca n both be said to be co nstants dep e ndin g o nly upo n t he impeller c harac te ri stics . N o te that sma ll laboratory scale test equipment needs to be individuall y assessed , as th e small diam ete r of the impelle rs o fte n m eans that the flow regime is transitio nal or even lamjnar. This means th a t the result o btained from the labo ratory can be diffi cult, if n o t impossible, to use fo r scaling up to the fulJ scale process. Th e imp ell e r Po wer number, N,, (equ atio n 8) is used to calculate the th eo re tica l powe r c onsumption o f an impell er of a partic ular size and speed (no t including gearbox losses, moto r losses etc). T his is required fo r correct motor and gea rbox sizing and is also used to c heck th e vessel average sh ear rate, or ve lo city gradie nt. Th e fl ow numbe r (NQ) is use d to calculate the pumping capacity of th e imp ell e rs and he n ce a bu lk fl uid velocity (BFV) ca n be calculated in the tank (equatio n 1 J). The Bulk Fluid Velocity is de fin ed as the fJow produced by the impelle r di vided by th e equival ent c ross sectio nal area of the tank (A') . The equi valent area of a tank is de fined as th a t of a pe rfect 'square' tank , i. e . when th e diam eter of th e vessel equals the fluid depth. In most fJo cculatio n application s th e tank w ill no t be round in cross sec tion so equation 10 should be used to calculate A '. This is defin ed by th e e quatio ns belo w :

VJ · nX

A'= - - -

4

BFV

=¾,

(10)

( 11)

The bulk fJu id ve locity is an average velocity important to prevent bypassing o r stag nant zo n es form ing in th e tank, an d can also be a use ful indica to r o f the shea r distribut io n (G.\,/,,.JC) . The high e r the fJo w o r BFV for a given velocity gradie nt, the g rea te r the shear distributio n w ill be. This w ill


WATER

Figure 5. Diameter of spherical aggregates and length of chain vs maximum tension in elastic link between nodes.

w

Spherical agg regate Length of chain

-Dimensions ------;~

both increase the floe size by reducing (shear (from equation 6), and by increasing the number of collisions, especially w hen further away from the point of application of the energy. It will also help to reduce t he floe size distribution, makin g downstream separation steps easier to control.

We must also introduce the concepts of macromixing and micromixing as important parameters for the flocculation process. Macromixing can be thought of as the bulk movement of fluid i.e. large scale pumping, and micromixing is movement at a very small scale, often represented as small eddies (Bourne and

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Ravindranath, Oldshue, 1983 ). Every impeller (both style and size) produces a combination of these two types of mixing in different proportions. Zones around the impeller are the predominant area for microm.ixing, as this is where the majority of energy is applied to the fluid. However it is not often explained what these facto rs mean in terms used by the water industry. Researchers investigating mixing types (Oldshue, 1983, Ruszkowski and Muskatt 1994, Oldshue and Mady) have shown that there is a specific dividing line between micro and macro scale mixing. This occurs at a certain size of turbulence or velocity fluctuation. T he ultimate size limit fo r a given duty can be related to Kolmogoroff microscale of turbulence, which in flocculation can be related to constants associated with the physical properties of water, and the energy dissipation rate at that particular point within the flu id. This is very similar to the approach used to calculate the velocity gradient by Camp and Stein . The shear rate is often measured in the water industry using the velocity gradient. H owever, it should be noted that the velocity gradient calculated with Camp and Stein's equation, (1 above), is the vessel average value, not that which occurs lo cally. In any agitation system you will get a range of energy dissipation rates and hence velocity gradients, depending on the fluids particular position in space in relation to the impeller (see fig. 1). At the surface of the fluid for example the velocity gradient will be close to zero . As described in the floe structure section above, we know that a floe consists of "an aggregate of primary particles bound together by intermolecular forces and encompassing a substantial fraction of fluid within its framework." In other words when a floe has grown , unlike other more solid particles, it will still be affected by both micro and macro scale mixing. Macro scale mixing will encourage floes of similar size to interact whilst micromixing w ill erode larger particles and aid the collision of small particles with larger particles. This often seems to overlooked by mixing researchers who tend to be more used to dealing with the solid particles associated with the chemical industry, o r batch flocculation, when floe sizes will be relatively consistent du e to the even residence time of each floe in the tank. The continuous process used in industry has both the normal difficulties of flocculation, and the added problem of an uneven floe residence time giving a wide range of floe sizes in the tank at any one time.


WATER

TraditionalJy flocc ulator agitators have been of the gate or picket fen ce types. These consist of a number of vertical blades, made of wood o r metal. When insta lle d in a tank they produce a rota tional flow patte rn , and micro scale mixing concentrated in alternating vortices at the back of th e impell er blades. By controlling the rotational speed of the impell er, the process can be optimised by altering the proportions of micro and macromix.ing in relation to each other (as bulk flow varies with rotational speed, and local shear w ith rotational speed squared fo r a g iven agitation syste m). The success of the picket fence impell er can be attribu ted to this controlJability. Although th e flow patterns created by a picket fe nce impell er leave a lot to be desired in terms of m ix ing efficiency, bu lk control of the vessel is never lose, as th e impeller will always travel through the majority of the rank. The more modern turbine or hydrofoil impe!Jer on the other hand must mai ntain a mi nimum agitator speed in order to maintain the whole of the tank contents in motion. If the speed is reduced below this limit, the surface of the tank will beco m e co ntrolled by the flow through th e tank rather than the mi xer. T h is reduces the effectiveness of th e flocculation process and th erefore w ill make downstream separation more problematic, and will often be co mpe nsa ted for by higher coagulant doses than predicted in th e laboratory . The successful appl ication of hydrofoils, or any oth er flow producing type of impeller to the flo cculation process, w ill th erefore depend upon them being able to maintain control of the tank, by produ cing sufficient flow, whilst keeping sh ear rates and shea r distributi on within acceptable levels. This makes the ini tial design of turbi ne or hydrofoi l imp e ll er size an d sp ee d extremely important. To enable us to investigate the agitation syste m more fu ll y, we need to exam in e not just the vessel average shear rate (veloc ity gradient) but also to get an indi ca tion of the maximum shear (to ensure that th e flo es w ill no t be frac tured and become too small), and c hec k the shear distribution within th e vessel (to ensure that the floes w ill be of a relatively even size distributi on). R. Geisler, R. Krebs and P. Forschner (1994) have exam.ined the local shear stress and d erived th e equation:

(~J p·v "P.

Max

= 0.03. N~

(12)

T his relates th e maximum turbulent shear stress occurring w ithin the fluid, 'tTurb, w ith the tip speed, v,;p, and power number, Np, of the impelJer. It is well kn own that there is a relationship between floe break-up and tip speed, but the power number relationship is less well known. As a practical explanation it can be said that th e maximum shear stress will develop close to the point in the fluid w here the induced velocity is at its highest. T his w ill , in general, occur at the point at wh ich the impell er blade is travelli ng at its maximum speed, w hi ch is at its tip. The fluid velocity produced and the amount of power applied are both a function of th e blade angle and geomet ry, an d t hi s is represented reasonably well by th e impeller power number. It should be noted that hydrofoil impell ers have a variable angle along th e impell er blade, with a small angle to th e plane of rotation at th e impelle r tip . This would mean that the point along th e blade where m ost power is appl ied may be at a position oth er than at the tip of the blade. However the correlation stated above wilJ provide a conservative estimate in this case, and so wo ul d still be a reasonable method of comparison. W e can relate e quation 12 co a localised maximum velocity gradie nt (G,,rn.J, which w ill he lp us examine th e effectiveness of the impeller system for shear distribution. This can be don e by using the equati ons fo r flu id viscosity as shown below:

-r = -µ . d½ty

= -µ . G

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(13)

Comb ining equations 12 and 13 gives

Cnr McCauley & Australia Av

us: v2 . p

G Max =O ' 03·N I'¾ . ....!!L...!:.. µ

(1 4)

As the viscosity of water at ambi ent temperatures is around 1cP (0.001 Nsm-2), and the de nsity is around 1000 kg/m3 this can be simplified to: G Max

= 30000 · N 1:9 ·V r~p

(15)

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Remembering the e lastic floe model, and knowing that fo r chis model co be true the bonds must ha ve a yield stress, we can use equation 12 or 15 not only to compare dissimilar agitators, but also pote ntially to more accurately predict flo e size, although chis will be an area for future work. Maintaining Minimum Fluid Motion

Interestin gly equation 15 ca n be combined with equations 8 and 9, to show that fo r a given system (tank size and imp eller typ e) designed to produce the WATER SEPTEMBER 2003

49


WATER

same velocity gradient ( C) the maximum shear rate in the tank is inversely proportional to th e flow produced by the impeller:

GMa., -_ 0.2325 · l.!:!.si.. N{9

j_(

Np

Impeller

Picket Fence

2 .

