__MAIN_TEXT__

Page 1


ISSN 0310-0367

Volume 19, No. 5, October 1992 AUSTRALIAN WATER & WASTEWATER A.SSOCIATION

CONTENTS

Australian Water & Wastewater Association Incorporated

ARBN 054 253066

3

My Point of View

Association News 4 5 6 12

FEDERAL SECRETARIAT Execut ive Director - Chris Davis Business Manager - Margaret Bates PO Box 388, Artarmon 2064 Telephone (02) 413 1288 Facsimile (02) 413 1047

Seminar Reports

FEDERAL PRESIDENT Barry Sanders, Phone (09) 420 2453

' SECRETARYfTREASURER Greg Cawston, Phone (042) 29 0236

News from the Executive It Seems To Me Association News Industry News

16 AWWA Seminars, N.T. and W.A. Community Consultation 17 Sewage into 2000 . .. A European View 18 A Festival to Water 19 AWWASEEK and WATMOD 22 International conference on the Role of Concrete Kilns in Waste Management

Features 23

BRANCH SECRETARIES Canberra, ACT Alan Wade, PO Box 306, Woden 2606 Phone (062) 513 368 New South Wales Nick Aposto lidis, GCEC, 39 Regent Street Railway Square 2000 Phone (02) 699 9922 Victoria John Park, Cl- Water Training Centre, PO Box 409, Werribee 3030 Phone (03) 417 7411 Queensland Don Mackay, PO Box 412, West End 4101 Phone (07) 840 4844 South Australia Neil Palmer, Cl- State Water Laboratories, E&WS Private Mail Bag, Salisbury 5108 Phone (08) 381 0268 Western Australia Ralph Henderson, WAWA PO Box 100, Leederville 6007 Phone (09) 420 2623 Tasmania Jeff Lawerence, GPO Box 78A, Hobart 7001 Phone (002) 30 3544 Northern Territory Lindsay Monteith, PO Box 351 , Darwin 0801 Phone (089) 81 5922

EDITORIAL CORRESPONDENCE E.A. (Bob) Swinton, 4 Pleasant View Crescent, Glen Waverley 3150 Office Phone-Fax (03) 560 4752 Home (03) 560 9306

ADVERTISING Ann Sykes-Smith, Appita, 191 Royal Parade, Parkville 3052 (03) 347 2377 Fax (03) 348 1206

Coke Ovens By-Products Wastewater Treatment Plant at Port Kembla D. Paris and A. Jell 29 The UASB Process T. Lawson 31 Recovery of Dissolved Metals from Dilute Effluents C. Hertle 33 Recovery of Mercury from Industrial Wastewaters N. S. C. Becker and R. J. Eldridge 37 Appropriate Control Method for Chlorine Disinfectidn D. W. Braden

Of Interest 42 44 45 48

Industry News, Personalities Book Reviews Product Information Conference Calendar

OUR COVER ENGINEERING EXCELLENCE Our cover shows the wastewater treatment plant at the BHP Steelworks in Port Kembla, in the course of construction , against a background of heavy industry, The plant, which is owned and operated by Thiess Environmental Division, under contract to BHP, was designed and constructed by Linde Australia Pty Ltd, and has now been commissioned successfully, as described in the paper on page 23. Despite the toxicity of some of the numerous chemicals present in Coke Ovens wastewaters, the plant was designed without the benefit of pil ot plant tests on the actual effluent, by utilising the experience of the German parent company. Eight process steps are employed ensuring the protection and stability of the biological stages. The leve l of treatment achieved sets a new world standard for this type of hazardous wastewater. In September, the Institution of Engineers, Sydney Division, awarded Linde the Engineering Excellence Award, Environmenta l Category.

PUBLICATION Water is bi-monlhly. Nominal distribution times are the third weeks of February, April , June, August. October. December.

IMPORTANT NOTICE

PRODUCTION EDITOR John Grainger, Appita, 191 Royal Parade, Parkvi lle 3052 (03) 347 2377 Fax (03) 348 1206

The views expressed by the contributors are not necessarily endorsed by the Australian Waler and Wastewater Association . No reader should acl or fail to act on the basis of any material contained herein. No responsibili ty is accepted by the Association , the Editor or the contribu lors for the accuracy ol information contained in the text and adve,tisements. The Australian Water and Wastewaler reserves the righl to alter or to omit any article or advertisemenl submilted and requires indemnily from advertisers and contributors aga inst damages which arise from ma1erial publ is hed. All material in Water is copyright and should rot be reproduced wholly or in part wilhout the writ1en permission of the edilor.