T Q

.

J

z (16)

This is theoretical confirmation of w hat we know happens in practice. When small impellers running at high speeds are used in flocculation tanks, they produce a much lower flow than a large impeller at the same velocity gradient. This m akes Q smalJ , and t herefore C.llax large . In these cases, although the ave rage velocity gradie nt (C) is the same, the maximum velo city gradie nt is far higher in a tank with a smaller impeller. This w ould in turn make the equilibrium. floe sizes dramatically sm aller an d solids removal far 111.ore difficu lt. E quation 16 can also be used to in combination with equation 11 to define the ratio of maximum to average velocity gradients fo r agitation to be maintained in the flocculation basin. The following equation is based on our experience in the design of flo cculators:

1 .08

Pitched Blade Tu rbine Old Style Hydrofoil

0.78 0.67

0.6

Operating

T' ·(NQ N J(17)

GMax) 52 4 c o,47 · ( <1 _ = · · flou·

Nq

4.4

Operating

2

G

Table 1

p

The value of th e velocity gradient (C) 1s defined by th e standard m ethods. It should also be noted that T* is the equivalent diam eter based o n an imaginary 'square' vertical cylind1; caJ tank with equal volume to the actual tank. Note that equation 17 is a max imum shear rate distribution m easure and is not meant to define the distributio n of shear required for adequate floe sizes, but the distribution required fo r adequate bulk movem ent of fluid and available turndown before control of th e mj xing system is lost. For this reason the shear rati o has been bracketed as flow.

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Speed (rpm)

0.5-1.5 1.0-1.5 3-12 3.5-11 7-11

po.s (aG)

NpB/ 9.vul

5.9-30.56

0.116-1.037 0.46-1.037

16.60-30.56 2.74-21.95 2.60-14.4 7.33-14.4

0.132-2.099 0.107-1.053 0.426-1.053

In practice this ratio should be used as the practical maximum required for th e system design. A lower ratio w ill increase bulk flow from the imp eller, w hile at the same time reducing lo cal sh ea r rates for a given velocity g radient. This will have the effect of making th e sys tem able to cope with higher levels of turn dow n , improving the size distribution of floe particl es, and reducing break up of floes, which in the end results in a larger more even flo e size and improved remo val.

Practical Experience Studies ha ve previo usly been done to compare th e use of flow type impellers with existing picket fence style impellers, notably using pitched blade turbines (Paris, 1977) and an o lder type of

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WATER SEPTEMBER 2003


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hyd rofoil impe ller (Oldshue and M ady) . Th e authors published data that we ca n now use to validate the eq uations above. R e- examinin g th e data given using the maximum shear stress technique gives the results shown in Table 1. T he table shows the impe lle r types in th e first colum n. Th is ran ges fro m the traditi onal pi cket fence type arran gem ent, though Pitched Blade Turbine, w hich arc mu c h small flow producing impellers chat have flat blades angled usua lly at around 45°. Th e last row s are fo r an earl y style o f hydrofo il impeller, w hich is again flow p roducing, but has tape red a nd twisted blades, in an effort to inc rease th e flow ca pacity of the impell er and re duce localised energy loss due to stall ing behind the blades. The second and third columns sho w the power n um ber and pumpin g n um be r res pec tivel y . Note that n o pump ing numbe r is show n fo r th e picket fe nce impeller, as this is usually not m easurable due to th e fl o w pattern produ ced . Th e fo urth co lumn sho w s th e ope rating speed wh ic h was set origin all y to g ive the required velocity gradie nt (fift h co lum n gives an in dication o f this), but whe n o perating conditio ns were set

co give the o ptim um process result. T he fin al column sho w s th e maximum shear stress technique as recommended by this paper. The partic ular fi gures to loo k for are those in th e o perating condition (whi ch give t he best p rocess result) As you can see, th e pi c ke t fe nce, whic h w as th e control impelle r , produced a signi fica ntly lo w er m ax imum sh ea r than t his particular design o f pitched blade turbine, and a very similar level to that of the hydrofo il. The conclusion o f the paper co111pa1ing pi cke t fe nce impe llers w ith pitched blade turbines (Argaman and Kaufiirnn , 1970) w as that the turbin es w orked adequately at lo w er speed , but seem ed to shear the floes at higher speed. This is exactly what w o u ld be expec te d fr om the data sho w n above based on the maximum shear calculated in the vesse l. N o te that using the s t a nd a rd ve lo c i ty g radi e nt d es ig n procedure you would have predicted tha t the turbin e sho uld have been run at even hi gher speeds, furthe r in c reasing th e shearing e ffect. The comparisons w ith the hydrofoil type im pellers speak fo r themselves in that o pe ratio nal values found by trial and error

could have been predicted almost exactly usi ng the maximum shear techniqu e. Again there would have been simi lar problems to that fo r the turbin e if predictio ns had bee n based on the velocity gradient alone. Unfortunately these papers did not give in d ications of the basin size used fo r the tests, so the flow requi rements cannot be validated w ith th is da ta .

Discussion T he logical conclusion w hen design ing a floccu latio n system wou ld be for t he designe r to spec ify t he agitator using (C _\/,,.JC)M." and 'tM,x or G_\/.,x as a design param eter as well as the standard velocity gradient. T his would allow manufac tu rers to utilise t h eir particul ar designs of hydrofo il, and w ould not excl ude any pa rticu lar im pelle r type . Note that the {C_\/.,_JC)M.ix would specify the minimum Bu lk Fluid Veloc ity, as d iscussed earlie r, an d sh o uld no t be rega rd e d as a n ind icatio n of floe stability. T h e parti cu lar valu e of 1:M,. o r C.H...,. to speci fy would be dependent on various factors, such as type of coagulant, temperature, w ater qu ali ty etc ., and is best assessed by looki ng at existing full-scale

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flocculators, or pilot scale tests. The author is presently attempting to arrive at reco mmendations based o n standa rd operating speeds fo r Picket Fence type impellers and published data o n the strength of fl.oc pa rticles. Any operating data for flocculators would appreciated to help provide as comprehensive data set as possible. This work will be published at a later date. Care should be taken when examining an existing flocculator, as it is often tempting to multiply out equation (2) so that: (18)

Thus by taking a reading of the power drawn from the agitator and knowing impeller diam eter and speed, 'tMax can be estim ated. This should not be done, as the power consumption wou ld include gearbox, motor and other losses, which are likely to be comparatively large, and would therefore lead to ve1y inaccurate results. The manufacturer of the agitator sho uld be asked for details of the theoretical power draw from the impeller, or estimations can be made from equations 7 and 8.

The Use of Hydrofoils for Flocculation Any flo w-inducing type of impeller has a considerable advantage over a gate or picket fence agitator, in chat they are ap pli cable co any tank shape. T h e impellers mix by inducing flow within the tank rather than having to physically travel through the liquid then1selves. Thus only one agitator would be required for tanks with w idth to length ratios of2: 1. Hydrofoil type flocculator agitators are often cheaper in capital cost due co the lower torqu e requirements for the gearbox and shaft system. Although hydrofoil impellers are expensive to manu facture due to their complex shape, th e cost of this item usually compares favourably with a picket fe nce due co its very much smaller physical size.