WATER October 1992

1


INTERNATIONAL CONFERENCE ON THE ROLE OF CEMENT KILNS IN WAS.TE MANAGEMENT A Report by David Moy An international conference was held on September 10-11, 1992 in Brisbane on the role of cement kilns in managing our solid and hazardous wastes. The conference was organised by the Waste Management Research Unit of Griffith University and supported by the Cement Industry Federation of Australia and the waste industry. Registrations numbered 164 which included 135 registrations from Australia and the remainder from Canada, United States, England, Belgium, Norway, New Zealand, Spain, South Africa, Korea, Indonesia and Ho·ng Kong. Opening and Keynote Introducing the conference, Professor Philip Jones, Head of the School of Environmental Engineering, Griffith University, reported that many countries having far stricter environmental standards than Australia are using cement kiln technology for managing wastes and at the same time recovering the energy stored in them. He stressed the importance of mounting a conference to bring together all the world's experts in this area to present their experience in the use of this technology in an open forum to the public, interest groups and the politicians to gain support in introducing this benign and environmentally friendly technology to Australia. The conference was then officially opened by Dr Ian McPhail, Executive Director of newly-formed Commonwealth Environmental Protection Agency in Australia. He described in detail the functions of his organisation and the steps undertaken to implement the recommendations of the Independent Panel on Intractable Wastes in Australia, one of which is the investigation of cement kiln technology for disposal of intractable wastes. The Keynote Address of the conference was delivered by Charles Coles of St. Lawrence Cement Company in Canada. He stressed the importance of determination to withstand the possible criticisms of this technology and to aggressively address the communication, public relations and political aspects of the project. Europe After an introduction to cement manufacture by Don Woodcroft of Queensland Cement Limited, the session on European experience was presented with experiences from Belgium, Norway and Switzerland. Pierre Degre of Ciments D'Obourg in Belgium reported that his company currently diposes of 172 000 tonnes of hazardous and non-hazardous wastes per year, replacing up to 35% of its enegry requirements, increasing up to 50% in 1994. Holderbank in Switzerland has achieved 15% energy substitution from incinerating hazardous wastes and dried sewage sludge in their plant. Experimentation using waste timber is underway. Kare Karstensen from Centre for Industrial Research in Norway reported on the studies undertaken in recent years to measure particles, metals and organic micropollutants such as PAH, PCB

22

WATER October 1992

and PCDDs from hazardous waste incineration in cement kilns and concluded that these emissions are more influenced by operating conditions than the fuel incinerated. North America The session on North American Experience was opended by Mike Benoit of Cadence Chemical Resources, USA with a historical perspective and current overview of this technology in the United States. He reported that year 1992 marks the 20th anniversary of the first experimental use of chlorinated waste solvents as fuel substitutes in North American portland cement kilns, where 25% of its plant locations across the United States burn hazardous waste as fuel. Brian Dawson of Cemtech, USA, reported that over 70% of waste directed to landfills has sufficient potential energy value to be utilized in cement kilns and described in detail the technologies developed by his company in this regard. According to Brian, the raw material substitution (RMS) from waste material such as fly ash, petroleum contaminated soils, sludge waste from paper mills and foundry sand is a growing trend in the United States. Eric Hansen of Ash Grove Cement, USA, who is also a President of the Cement Kilns Recycling Coalition in the United States, reported on an innovative method to use solid waste derived fuels such as whole tyres and containerised hazardous waste to replace up to 40% of the energy requirement in the cement kilns by introducing the fuel directly into the mid kiln. Robert Schreiber of Waste Management International, USA reported on the activities by his company to focus on technical development, operations, permitting and regulatory compliance for waste recycling programs in conjunction with the cement industry. Robert Holloway of the United States Environmental Protection Agency reported that his agency has no data that indicate that emissions from hazardous waste burning in a cement kiln pose a hazard to human health or the environment, or that there is a measurable increase in the level of toxic metals in the cement product. The North American Session was concluded by Dr Kathryn Kelly of Environmental Toxicology International, USA. She reported on a largest-ever exposure study conducted in Midlothian, Texas which is believed to be the site of the highest concentration of cement plants burning hazardous waste in the world having three cement plants all within 5km of each other burning hazardous wastes. This detailed study has clearly indicated that no adverse health effects would be expected in a community due to waste incineration in cement kilns. Pacific Rim This was followed by a Session on Pacific Rim Experience. Brian McGrath of Blue Circle Southern Cement in Victoria,

Australia reported on the initiatives undertaken by his company to use waste tyres and used lubricating oil as kiln fuels. Ron Pilgrim of Morrison and Cooper, New Zealand, stated that many of the problem wastes generated in New Zealand can be destroyed by this technology. However, he stressed the importance of educating the public in this regard. Workshop The technical sessions were followed by a panel session and a workshop with involvement of respresentatives from the Queensland Department of Environment and Heritage, Queensland Conservation Council, Environment Management Industry Association of Australia, Waste Recycling and Processing Service of New South Wales, Brisbane City Council, Cement Industry Federation, Pacific Waste Management and Environmental Toxicology International from USA. The lively interest and enthusiastic participation of delegates during this session clearly indicated the importance of implementing this proven and environmentally friendly technology in Australia.

'lole: The prinled proceedings from lhe cont'crence are available al a cosl A$100 plus 110slage. In addilion, a video prepared in parl from lhe presenlalions will he available shorlly. The preproduclion price of lhis video is A$150 11lus poslage. Please conlacl Dr David Moy on lelephone (07) 875 5506 or •·ax (07) 875 5288 for furlher delails.

Public and Media The innovative free public session following the above sessions proved to be highly successful. The session, moderated by Jan Taylor of ABC Radio, included several overseas speakers available to answer the public's questions and concerns in using this technology. Prior to the conference, a luncheon was held in Brisbane to brief the media on the use of this technology and to present the objectives of the conference. The coverage given by the media was satisfactory given the fact that Queensland State Election was imminent. The outcome of the conference clearly demonstrated the need to inform the public, interest groups and the politicians of the potential of this technology through open forums. The Chairman of Australia New Zealand Environmental Conservation Council and the Minister of the Environment for New South Wales, Hon Chris Hartcher was the dinner guest speaker and the Minister of Environment and Heritage in Queensland, Hon Pat Comben spoke as the conference lunch guest speaker.


TECHNOLOGY

Coke Ovens By-Products Wastewater Treatment Plant at Port Kembla by D. PARIS and A. JELL Danny Paris is the Manager - Wastewater Treatment Division of Linde Australia Pty Ltd and was the Project Manager of the Coke Ovens By-Products Wastewater Treatment Plant project. As well as managing the project he was also involved in the Process Design and Commissioning of the plant.

SUMMARY The Coke Ovens By-Products Wastewater Treatment Plant at the BHP Port Kembla Steelworks (Slab and Plate Products Division) was designed and constructed by Linde (Australia) Pty Ltd. The comp lex nature of the plant utili ses both biological and physical/chemical processes due to stringent effluent requirements. During the contract pilot plant studies were performed to set design criteria and establish detailed start up procedures. The plant went into commercial operation in May 1992 and the results achieved confirm that by careful design, biological treatment of wastewaters normally regarded as difficult to treat can be successfu lly accomplished.

Anton Jell is a Process Design Engineer from Linde Australia's parent company Linde VA Munich Germany. He had overall responsibility for the Process Design and Commissioning of the plant.

INTRODUCTION In April 1990 Linde Australia Pty Ltd was awarded the contract to design and construct a turnkey wastewater treatment plant to treat wastewater emanating from the BHP Coke Ovens By-Products plant at their Port Kembla Steelworks. The plant, designed in conj unction with Linde Australia's parent company Linde VA of Germany, includes a number of Linde proprietary process technologies. A significant constraint on the plant design was the relatively small site area (50m x 110m) available for the complete installation . The plant construction was completed within 16 months, with the commissioning of some biological steps commencing I month prior to the ready-for-commissioning date. The paper describes the plant's eight process steps necessary to achieve the stringent effluent limits, then follows with details of pilot plant studies, actual commissioning experiences and performance data.

WASTEWATER SOURCE - COKE OVENS BYPRODUCTS PLANT At Port Kembla, BHP convert approximately 10 000 tonnes of coal to coke every day. For each tonne of coal coked 320 litres of effluent is produced . This effluent comes from water in the coal, water generated in the coking process and from cooling water in the gas cleaning and by-products and pretreatment processes. (ByProducts produced by BHP include BTX, a mixture of benzene, toluene, xylene and naphthalene and tar) . A significant wastewater stream is thus generated from the byproducts plant, contaminated with cyanides, phenols, nitrogen compounds and hydrocarbons. Historically this water was consumed in quenching the coke but intermittently it overflowed from the quench basins to the steelworks main drain . Other wastewater sources are the ammonia distillation plant, benzene distillation plant and the coke ovens gas (COG) compression plant. The combined flow and load from these sources form the basis of design, shown in Table 1 along with the effluent limits required. The treated effluent is primarily reused as quench water for the coking process, however, BHP have the option to discharge to stormwater if the water quantity exceeds quencher demand.