Procedure for Flocculator Specification The following procedure is based on the discussion in th e sections above . However it should be noted that although it w ill produce a far tighter specification on the flocculator, no attempt has been made to compensate for the strength o f

floes fro m particular water, or a set temperatures. H owever the design will ensure chat the speed can be turned down, co optimise the local and average sh ear, to an extent that the control of the vessel contents is not lost by the mixing system. The resu lt of this will be a fa r narrower floe size distribution, and often, larger more stable floes, which are easier to separate. The result of specify ing these new parameters to agitator manufacturers should be that the flocculators quoted are roughly equivalent in all cases. With the current specification , and over reliance, on the velocity gradient only, the agitator manufacturer needs only to design to draw a certain power level, w heth er the mixer is really suitable for the application or not. In all cases the specification should include a phrase that ensures that the final manufacturer chosen is happy that the design is suitable fo r the application, and will give the required process result. This is particularly important if specify ing the velocity gradient alon e. This specification method is suitable for all flow producing impellers, such as turbines and hydrofoils. For picket fence

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type impellers the veloc ity gradient an d a tip speed are usually sufficient. • Calculate the required velocity gradient or velocity gradient range for each basin using your preferred m ethod. • Using equation 17 calculate and specify t he required (G1110 .JG)F1ow· This will spec ify the minimum flow that the impeller can produce for the given velo city g radi ent, and w ill therefor e set a minimum impeller size, and m axi mum tip speed for each impeller type. • R equest inform ation on the power number (Np), pumping number (Nq), imp eller size and operating speed (both th e actual from the gear ratio and the recommended ru nni ng speed) from the suppl iers. • O n ce th e quotations have been received, check the velocity gradient at the recom mended running speed, using equations 8 and l. This should match your spec ification , and cells you that you have the requi red average shear rate . • Also at the run nin g speed check the (G 111 ,,,./G)F1ow figure from equation ·17. This should be equa l coo o r less than the valu e chat was specified and cells you that the flocc ulator has been designed with enough flo w capa city to al low for a reasonable amount of turn down (shou ld the flo es be m o re or less stable than yo ur test results indicated). • La stly check the val ue fo r G,,,,,x from equ ation 15. T his will cell you w hi ch des ign is 'safer'. The lower the value the safer the design will be. If the re is a great d iscrepan cy at chis point (and yo u wi ll probably have also noticed it in the (G 11111x/G)F1ow) you can ask the releva nt co mpany to re-evaluate their offer, or put it to one sid e. • T he end resu lt shou ld be a list of com.pani es which m eet the req ui red

velocity gradient speci fi catio n , and th e required m inimum flows. You c ould rank these in term s of G,,,,,.,., w h ich will give you a safety ranking against pri ce . H owever it is diffi cult at this point to give a definitive recom mendation as it w ill depend on how stable floes at your particular location are known to be. To evaluate this yo u can look at existing fu ll size eq uipment, and ca lculate the G""'·'· at the o perati ng speed and use chat to make your selection. Otherwise if your plane operacor can give you an indication of how sensitive they are you can make your selectio n accordingly. Th e more sensitive and fragile , th e lower G,,,,..,. you sh ould select for the given velocity gradien t. You may note that ch is proced ure is based on 'standard ' flocculation basin velocity gradient test. Th is is obvio usly a weakness in that experience indi cates chat there are usually some rules of thumb, w hich allow sca le up fro m lab. scale size flocculacors. As menti oned previously lab. sca le flocculators often do not work in the same flow regime as full scal e equipmen t and therefore are noto riously unreliabl e. If at all possible full scale data from existing equ ipment should be used.

Conclusion Although the concept of the velocity grad ien t ha s been prove n to be a successfu l method of specifying flocculacor agitators in the past, recent papers have indicated that for new impe ller syste m s the ve locity gradie nt alon e is in sufficien t to fully specify an agitation system. This paper has attempted to give an alternati ve procedu re that is easy to use, and incorporates sta nd ard floc culatio n d esign procedures. Th e fo ll ow in g points summarise the information above :

1. Camp and Stein 's velocity gradient concept assumes a perfec t distribution of shear w ithin the vessel. This can never be the case in a m ec hanically agitated system. 2. The velocity gradient has been adapted to fit the standard impeller used in the past for flocculation, the picket fence, by the specificati o n of a maximu m tip speed, which gives some idea of shear distribution . 3 . N ew hydrofoil type impellers offer some advantages for th e flo cculation process, but do not necessarily fit the G factor and tip speed parameters used in the past. Therefore a more general design procedure is required to prevent flocculation problems. 4. Geisler's equation for maximum shear stress has been recommended as a general and easy co use method com patison of floe break up, and hence the overall pe rformance produced in an agitated tank. R ework ing of previously published test data has shown this m ethod to be reasonably rel iable. 5. T he ratio of (G,11,,./G)M,x has been recomm ended as a design paramete r for flow produci ng impell ers. No te that ch is rati o is no t an indication of flo e stability, rath er the shear distri bution required to prevent stagnant areas and bypassing in the tank. 6. Low tip an gle hydrofoils and hi gh effi ciency impellers have t he advantage of produ cing a high bulk flow with a low shear stress, compared with other axial flow impellers. H owever any impell er type, if of the correct size, ca n produce the requ ired shea r races fo r success ful floccula tion. 7. ln general, to keep wit hi n shear stress li mits equivalent of a picket fence w hile main taining a minimu m Bulk Fluid Velocity, a large impell er at a relati vely

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low tip speed (for this type of impeller) would be required.

Nomenclature A' = Equivalent Area of the T ank (1112) BFV = Bulk Fluid Velocity (m/s) B = Break-up constant C 5 = Floe strength coefficient D = Impeller diameter (m) d = Floe diameter (m) d5 = Stable floe diameter (111) G = Velocity gradient (s- 1) GMax = Localised maximum velocity gradient (s- 1) h = Coefficient depending on strength of flo e bonds k = Constant in equation 16 KA = Aggregation constant K 8 = Break-up constant L = Coefficient depending on break-up mode and size regime of eddies m = Coefficient depending on size regime of eddies N 1J = N umber of co ntacts of primary particles (1), and floes (f) N = Impeller speed (s- 1) n = Number concentration of primary

particles (1), and floes (f) NR, = Reynolds number Np= Power number NQ = Flow number P = Power (Watts) Q = Flow rate (1113/s) R = R adius of floe particles (m) r = R adius of prima1y particles (1), and floes (£) , (m) T = T ank diameter (m) T' = Mean residence time (s) T' 11111.,. = Largest force in stru cture T* = Equivalent tank diameter from equation 17. u = Velocity (m/s) V = Volume (1113) v ,;p = Impeller tip speed (111/s) z = Liquid height in tank (111) ~1 = Viscosity (Nm/s2 ) 't = Shear stress, (N/ m 2 ) k 1 = Force proportionality constant p = D ensity (kg/1113)