equalisation pure oxygen activated sludge including denitrification clarification and sludge handling. nitrification filtration activated carbon adsorption. The multistep design of the biological system was purposely selected to provide greater protection in the event of toxic surges on the plant. From the very nature of the wastewater it can clearly be seen that many of the contaminants are normally toxic to a conventional biological plant. However, by careful design and adaptation of the biomass, the plant treats the majority of the contaminants. The simplified schematic Figure 1 and fate of pollutants diagram Figure 2 provide a general plant overview. • • • • • •

Pretn:atment

Due to cyanide concentrations of 800-lO00mg/ L, the coke ovens gas compression effluent stream is independently treated. By chemical reaction the cyanide concentration is reduced so that the combined wastewater cyanide level is, after biomass adaptation, biologically treatable. Table I Component

Flow

Overview

Ammonia TKN CN SCN Phenols

The plant has eight process steps comprising: • pre-treatment of the COG stream • cooling

Soluble Oils BOD 5 COD

PROCESS DESCRIPTION

Footnote: This paper is an updated version of the paper presented to the First National Hazardous and Solid Waste Convention, March 1992.

ss

pH Temp ( 0 C) NO,-N Colour

Basis of Design

Combin ed lnflu en l

160.4m 3/

hr 272mg/L 330mg/L 93mg/ L 184mg/L 333mg/ L 71mg/ L 230mg/L

Effluent Limil

25mg/ L 0.5mg/ L 0.5mg/ L 0.lmg/ L 50mg/ L

610m g/ L 2 200mg/ L

25mg/ L 50mg/ L

9

8

78-80°C 100mg/ L Visually Clear

WATER October 1992

23


Dz

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Coke ovens by-products wastewater treatment plant simplified schematic

Bio Reactor - Early Warning Toxicity A larm System To protect the plant biological systems from toxic surges a small online bioreactor is located prior to the equalisation tanks. The bioreactor continually draws a small amount of raw wastewater and return activated sludge. If toxic substances are present an alarm is activated alerting operators so that preventive action can be implemented. It must be remembered that in order to degrade some of the specific influent contaminants (C-N -, SCN, etc) the biological system of this plant is quite specialised requiring longer

than normal cjevelopment time, so that it is imperative to protect the biomass. Cooling Following the pretreatment of the COG stream, t he wastewater streams are combined and screened in a duplex filter. This prevents plugging of the plant heat exchangers which cool the wastewater to a biologically treatable level (30°C). A cooling water circuit including a cooling tower is incorporated in this step. -f

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WATER October 1992

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Fate , of Pollutants In a Multlstep Waste Water Treatment Plant Fig. 2 - Coke oven waste water treatment

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24

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Equalisation Two equalisation tanks capable of operating either in series or in parallel provide flow buffering. The tanks are equipped with a mixer and pH control and one of the tanks can be operated as· an additional denitrification tank under abnormal or maintenance shutdown conditions. Backwash water from the sand filters and the activated carbon columns along with belt press filtrate are returned to either or both tanks. Prior to equalisation a sample station samples influent proportional to flow. Design Information - Equalisation Tanks T1411/21 2 Number of Tanks Effective Volume per Tank 1950m 3 Total Volume 3900m 3 Secondary Treatment by Pure Oxygen Deep Tank The activated sludge biological treatment system comprises three aeration tanks in series, each 10 metres diameter by 12 metres deep. Aeration is via the Linde Solvox pure oxygen system which comprises fine bubble aerator tubes located on the tank floor. A bridgemounted vertical shaft mixer ensures fully mixed conditions . The tanks are equipped with DO, pH and temperature monitoring. Oxygen supply is automatically controlled dependent on the DO, similarly pH is automatically controlled. The required nutrient ratio for biological treatment is adjusted by addition of phosphoric acid. Activated carbon slurry dosing is provided to absorb hazardous components in the event of a toxic surge. Wastewater and return activated sludge are fed into the first of the three tanks (the option also exists to feed the second tank). The first tank normally operates under anoxic conditions as a denitrification tank with nitrite/ nitrate rich wastewater recycled from the downstream nitrification step. In this treatment step the main removal of COD, BOD 5 , phenols, cyanide and thiocyanate is achieved. Design Data - Aeration Tanks T2111/21/31 3 Number of tanks Effective volume per tank 850m 3 Total volume 2550m 3 Volumetric loading (BOD~) 0.99kg/ m 3 .d Volumetric loading (CODJ 3.43kg/ m 3 .d Volumetric loading (Phenol) 0.50mg/ m 3 .d MLSS 4000- S000mg/ L Operational Data - Aeration Tanks T2111/21/31 Dissolved Oxygen 6mg/ L (except denitrification tank) 7.0 pH 28-32°C Temperature Clarification and Sludge Handling The aeration tanks are followed by a clarifier in which the biological sludge and treated water are separated. The clarifier consists of two trains each equipped with lamella plate packs and a vacuum operated sludge withdrawal system. The lamella plate system was selected due to space limitations on the site. The majority of the sludge is returned to the aeration tanks with excess activated sludge diverted to a sludge holding tank prior to dewatering using a belt press.