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References Argaman Y ., and Kaufman, W.J. (1965) "T urbulence in Orthokinetic Flocculation," SERL Re p ort No 68-5, San i ta r y Engineering R esearch Laboritoty, U niv. of California, Barkeley, Calif. Argaman Y ., and Kaufman, W .J. (1970 )"Turbulence and Flocculation", j o11mal of the Sar1itary E11gi11eeri11g Divisio11, ASCE, 96 No SA2, Apr., pp 223-241. Bourne, J .R. and R avindranath, K. "Compaiison of Finite Difference and Collection Methods for Micromixing Calculations". TechnischChem.isches L1b. ETH , C H -8092 Zurich, Santz. Camp, T.R. and Stein , P.C. (1943) "Velocity Gradients and Internal Works in Fluid M otion", jo11mal of the Society of Ci11il E11gi11eeri11g,. 30, Oct., pp 219-237. C hemineer Inc., Amer. Chem. Eng., McGrawHill, Inc., New York, N .Y. Firth, B.A. and Hunter, R.J ., (1976), J. Coll. lnte,f. Sci., 57 pp 248-275. Francois, R..J. and Van Haute, A.A., (1983) " Floe Strength M easurements Giving Equipment Support for a Four Level H ydroxide Floe Structure", Katholieke Universiteit Leuven, Inst ofindust. C hem de Craylaan 2 B-3030 Heverlee, Belgium. Geisler, R.. , Krebs, R.. , and Forschner, P. (1994)." Local Turbulent Shear Stress in Stirred Vessels and its Significa nce for Diffe rent Mixing T asks". Ekato Ruhr- und Mischrechnik G m bH Eighth European Conference on Mixing, Chemical Engineers and the Fluid Mixing Subject Group, Sept 21-23 . Lagvanker, A.L. and Gemmel, R..S., (1968), J A,11. Wat. Wks . A ss., 9 pp 1040-1046. Michaels, A.S., and Bolger,J.C. (1962) l11d. Eng. Chem. F1111d., 1 pp 24-33, 153-162 . O ldshue, J .Y. ( 1983) Scale-up of U nique Industrial Fluid M ixing Processes", Mixing Equip. Co. R ochester, N ew York. Oldshue, J.Y. and Mady, O .B. " Flocculator Impeller: A Comparison", Mixing Equip. Co., R ochester, New York. Paris, D. (1977)."Evaluation and Performance of Flocculators", NEWWA, N. H Meeting Apr 21. Ruszkowski, S.W. and Muskatt, M .J. , (1994) Comparative Mixing Times for Stirred T ank Agitators", FM P, BHRA, Cranfield, Beds. Speilman, L.A., (1978). "Hydrodynamic Aspects of Flocculation." in K.J. Ives (ed), The Scientific Basis of Flocculation, Sij thoff and Noordhoff. T ambo, N. and Watanabe, Y., (1970) "Turbulence and Flocrnlation",jo1-1ma/ ef the Sa11itary E11gineering Di11isio11, ASCE, 96 , No SAZ, Apr, pp 223-241. Tomi, D., and Bayster, D.F. (1975), Chem . E11g. Sci., 30 pp 269-278. Van de Ven, T .G. and Hunter, R.J. , (1977), Rhocel. Acta. 16 pp 534-543.


II

ENVIRONMENT

STORMWATER RUNOFF TREATMENT USING CONSTRUCTED WETLANDS P Geary, M Saunders, D Waters Abstract This paper reports on the performa nce of two urban stormwater managem ent syst ems whi ch incorporate constru cted wecland system s in the treatment train. Th e storm water runoff is treated prio r to its discharge to di ffere nt estuarin e lake systems located on the NSW Cen tral Coast. At Croud ace Ba y on Lake M acquarie, the system incorporated a surface flow constructed wetland, w hile at Blue Haven near Wyong, a sub-surface flow constru cted wetl and was incorporated into the stormwater management system. In each case the local autho rity was re s pon sible for th e d es ign, construction and management of their st ormwater t reatment sys tem. The ren1oval of pollutants in stormwater ru noff was monitored by two Bachelor of Environmental Sc ience H ono urs st u d e n ts fr om th e Univ e rsity of Newcastle .

Introduction A constru cted wetland (CW ) is a wastewater treatment system which uses nacural p rocesses to imp rove water quality. They have been used for successfully treating, and particularly polishing, municipal wascewacers and stormwater runoff w here sys tems receive low hydraul ic and constitu ent loads. C W s trea t stormwaters by passing it horizontally o r vertically th rough a permeable media. They may be underlain by either an imp ermeable m embrane or clay material and are usually planted w ith eme rgent wetland plants (Kadlec and K night, 1996). Constructed wetlands allow many nawral wetland pro cesses to be actively managed and manipulated such that they may ach ieve substantial pollutant tran sform atio n and removal rates in excess of chat w hich would have been expected fro m a nawral wetland of similar size and loading rate (Reed et al. , 1995). They are able to effectively remove water co ntam in ants, th us reducing biological Ox')'gen demand (BOD), suspended solids, nu trients, trace organ ics and heavy metals, while keeping constructio n and main te-

Plate 1. Phot ograph of Croudace Bay surface flow wetland .

nance costs relati vely low (Bastian and H ammer, 1993). Constructed wetlands can be classed into two major categories; surface flow (S F CW) and subsu rface flo w constructed wetl ands (SS F CW) . The difference between the two types of constructed wetlands is the presence or absence of su rface water. Surface flow constructed wetlands have free water visible on the surface, w hereas the water is entirely contained below the surface for SSF co nstru cted we tlands. Th e sub sur face CW w hil e less common has several advantages over its su rface flow counterpart. The water is maintained below the surface of th e substrate so there is little risk of odou rs and public or animal exposure to pollutants. Insec t vec tors and mosquito breeding problems are also limited in SSF CWs and th e rh izo mes and substrate within the SS F C W provide a huge surface area fo r microbial attachment and trea tment of the water. T his allows the SSF CW to have smaller dimensions requiring less space, however, a major drawback is the relative high cost of

construction whe n co mpared w ith su rface flow wetlands, due to the cost of the substrate and the higher degree of complexity required in their design and construction .

Site Description - Lake Macquarie In 1998, in response to m creasing community co ncerns regarding deteriorating water quality and environmental degradation , Lake M ac q uarie C ity Council implemen ted a program o f works designed to minimise the impact of urbanisation on the lake en vironment. The program involved the construction of vario u s stormwate r qua li ty improvement devices including CW s at selec ted locations surrounding the Lake . O ne such device is located with in th e grou nds of a public recreational area at Cro udace Bay. which treats stormwater runoff from an urbanising 68 ha subcatchment of the greater Lake Macquarie catchment, eventually discharging into Lake M acquarie . It consists of two o fflin e sto rmwa ter treatm ent trains constru cted in parallel and includes a surface flow CW. WATER SEPTEMBER 2003

55


ENVIRONMENT

The m ajor components of each of the Croudace Bay treatment trains are a coarse sediment trap (CST), gross pollutant crap (GPT) , sedimentation basin (SB), surface flow wetland (Plate 1) and a high flow diversion channel (Figure 1),

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Site Description - Tuggerah Lake Wyong Shire Council (WSC) implemented an Urban Stormwater Qua li ty Management Plan in l 999 in an effort to improve urban storm water quality. The plan, which provides the framewo rk for further improvements in sto rm water management, highlights the cumulative impact that urban stormwater has on the Tuggerah Lake syste m . T he plan recognises the importance of wetland systems in the treatment of storm water runoff. Th e Blu e H aven treatment fac ility drains an urban area of approximately 21 ha and discharges into Wallarah Creek wh ich in turn flows into Lake Budgewo i (part of the Tuggerah Lakes system) . The maj or compone nts comprise a coarse sedim.ent trap (CST), gross pollutant trap (GPT), sed imentation basin, t wo subsurface flow wetland cells (Plate 2) an d a hi gh flow diversion chan nel. (Figure 2) . For the cost reasons previo usly mentioned, SSF CW systems are not commonly used in stormwater runoff treatm ent and this syste m is unique with in the Wyong Council area .

Monitoring A monitoring/analytical program was undertaken to allow the examination of each facility independently and under the fu ll ra nge of operational flow conditio ns.

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Sampling Site

Energy Dissipator

Overflow Weir Inlet monitoring sites were established at the top of eac h treatment train Figure 1. Schematic drawing of part of the Croudace Bay stormwater treatment to sample water discharging directly from facility (not to scale). eac h of th e sub-catchments. Outlet monitorin g sites were established at the outlet structure of each Concentration (EMC). M ass R e moval Efficiency (MRE) was calculated as: facili ty to sample water (Figure 1 and Figure 2). M on itoring and sampling equipment were housed in concrete bunkers placed ( Vo/11111e i11 x EMC i11) - (Vo/11111e 0111 x EMC 0111) I00 on-site for security and storage. MRE (%) = x (Vo/11111 e i11 x EMC iii) The removal efficiencies of the principal pollutants in the urban stormwater treatment systems were calculated fo r the event Croudace Bay Surface system mean concentration (EMC) and, using the recorded flow data, as mass of pollutant (MRE) removed for each monitored event. Fieldwork was undertaken between August 2000 co March Event mean concentration was calculated from the sample series 2001, however, due to material and equipm ent constraints, collected during the monitoring program. It is described as a rainfall, inlet/ outlet discharge and storm event water quality flow proportional average concentration of a given parameter sampling did not begin until October 2000 . Prior to this, only d uring a storm even t. EMC removal efficiency was calculated dry-weather water quality sampling/analysis and fie ld mappin g as: were undertaken. During the period November to March rainfall and inl et/ outlet discharge data were monitored, recorded, I_ OutletEMC ] _ downloaded and analysed. D1y-weather (baseline) water quality EMC Efficiency (%) = [ lnletEMC x 100 samples from each treatment fac ility were collected and analysed In the MRE method a mass balance equation is used to on a monthly basis and water quality samples were coll ected for determine the pollutant mass removal efficiency during seven (7) storm events. Rainfall (pluviometer) and inl et/outlet individu al storm events. The calculations are based on the use discharges (pressure transducer/magnetic flowmeter) data were of total inlet/ outlet volumes fo r each sto rm and Event M ean also collected during thi s period and ana lysed.

56

WAT ER SEPTEMBER 2003


ENVIRONMENT

Each stormwacer water sample was analysed for seven parameters: Total Ph osp h orus, Total Nitrogen, Tota l Suspended Solids, Elect1ica1 Conductivity, Temperature, Dissolved Oxygen and pH . T he variation in concentration of the principal pollu tants in urban stormwacer (to t al suspe n ded sol ids (T SS), tota l ni trogen (T N), and total phosphorus (TP)) along with flow measu rements were examined to determine the re moval effi ciency of the storm wacer system. T he equations fo r event mean concentratio n (EMC) and mass removal effic iency (MRE) were utilised in chis procedure.

Blue Haven Sub-surface system Dry-weather sam ples were collected (grab samples) at approximately monthly intervals between March and August 2002 co establish baseline co nditions fo r the syste m and fo u r (4) storm events were sam pled (automatic water samplers) fo r r emov al effic iency ana lys is. Thr ee mo n itori ng sites we re established and rainfa ll (pluviom eter), evaporation (Class A p an), inl et/o utl et disc harges (magnetic flowmecer) data were also coll ected and analysed. T he stormwace r fo r each storm event analysed was " tagged " as it en tered the trea tm en t train with a lith ium chlori de tra cer. T he sa mples used fo r the pollutant analysis were chosen based on the li thium concentration in the sam ple . T he tracer was also used to determ.i ne the ac tual residence ti me of the SS F C W cells. Each storrn water sa mple was analysed fo r n ine parameters: Lithium, T o tal Phosph o rus, Total N itrogen, T otal Suspended Solids, T ur b id ity, El ec tri cal Cond u ctiv ity, Temperatu re, D issolved O xygen and pH . T he remova l efficiencies o f th e prin c ip a l p o llu ta nt s in t h e u rban storm water system were calculated using the previously described methods.

Plate 2 . Blue Haven SSF CW cells.

l

\

North

Sedimentation Basi n High Flow Bypass Channel

Leak Cell I

Results For each system , th e average rem oval effic iency was calculated fo r all storm events using both removal efficien cy calculation me thods. The m ean rem oval effici ency (and standard deviation) of the prin cipal scormwa ter pollu tants (TN, T P and T SS) fo r bo th facilities are presented here. For detailed results o f the perfo rmance of th e C roudace Bay and Blue H aven fac ility during individual storm events, the reader is referred to Saunders (200 1) and Waters (2002) resp ectively.

Total Nitrogen Removal Nitrogen in stormwater exists in a nu mber of different form s and as the stormwater travels through che trea tment

Cell 2

Wallarah Creek Wetl and

C2m1220, 01

D D

â&#x20AC;˘

CST GPT High Flow Weir Inlet Weir

~

-+-

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0

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Piped Flow

â&#x20AC;˘

S2

Perforated Pipe Flow

0

S3

Figure 2. Schemat ic drawing of the Blue Haven stormwater treatment facility (not to scal e). WATER SEPTEMBER 2003

57


ENVIRONMENT

fac ility, the relative conce ntrations change resul ting in a net reduction in the TN present in the stormwater. The major removal mechanisms for nitrogen in SSF CWs are mic rob iolo g ica l ca talysed processes, including ammonifica tion, nitrifi cation and denitrification . M onitoring results show that the Blue Haven sub surface system removed TN from th e stormwater under vari ous hydraulic conditions. The entire treatment fac ility recorded a TN remova l performance of72% (±4.2%) (Figure 3). D uring the four m onitored storm events, the treatment facility at Blue H aven performed much better than the long-term average reported in the literature, w hich for almost 60 (primarily s urfac e) storm.water treatment wetlands in the U .S. reported by Sc hu ele r (1992), was approximately 25%. The superior performance of the Blue Haven system may have been due to the denitrifi cation process w hich is known to occur u nder low oxygen conditions more commonly e ncountered in SS F we tland system s. Comparatively, th e nitrogen (TN) ren1oval perforn1ance within the Croudace Bay surface system was found to be highly

• Blue Haven SSF Wetland D Croudace Bay SF Wetland

120 i100 > u

C: 80 Cl) ·c:; :E w 60

'iij

> 40 0 E Cl)

a: 20

0 TN

TSS

TP

Stormwater Parameter Figure 3. Reported pollutant removal efficiencies for both treatment facilities.

variable and lower than for Blue H aven. N itroge n removal du ring the monitoring peri od was within the range of -10% to 24% (±18 .0%) . This study also fo un d an apparent nitrogen export dming one storm event. It was concl uded that thi s export may have been caused by a combination of significant red uctions in su rface water leve l over the dry su mmer period,

resulting in significant nitrogen release from decaying vegetation within the wetland w hi ch was then exported from the system du ring the storm event.

Total Phosphorus Removal Phosphorus exists in three major forms in wetlands, categorised as dissolved P, solid mineral P, and solid organic P

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(Kadlec and Knight, 1996). Ph osphorus is removed th rough the interaction of both abioti c and biota processes within CWs. Phosphorus may be removed by processes such as substrate sorpcion (depending on its physico-chemical properties), plane uptake and prec ipitation. Storage in plant tissue is normally restricted co a narrow range and the principal removal mechan ism is typically storage in substrate rather than biomass (Kim and Geary, 2001). Phosphorus removal is also highly sensitive to loading rate and declines after an initi al equ ilibration period as saturation occurs. The study results show that the Blue Haven subsurface system removed TP from the stormwater under various hydraulic conditions. T he entire treatment faci lity recorded a TP removal performance of66% (± 10.7%). The long-term stormwater constructed wetland phosphorus removal rate for sto rm wacer wetland systems has been esti mated by Schueler (1992) to be approximately 45%, although chis signi ficantly vari es according to the nature of che subs crate and may range from 20% to 70%. Phosphorus (TP) removal performance within the C roudace Bay surface system was also found to be ve1y good an d was reported in the range 56% to 70% (±26.0%) during th e monitoring period. Phosp horus removal was very well correlated with suspended solids reten tion, hi ghlighting the importance of sedimentation as a P removal process in wetland systems. Both treatment facilities performed we ll above the reported long-term perfo rmance estimates for the monitored storm events, suggesting that the short-term p hosphorus rem ova l mechanisms are still actively rem ovi ng phosphorus in each case .