Sludge Storage The sludge holding tank comprises an oxygen aerated zone and a settling zone. The tank is designed so that the aerated zone always provides a known quantity of live biomass that, in the event of an emergency (e.g. toxic shock), can be used to re-seed the plant thus ensuring a rapid plant re-start. Sludge which overflows to the settling zone is fed to the belt press. Design Data - Clarifier T231l/21 Number of clarifiers 2 (in parallel equipped with lamella plates) Volume per clarifier 600m 3 Total volume 1200m 3 Nitrification Clarifier supernatant is then pumped to the nitrification tanks. Biological nitrification and the reduction of residual organic pollution occurs in the two parallel operated nitrification tanks which use the Linde proprietary Linpor-N system. This system comprises small foam cubes in which the nitrifying bacteria grow. As the biomass is almost totally fixed in the Linpor cubes, and because

26

WATER October 1992

of the negligible excess sludge production due to slow growth of the nitrifiers, the water in these reactors has ~ry low suspended solids and residual biomass or solids can be readily filtered. This negates the need for tertiary sedimentation tanks eliminating the phenomenon of floating sludge due to uncontrolled denitrification in these sedimentation tanks. These reactors are equipped with the Solvox pure oxygen aeration system, a bridge-mounted vertical shaft mixer and specialised equipment to retain and regenerate the Linpor N cubes. DO and pH are also automatically controlled. As indicated in the design data the Linpor-N system allows much higher ammonia loads compared with conventional systems. The majority of the nitrate/ nitrite rich wastewater is recycled to the aerobic reactors for denitrification. Design Data - Nitrification Tanks T3lll/21 Number of Tanks 2 1950m 3 Effective Volume per tank Total Volume 3900m3 Ammonia load 1410kg/day Linpor Volumetric load 0.36kgNm 3 .d Linpor material carrier 20% of tank volume volume Operational Data - Nitrification Tanks T3lll/21 Dissolved Oxygen Cone. 8.0mg/L pH 7.5 28-32°C Temperature Sand Filtration Following nitrification, the equivalent to plant influent flow is directed to four multimedia sand filters which remove residual biomass and suspended solids. The filters operate in parallel and are backwashed automatically using water from a clean water storage tank. The dirty backwash water is pumped to the equalisation tanks. Design Data - Sand Filters F331l/21/31/41 Number of filters 4 Design flow 180m 3/ h Flow rate 6.4m/ h Activated Carbon Adsorption Due to the high COD/ BOD ratio of the raw wastewater and the need to meet the required COD effluent concentration, a COD removal rate of 98% is necessar:i1- which cannot be achieved by biological treatment only. Final COD reduction and colour removal is attained in the last treatment step, the activated carbon adsorption unit. The activated carbon adsorption unit consists of three columns filled with a granular activated carbon (GAC) bed. The water passes through two of the GAC columns in parallel (the option also exists to run the columns in series, ie lead, lag) . The spent carbon is reactivated in a multiple hearth regeneration kiln . Treated wastewater then flows to the cleanwater tank. From here it is pumped off site to the quencher basins. A sample station located on the cleanwater pump discharge collects treated wastewater for analysis. Design Data - Activated Carbon System Activated Carbon Adsorption Unit F4111/21/31 Number of columns 3 Design flow 180m 3/ h Multiple hearth regeneration 500kg/ hour kiln

PIWT TRIALS The very stringent requirements for the effluent and the problematical nature of the plant influent for biological treatment would have meant that under normal circumstances pilot plant trials would have been conducted to collect data and prove nominated processes. In this case this was not possible for a number of reasons. Firstly, BHP's licence requirements nominated that the plant had to be completed within a time frame that allowed no time for pilot trials. Secondly, in nominating the specified influent BHP predicted a reduction in some of the contaminants which they anticipated would result from improved operation and scheduled modifications to the By-Products Plant. Thirdly, the coke oven gas compression plant was not complete and the influent from this source had to be estimated. Therefore the actual specified influent was not available.