Total Suspended Solids Total suspended so lids are defi ned as the organic and inorga nic solid materials present in suspension in the sto rmwater. T he typica l size fraction for suspended solids is particles greater than 0.45µ111. Gravitational settling and flocculation are th e primary removal mechanis ms fo r TSS and remove all o f the T SS size fract ions in SSF CWs. Th e other maj or TSS remova l mechanisms are fi ltration and adhesion, wh ich remove much of the co lloidal size fract ion of the TSS loa d. The study results show that the Blue Haven subsurface system removed T SS from the stormwater it receives u nder various hydraulic cond itions. The treatment facility recorded a TSS removal performance of 95% (± 1.0%). Long-term TSS remova l performan ce rates for storn1vvater CWs ha ve been estimated to be 75% (Sc hu eler, 1992), with the ran ge gen erally being between 50% and 80%. T otal suspended so lids (T SS) rem oval performance within the Croudace Bay su rface system was also found to be very good and was reported in the range 84% to 87% (± 10 .0%) during the monitoring period. Both treatmen t fac ilities performed much better than the long-term average reported in the literature.

Conclusions Th e assessment of the performance of a treatment wetland is usually based on measured concentratio ns or mass of pollutant entering an d leaving. Most of the work w hich has been undertaken on the performance of treatment wetland systems reports significant removals ofTSS, with often variable results fo r TP and T N . The research results indicate that both the stormwater treatment fac ilities at Blue H aven and C rouda ce Bay w hich incorporate constru cted we tl ands in the ir design are

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performing very well for the principal stormwater pollutants TP (66% and 70% respectively) and TSS (95% and 87% respectively) . However, with respect to TN removal, th e subsurface wetland treatment system appears to be removing more nitrogen from stormwa ter (possibly as a result of denitrification) than is reported in th e surface wetland system (72% and 24% respectively) whi ch on one occasion ex ported nitrogen. It must also be stressed that these are short-tem1 results, and w ith respect to all the pollutants m onitored , are typically better than migh t be expected in the longer term, particularly for phospho ru s w here substrates may desorb phosphorus over time. The results are however qu ite enco uraging given th e commitment that both Lak e Ma cq uari e and Wyong Councils have shown t o installi n g stonnwater treatment facili ti es which incorporate eith er SF or SS F co nstructed wetlands in their design.

Acknowledgements

H aven project was pro vided by Wyong Shire Council, while the Croudace Bay work was funded by Lake Macquarie C ity Council. The s upp ort of Greg Walkenden, Sian Fawcett and Adam Mularczyk (all W yong Council) and Symo n Walpole (La ke Ma cq uari e Council) is acknowledged. Assistance with instrumen tation at each of these project sites was provided by Ch ris Dever and Peter Lough ran from Th e U niversity of N ewcastle.

The Authors Phillip Geary is a Senior Lecturer in the School of Environmental and Life Sciences, The U niversity of N ewcastle , Ca ll ag h a n , NSW, e m ail ggpm g@a linga.newcastle .edu.au . Mark Saunders is a Project Managem ent Officer at Lake Macquarie C ity Council, Speers Point, NSW, and Daniel Waters is an Environmental Health Officer fo r the Blue Mountains C ity Council, Katoomba, NSW.

References

Financial ass istan ce to unde rtake monitoring work associated with the Blue

Bastian , R.K. and Hammer, D.A. (1993) Tlte Use ,!f Co11stmrred l1Verlm1ds fi,r l1'1as1ewater treai111rnr

a11d R ecyc/i11g, i11 Co11stn1cred We1/a11dsfor l1'1ater Q11ali1y l111prcwe111e111, Moshiri, G .A. (ed), Lewis Publishers, Boca Raton . Kadlec, R.H . and Knight, R..L. (1996) Treat11u•111 Wetla11ds, Lewis Publishers, Boca Raton. Kim , S-Y. and Geary, P.M. (2001) The Impact o f Biomass H a1v esting on Phosphorus Uptake By Wetland Plants, vVater Scie11ce a11d Ter/1110/ogy, 44, I I, 6 I -6 7. R eed, S.C., Crites, R.W. and Middlebrooks, E.J. (1995) Natural Syste111sfor Waste 1\lla11a,ee111w1 a11d Trrnt//lmr, 2nd ed., McGraw Hill, N ew Jersey. Saunders, M (2001) T h e Effectiven ess of Constru cted We cl and s for Scormwater Po llutant Reduction: A R eview of Design , Hydrology and Management at C roudacc 13ay, Lake Macquarie, unpublished B.Env.Sc. Honours thesis, School of Environmental and Life Sciences, The University of N ewcastle. Schueler, T.R. (1992) Design of Stonnwater W etland System s: Guidelines fo r C reating Diverse and Effective Stormwater Wetlands in the Mid-Atlantic region, Metropolitan Wash ington C o uncil of Gove rnm ents, W ashington, D C. Waters, D. L. (2002) The Effecti veness of a Subsu rface Flow Constructed W etland for Scorrmva tcr Pollu tant Redu ctio n, Blue Ha ve n , NSW , u n p u bl ished B.En v.Sc . Honours thesis, School of Environmental and Life Sciences, The University of N ewcastle.

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THE USE OF SEWER PRESSURE SYSTEMS FOR MOUNTAINOUS REGIONS K Logan, M Cavaney

PUMP UNIT !HJECT!HG !HTOJ RETICULATION SYSTEM ON STREET SMALL SHALLOW

RETICULATION PIPE

"-PUMP UNIT IHJECTlNC I NTO RET I CULA TJOH SYSTEM ON STREET

Figure 1 . Schematic of a sewer pressure pump station .

Abstract Pressurised Sewerage Systems, whi le relatively new co Australia, have been used over the last 30 years in va rio us countries throughout the world for vario us reasons and are now starting to be used by Water Au thori ties in Australia on a larger scale. Connell Wagner have completed investigations for sewering options for various mo u ntainous areas with steep undulating terrain. The investigations have compared the use of gravity and sewer pressure reticulations systems for servicing mountainous areas which are traditionally expensive to service due to the terrain. Based on a cost benefit basis the fina l preferred systems are usually a combined gravity and pressure sewer reticulation system which can pot entially provided 30% cost saving and other added benefits over a more traditional gravity sewer system. This paper provides a brief description on sewer press u re sc h em es, some backgro und to w hy pressure systems are considered, and, a process that can be used to compare gravity and sewer pressure systems and develop a combined gravity and sewer pressu re system. This paper demonstrates the potential cost savings of combi ned gravity and pressurised systems in mountainous areas an d steep terrain. This coupled with the env ironmen tal advantages of t h ese schemes ca n result in a proposed com bined system using gravity systems for

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larger, flat catchments and pressure systems for smaller, mountainous catchments. The motive behind the combined systems is to utilise the benefits of each system in t h e vastly diffe ring landscape in m ountainous areas. As Water Authorities become more aware of the benefits and savings these schemes can provide in difficult locations there can expect to be an increase in the number of these schemes being developed in Australia.

Introduction Connell Wagner (CW) have completed investigations into sewer pressure systems both in Victoria and rural New South Wales. As part of these works we have completed investigations and functional design into what would be the most appropriate reticulation system to service a large number of communities located in mountainous areas and steep hilly terrain. Typical sewer reticulation options available in Australia are: 1. Traditional Gravity Systems 2. Vacuum System 3. Sewer Pressu re System 4. A combination of any of the above.

Due to the steep terrain of mountainous areas vacuum sewerage system s are usually deemed unsuitable due to the low static heads these systems need to operate under. Therefore they are usually not pursued fu rther after the initial stages of investigation. Therefore, investigations became a comp arison between sewer pressure systems, the more traditional gravity sewer system, or a combination of the two. The resulting proposed systems are usualJy a combined pressure and gravity scheme which can resu lt in a potential saving of up to 30% over a more traditional gravity scheme, with less environmental impact for what is usually an environmentalJy sensitive area.

What are Pressure Sewer Systems Pressurised Sewerage Systems have been used in the United States since the 1970s and have also been in use in Europe. Th ey co n sist o f a sin gle positive displacement pump vvith a grinder or muncher facility located in a small well at each lot in a sewer scheme. Each pump injects into a pressurised pipe network reticulation system. Th ese sc hem es

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typically cost between $12,000 and $16,000 per lot depending on terrain, layout and other site factors. There are a number of specialist suppliers fo r these systems now available in Australia. A schematic of the system is provided in Figure 1. Sewer Pressure systems are commonly used overseas in locations where the ground excavation condi tions are expensive, semi-rural subdivisions (low lot to reticulation length ratio), small hamlets and difficult terrain (steep) . They have not been used extensively in Australia. To date most large scale applications of sewer pressure systems in Australia has been done by the City of Wagga Wagga and South East Water Ltd (Tooradin, Cannons Creek and Warneet). Sewer pressure systems for these locations have been chosen over gravity sewer systems to address expensive grou nd conditi ons and small isolated conrnrnnities where the cost of a collection pump station and rising m ain pushes a gravity scheme to be cost comparative with pressure systems. The advantages of sewer pressure schemes over gravity sewer systems can be summarised as follows: â&#x20AC;˘ the reticulation pipe is much smaller (typicalJy a DN 32 to DN l 10 polythelene pipe) , laid at a shallow depth and has no maintenance shafts - providing substantial savings in the reticulation costs; â&#x20AC;˘ being a pressu re pipe, the location and gradient of the main is not as critical as a gravity main and can fo llow the undulations of the land; and, â&#x20AC;˘ the pressure w ithin the pipe limits infiltration; due to the potential lack of infiltration the peak loads from these reticulation systems can be significantly lower than gravity systems where wet weather in filtration occurs. The system operates on a "trigger" system where only a ce rtain number of pumps can operate at any one time. Once the maximum number of pumps per line are operating, the next one that tries to start up will trigger out, after a pre defined time (eg 2 nuns) it will try to start up again and if another pump in the system has not shut down during that time, it wilJ trigger o ut again and attempt to start again. This mechanism also serves to restrict the maximum flow in the lines. There are several issues that Water Authorities need to consider when impl ementing sewer pressure systems. They contain e lec tri ca l and mechanical equipment that needs to be maintained and replaced. Issues that need to be addressed in clude ownership of th e equipment and access. M.aintenance


WASTEWATER

programs and emergency breakdown and repair syste ms must be in place to adequately service the sc hem e.

Typical Cost Distribution Gravity Vs Pressure in Mountainous Areas

Investigation Area Backgrounds Mountainous areas typically have clayey soil, undulating terra in, and a high an nual rainfall. They are also tend to be areas of high environmental sign ificance. Due to the cost fo r developing these areas they histori cally have had limited, or sparse developm ent. T h erefore, th ey tend to have environmentally sensiti ve areas and significant amounts of vegetation whic h local communities tend to have a strong sense o f ownership wi th . M uch of th ese areas have been develop ed in a seemingly spasmodic, ad hoc fas hion throughout the previous decades. The main attraction fo r n ew residents is th e abu ndance of natural landscapes and wildlife and, combined with the general population growth , the provision of sewerage infrastructu re has not matched the influx of residents and visitors into these areas. C urrently a large number of houses in these areas are unsewered and are serviced

0

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Med ian

Pressure Systems

o

75th Percentile

Gravity Systems

95th Percentile

I

Figure 2. Typica l Cost Distribution of Pressure Vs Gravity Systems.

by septic tanks, usually du e to the high cost of installing a traditi onal gravity system in these areas. Because of the inherent characteri st ics of th ese mou n taino us areas, this m eth od of servicing is highly undesirable . Due to high rainfall, poor soil conditions and lack

of m aintenance, over flows occu r from the septi c tanks, wh ich has in turn led to the pollution o f the lo cal waterways. Enviro nmentally, socially, and politically, this is an undesirable situation. Several constraints are imposed on the design and construction o f any proposed

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system in moun ta in o us and steep hilly areas. Firstly, the areas typically have a particularly high d ensi ty of native vegetation du e to high rainfall and steep slopes difficult to develop, with wildlife that contributes to the area's character. As such the alignment of pipelines can be restricted to road reserves and unvegetated locations in many areas. T he impact of construction techniques is also o f high consi deratio n du e to loca l coun cils' requirement that trees are impacted as little as possible. Consultation with interest groups both initially and periodically throughout the design process to ensure all local issues were identified and addressed is important. T here is a need to develop sewering strategies for these vastly different landscapes in this terrain over the course of the next 50 years as they become subject to fu rther development and environmental p ressures from population growth, particularly as surrounding more easily developed land is built out.

Methodology for Developing Combined Sewer Systems The initial approach for developing a com bined sewer system is to compare a

â&#x20AC;˘ â&#x20AC;˘

traditional gravity reticulation system with a sewer pressure reticulation system for t he e ntire catchment area. An extensive comparison is undertaken in terms of capital costs, operational costs, environmental impacts and construction timing. For large scale areas some assumptions and comparisons with representative catchments needs be made and applied. Gravity System

Catchment size typically varies from as large as 350 lots to as small as 3 lots. Anything smaller than this is usually deemed unfeasible to service by a gravity system. This implies the number of catchments are smaller in area than a typical flat undulating terrain which occu rs in most gravity sewer systems. The gravity systems are typically set up in a "piggy back" approach. That is that one gravity ca tchm ent pump ed into the next catchment which pumped into the nex t catchment and so on until it could be discharged into a trunk transfer pipeline. T his means that downstream catchments generally require larger trunk mains, larger pump station flow ra tes and larger em ergency storage's compared to sewer pressure systems.

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Th e sewers can be plotted into a GIS system and total lengths of each pipe size ca lculated . Typ ically two sa mp le catchment areas are investigated in detail and the findings applied across all catchn1.ents. We have typi cally inferred for these system s that the average depth of sewer is assum ed to be 2.Sm, a ma nhole would be required an average of say eve1y 50m of pipe length and that there would be in the order of 20% rock excavation . Pressure Syst em

For pressu rised systems, the same area is divided into distinct catchment areas and can va1y between 200 lots to 2500 lots. D ue to the steep terrain in these regions it is not usually possible to develop a standard sewer pressure system which reli es only on the head fro m t he household pu mps to be able to transfer sewage out of the ca tchment. Therefore each catchment area usually needs to be collected at an intermediate pump station, the same as typical kerb side pump station for a gravity scheme. This is then pumped into a bulk transfer pipeline. For the entire pipeline network the average depth can be assumed to be 1.5 m and the proportion of rock was kept at 20% to provide a comparable cost with gravity systems. Ongoing operating and maintenance costs are also considered for these systems. Once the overall p ressure system fo r the entire catchment has been developed, each pressure catchment area is divided into the same small catchment areas that the gravity system was based on. The costs of the intermediate pump station and associated trunk mains are then proportioned off to each sub-catchment. The operational costs are based on recent experience and i nformat i on from suppliers. It needs to be recognised that the m echanical and electrical equipment associated with the pressurised system would require more freque nt servicing and replacement than the civil and structural component. It is also assumed that mechanical and electrical equipment require complete replacem ent every 20 years, and regular servicing every 5 years. This provided a reasonable p relim inary cost comparison between the two systems. Based on investigations completed into these systems, Figure 2 shows a typical indicative cost distribution for the two different sewer reticulation systems in mountainous areas. As this demonstrates, there is little variation in sewe r pressure system catchm ent costs. In comparison for gravity sewer catchment costs there are significant differences between the 5th and


WASTEWATER

the 95th percentiles. T his is du e to a nu n1ber of factors. The high cost va1iances associated with the gravity system are due to: 1. The number of ho uses within the catc hment: Due to steep hilly nature of the terrain, several catchments can be as sm a!J as 3 lots. Thi s then follows that th e co sts of the pump station w ill be proportio n ed to three lots, thus increasing the cost per lot substantia!Jy; 2. The number of catch m ents upstream: Th e higher the number of gravity catchm e nts upstream , the larger the flow rate and emergency storage that was required down stream. This increases the costs per lot of the do wnstream catchm ents; and, 3. Number of lots per length of reticulation sewer: Several areas were on steep terrain resulting in a sewer being required on the low side of eac h lot ie, a sewer in th e street and a sewer at the rear of the lot. In compariso n, fo r relatively fl at terrain, a m ore efficient design was possible where a sewer could se rvice both sides of a street, ie only half the length of reticulation sewer would be requi red. Development of a Proposed Combined System