The process design was therefore done utilising our extensive data base of biological processes and behaviour from past Linde projects. However, during the course of the contract three sets of trials w:re performed, two at Linde Australia's parent company's laboratories · in Munich Germany, the third on site during the commissioning. These trials helped set and confirm design parameters, obtain detailed information for plant start-up and provided additional commissioning data. Pilot Plant Trial No. 1 - Activated Carbon Adsorption Using pretreated coke ovens wastewater obtained from a European coke ovens plant, adsorption, reactivation and readsorption trials were performed on granular activated carbons from a select number of suppliers. The trials set the following design parameters: • Activated carbon loading factor (ie. mgCOD/ g AC) . • Carbon wavefront length. • Suitable brands and grades of activated carbon. The trials also confirmed design parameters for : • COD and other contaminant reduction. • Colour removal. Pilot Plant Trial No. 2 - Biological System The second trial involved the whole biological treatment section of"the plant from the first stage reactors in sequential process step through to the nitrification tanks. This plant was quite sophisticated as the trials were carried out over a six month period. Actual BHP wastewater supplemented by effluent from a German coke ovens' by-product plant was used. The trial confirmed Linde's basis of design and most importantly developed a detailed start-up procedure establishing: • Optimum start up parameters. • Threshold limits for inhibitors. • Load increase rates and limits. • Best type of seed sludge for the aerobic and nitrification reactors. • Indicated maximum loading rates that could be achieved. The information obtained about the nitrification step, which traditionally is the slowest to start up, was especially useful. Pilot Plant Trial No. 3 - Linpor N The third trial comprising two Linpor N reactors fed from the plant clarifier supernatant which: • Confirmed full ammonia load and effluent NH 4 and NOx-N requirements. • Provided more data on loading rates. • Enabled inhibition studies to be performed setting threshold limits. • Provided an early warning system if inhibitory substances were found in the influent.

to the tanks. A pH control system added lime as required to maintain a preset pH of 7.5. Aerobic Reactors - Start-Up The aerobic reactors were started up by diluting the influent with mains water, seeding the tanks with activated sludge from an industrial wastewater treatment plant and by the addition of activated _carbon powder slurry, which assists in the initial adsorption of any influent component which may be hazardous to the start-up. The start-up proceeded extremely well with discernible biomass growth and contaminant reduction noticeable within one week. The activated sludge produced by the plant settles well with an SVI of less than 50mL/ g. There was however, a noticeable amount of fine colloidal solids present that would not settle which affected downstream processes (high pressure drops in the sand filters and carbon columns). The introduction of polyelecrolyte in the clarifier inlet channel coagulated these solids and they were removed in the lamella clarifier. The sludge also dewaters easily with typical dry solids concentrations of 25-30% after the belt press. The low SVI and good settling properties of the sludge is compatible with Linde experience on other plants where pure oxygen aeration has been used. The combination of deep tanks (12 m) and the fine oxygen bubbles produced by the Solvox system results in a high oxygen utilisation efficiency. A common problem with other coke ovens wastewater treatment plants is presence of foam and sometimes floating sludge. This has only been experienced once on site during the commissioning and was attributed to an influx of very highly concentrated oil. The low aerobic reactor energy levels due to the use of pure oxygen make the foam and floating sludge easy to control with first step recovering within one week from this set back(determined by the return of COD removal efficiency to normal levels). Any minor foaming which does occur is very easily controlled by simple water sprays. Plant Performance The plant went into commercial operation in May 1992 and to date has performed up to and exceeding expectations. Table 2 lists some influent and effluent data for a one month period and Figures 3 and 4 show the typical reduction of BOD and COD through each process step. Figures 5, 6, 7 graphically illustrate some results for 32 days during July-August 1992. Contractural Agreement The plant was also unique from a contractural viewpoint. In keeping with their core business philosophy BHP issued the enquiry on a Build, Own and Operate basis. • Linde (Australia) Pty Ltd was awarded the turnkey contract to design, construct and commission the plant with Thiess MAJOR PROCESS STEPS