A fter co mpl et ion of th e above assessment it is recognised that the best out come is a co mbined pressure and gravity system , making use of the inherent ad van tages of both sc h em es. When comparing the two sche mes an extensive compariso n needs to be underta ken in tern,s of capital costs, operational costs, environmental i mpacts and , constru ction timing. This then needs to be compared to the proposed adjoini ng system in order to develop a combined system that can work fro m an enginee1ing perspective and long term su itable operational basis when being developed. T his is an iterative process to arri ve at a final proposed sche me. Due to the lack of cost information regarding sewer pressure systems a conservative approach can be taken to make the initial selection to determine what type of catchment it would be. Catchments can be assessed and compared based on the proposed fo llowing decision making criteria: If the gravity system is: • g reater than say 120% of the pressure cost/lot the resultant system be a pressu re syst e m ; • b e tween say 120% and 110% of the pressure cost/ lot the resultan t system be a pressure system; • between 110% and 90% of the pressure cost/lot the resultant system was indefinite, consider either;

• between say 90% and 80% of the pressure cost/ lot the resultant system wou ld probably be a gravity system; and • less than say 80% of the pressure cost/lot the resultant system would definitely be gravity system. After each catchment area has been designated pressure, gravity or indefinite, the catchment is considered in relation to wh at was happening around it. The purpose of thi s is to determine an operationally logica l syste m. It would be inappropriate, for exa mpl e, to have a gra vity catc hment in th e middl e o f several pressure catchm ents. Fi nally each ca tchment needs to be considered in relati on to relative environmental impacts in these m ountaino us areas. Since the amount of excavatio n and general construction works is significantly less fo r a pressure system , it is preferable w here large trees, and se nsitive native habitat would be encountered an d potentiall y im pacted o n . In the few situations where th ere are no overriding factors in flu e ncing the type of system designated, a gravity syste m can be chosen beca use the syste m is more robust, reliable and doesn 't require O &

M maintenance of mechanical equipment. A typical split between gravity and pressure syste ms in a mountainous area is say 25-40% gravity and 60-75% pressure. Th e spli t between the two systems wi ll vary depending on local in fl uences such as steepness of the terrain, environmental infl uences, and existing in frastructure. A proposed combined system can result in the follow ing benefits: • A potential cost saving of up to 30% ove r a traditi onal gravity sc heme; • Less env iro nm e ntal impact in an environmentally sensitive area; and, • Lower peak flows from the reticu latio n system all owing the required ca pacity of bulk transfer system s and treatm en t systems downstrea m to be small er resulting in further capital savings which were not considered in these studies, b ut do provide a benefit to the system. There are however, seve ral issues w hi ch water auth ori ties need to consider for the development and imple mentation of these systems, including; ownership and access to equipment, maintenance requirem ents and provision of breakd own servicing requirements for electri cal and m ec ha n ica l syste m s whic h are not

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associated with a traditional gravity sewer system.

Conclusions Sewer pressure systems are a new development in the Australian market. To date they have typically been applied to fla tter terrain with difficult excava tion conditions making chem cost comparative with more traditional gravity systems. Sewer pressure systems are also a viable option for steep hilly terrain and environmentally sensitive areas. T he combined schemes can result in potential cost savings

of up to 30% with other added benefits over a traditional gravity sewer system. Australian Water Authorities can expect to see more of these systems as their po tential for cost savings in difficult areas and redu ced environmental impacts are assessed fo r regions w here traditional gravity sewer systems are not as suitable.

Acknowledgments The authors of this paper would like to acknowledge Pat Little and Peter Johns who have been involved in assessing these schemes.

The Authors Kristian Logan and Mark Cavaney are both experienced engineers with Connell Wagner who were responsible for delivery of these investigations. Kristian has five years of experience in rivers and waterways, sewer and water reticulation systems in both Europe and Australia. M ark has over ten years experience in the water an d wastewater industry in both Western Australia and Victoria. Ema il: LoganK@conwag.com, CavaneyM@conwag.com

BOOK REVIEWS The Handbook of Water Economics: Principles and Practice. Colin H. Green ISBN: 0-471-98571-6 Wiley & Sons. Hardcover Available AWA Bookshop by email bookshop@ awa.asn.au RRP $222.95 Water is vital to social and economic development whilst both arable land and water are scarce. Managing water is highly capital intensive, and capital is also sca rce . Simult an eo usly, th ere are environmental consequences to any intervention in the water cycle whilst the economy depends on the environment. Therefore, an integrated catchment, economic analyses must be undertaken on the analysis of the impacts of the proposed scheme upon the catchment as a w hole. This book starts with the Dublin declaration for defining sustainable water management and sets out the economic framework needed to support the implementation of its requirements. The book is divided into two parts: the theory and applications. The theory side sets out the nature of choice and decisionmaking, considering social and policy is s u es for water a nd resource management. The applica tions side provides the tools for the economic evaluation of water needs , the use of economic instruments and cost-benefit analysis. This Handbook of Water Economics adopts an integrated approach to managing land-water interactions. It includes good practice guidelines for each method along with a comparative summary of the advantages and disadvantages of each method. Case studies from projects in Egypt, South Africa, C hina and the United Kingdom illustrate th e application of various technologies. These case studies provide examples on

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wate r availability, sewe r age and wastewater treatment, tradable p ermits, flooding, and hydrometric data. H ence the approach throughout is essentially practical. T his book is suitable for those interested o r working in the following fields: environmental economics modules in Departments of Environm e n tal Management , Geog r aphy and Engineering, and r esea rch e rs in H ydrology. It could also be a useful resource for professionals and policy makers in water companies, water authorities, NGO's, government agencies and international agencies. Sustainable Water from Rain Harvesting. 3rd edition. Environmental Conservation Planning Pty. Ltd. ISBN 9 31711 802 0006. Available bookshop@awa.asn.au RRP $22 plus postage and handling. Four parts and 20 chapter sections covering all aspects of capturing rain water, choosing the right sized tank for a property and setting up the appropriate devices an d sys te ms to maximi ze collection of safe, useable water. This is the third edition of an increasingly professional, informative, grab-all booklet which targets the public, the water consultant and the dedicated layman wanting to comm.it to sustainable living. Part A, the Introduction covers the c ost of supply ing water to th e community and addresses the qu estion of whether or not it is safe to capture and use rainwater in a densely populated , urban environment. Not surprisingly, the answer is in the affirmative with a series of provisions and precautions that n eed to be borne in mind. Part B, titled 10 seep s co rain harvesting, tackles th e procedures and

actions chat need to be undertaken to install and draw water from a rain water tank to supplement che public supply. C hapters cover roof types - yes, some can be dangerous beca use of heavy metal shedding - roof gutters and downpipes, gutter outlets, mosquito/vector proofing systems and first flush water diverters. Particularly useful is the chapter on water storage tanks and sizing. Rainfall data from the local region, the dimensions of the roof collection area, end use of water collected and number of occupants to be se1viced, are all important. Maintenance, fire safety and water testing are also covered. Part C deals with setting up a sustainable system . Australia's climate extremes mean that planning for water storage and use needs to be developed over years not season to season. This part deals with dial retention and detention systems on a property, dual water supply systems, filt er pits and catchment domains , water pumps and pressure systems. The final Part briefly deals with septic tanks and sewage treatment systems. The inclusion of these asp ects of the total water cycle effectively close the loop so that the author, Rod Wade is able to honestly claim that his little book covers the sustainable use of water from the time it falls to its return to the environment. If you are a city dweller and think chat it is too hard to tackle the tantalizing prospect of installing a water storage tank in your own back yard - even just to pretend you live a simple country life this little booklet is what just what you need to pretend that life really is as simple as it once was. Diane Wiesner Science & Technical Information Officer, AWA

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Water Journal September 2003  

Water Journal September 2003