COMMISSIONING Commissioning procedures were established from pilot plant studies and previous Linde experience. The first step was to establish a 24 hour sampling and laboratory analysis program for the on site laboratory which is capable of performing all essential analysis. Nitrification - Start-Up The nitrification step being the slower biological reaction and typically requiring most care was commenced first. Start-up invol~ed isolating these reactors from the rest of the plant and commencmg nitrification using an artificial ammonia source and a nitrifying seed sludge. Once nitrification was established controlled amounts of clarifier supernatant were fed to the Linpor tanks, initially in conjunction with the artificial ammonia feed. Loading of the Linpor-N cubes, which occupy approximately 20% of the tank volume, commenced July 1991 with the tanks partially filled with potable water. The cubes, whose dimensions are approximately 15 mm cubic, initially submerged very slowly, however, after the addition of the seed sludge and the ammonia source, biomass development aided their submergence. During these early stages it was essential that the DO in the tanks. be kept_ below 8.0 mg/L, because at higher DO's the cubes, still with low biomass populations, could resurface due to buoyancy effects . With time, biomass growth negated this effect. As start-up commenced in winter with the mains water temperature approximately 15 °C the tank contents were heated to approximately 30°C by the direct injection of steam. This _was no longer possible after the process start-up and the temperature m th~se tanks was maintained by a coil type heat exchanger through which hot water was continually pumped. Heating was no longer necessary once nitrification was established and actual wastewater was pumped

COD CONCENTRATION mg/ L

PERCENTAGE REMOVED PER STEP

2200

0

AEROBIC STEP 1

CLARIFIER 80 - 851/. NITRIFICATION LINPOR-N 941/.

140

SAND FILTERS

ACTIVATED CARBON ADSORBERS

20

Fig. 3 -

Reduction of COD through the process steps

WATER October 1992

27


Environmental Services as plant operators and principal contractor for the BOO contract. Also included in the Linde/ Thiess contract is the provision by Linde of ongoing technical support for Thiess for the 15 years life of the BHP/ Th'iess operating agreement. ·

CONCLUSION Biological treatment has long been established as the most economic, effective and environmentally friend ly way of treating continued on page 40

PHENOL INFLUENT MAJOR PROCESS STEPS

B00 5 CONCENTRATION mg / L

PERCENTAGE REMOVED PER STEP

610

0

10

981/.

5

991/.

350 - -- - - - - - - - -- - -- -333

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PHENOL EFFLUENT EFFLUENT 0. 1- - - - - - - - - - - -- - -- - - - ~ LIMIT

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Fig. 4 -

Fig. 6

Reduction of BOD 5 through the process steps

SCN INFLUENT

NH4-H INFLUENT

220 - -- - - - - - - - - -- - - -- - - -

250---------------------, SPECIFIED M AX INFLUE~T 272 mg/L

210 200

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140

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NH4-H EFFLUENT 2 5 - - - -- -- --

-

- - - -- - - --

-'--'---'--'

1.0 - - - -- - - - -- - - - - - - -- - - - ,

-

EFFLUE'.\i LIMIT 25 mg/L

cg,

cg,

E

E

0 _5 ,_ _EFF~U_ENT_1:l~_UT. ... . ............... . .... . . . .... ....... .

0.1L--------------------1 o.,. .

Fig. 7

Fig. 5

28

WATER October 1992

le\"°') 1• 1 .. : .1 ......... ·--


••

TECHNOLOGY

--

THE UASB .PROCESS by T. LAWSON

THE NEED Most municipal sewage treatment plants receive a proportion of their influent load from industrial or 'trade waste' sources. The proportion is usually less than 20%, but in some cases may be up to 50% or even higher. Until comparatively recently, the full cost of treating this trade waste component has not been passed on to the industries concerned. In effect, local rate payers were subsidising their industries in their area and it has seldom been necessary for industries to install wastewater treatment facilities on their own site. This situation has changed in recent years as municipal authorities raise their charges to more economic levels and become more strict at enforcing their own regulations for accepting industrial wastewater into their sewerage system. Sewer charges in most Australian cities have risen markedly over the last five years, and are still rising. For example, current (simplified) charges in three eastern capital cities are as follows:

Volume charge, $/ m 3 8OD 5 Charge, $/ kg NFR Charge, $/ kg

Sydney

Melbourne

Brisbane

0.80

0.29

0.51

1.50-3.00

0.31

0.93

0.75-2.50

0.15

0.28

sewage and thus usually need a different process approach. Aerobic systems run into difficulty as wastewater strength gets higher and higher. High recycle rates may need to be used to dilute influent strength, and energy and land area requirements tend towards unacceptable levels . The activated sludge process is best suited to comparatively low strength wastes (say below 1000 mg / L BOD~) and has the disadvantages of comparatively high energy usage and substantial production of waste sludge. Biological (trickling) filters can handle higher strength influent with slightly lower energy consumption, but they still occupy quite large land areas and still generate a lot of sludge. The usual choice in recent times for high strength biologically degradable wastewater has been to go anaerobic. Such systems can combine very high volumetric loading rates (leading to low land area requirements) with very low energy usage and low surplus sludge production. Methane gas is produced as a by-product and may yield a plant energy surplus. The only main disadvantage of anaerobic systems is the comparatively modest effluent quality achieved, but this can be overcome (for a watercourse discharge) by tagging on a second aerobic stage downstream.

ANAEROBIC PROCESSES The Sydney charges are approximate only, and reflect the highest scale - for discharge to secondary STPs only. The Melbourne charges seem relatively low and may not be correct, in spite of diligent enquiries. All charges are expected to rise at inflation rates or higher. These new scales of charges now make it more economic for many industries to provide their own treatment plants than to pay the local authorities to treat an untreated waste stream. In some cases (eg in country areas) there may be no convenient local sewer to discharge into, so the company may be obliged to treat to a much higher standard to allow direct discharge to a waterway.

TREATMENT OPTIONS Unlike municipal authorities, industrial companies are not usually willing builders and operators of wastewater treatment plants. Their interests and priorities lie elsewhere: manufacturing a quality product, new marketing strategies, production schedules and so on. A wastewater treatment plant is about the last thing they want on their premises and they are seldom very knowledgeable about the process options available to them . In many cases land availability is a problem for them too. In general, industrial wastewaters tend to be much higher in strength than domestic

_The most easily understood anaerobic process is the so-called Contact Process, in which a fully mixed anaerobic tank not unlike a sewage sludge digester is linked to a downstream clarifier, with recycle of sludge solids back from the clarifier underflow. The flowsheet is akin to the activated sludge process, except that the biological reactor is covered and anaerobic, and that it exports energy in the form of sludge gas. Newer and more efficient anaerobic processes have been developed since the early success of the contact process. One of these is the anaerobic filter, which may be upflow or downflow and another is UASB, or Up/low Anaerobic Sludge Blanket reactor. Hybrid types have also been developed which aim to combine some of the advantages of each. The UASB system has now been far more widely applied than any other anaerobic process worldwide. In general, it allows much higher loading rates to be used than for the contact process, and has fewer operating problems than have been experienced with anaerobic filters. Some guide to the advantages of the UASB system in comparison with aerobic processes for treating high strength industrial wastewater can be found below. The figures given are approximate only. An activated sludge plant will consume

Tom Lawson is Managing Director of Aquatec-Maxcon Pty Ltd, having founded the company (as Aquatec Engineering) in 1981. Originally a civil engineering graduate from Glasgow University, he maintains strong personal links with a number of European companies, and has particular expertise in equipment and systems for biological reactors.

Volumetric Loading Rate, kg BOD/ m l.ct Sludge Production, kg/ kg 8OD 5 Energy Consumption , kWh / kg 8OD 5 Gas Production, kJ/ kg 8OD 5

Aerobic

UASB

0.5-1.0

5- 10

0.5-1.0

0.05

0.7-1.2

0.02 "20

more energy and produce more sludge than a biological filter plant, but volumetric loading rates will be similar. In general, a UASB plant can be loaded at up to 10 ti~s the rate (ie. the reactor a tenth of the size) of an activated sludge plant and will produce about a tenth of the sludge. Electricity usage is about one tenth of that of an activated sludge plant and the energy value of sludge gas available exceeds that of its electrical power consumption by a factor of about ten.

THE UASB SYSTEM The Upflow Anaerobic Sludge Blanket (UASB) system was developed in Holland in the late 1970's with the first full scale applications being built by CSM on sugar beet wastewaters. Much of the original theory came from the University of Wageningen, with Professor Lettinga much to the fore. Since that time, upwards of 200 full scale UASB plants have been built worldwide, with the chief sources of technology (licensors) now being Paques BV and Gist Brocades, both of Holland. Each organisation has a network of licensees, and when the writer attended a Paques conference in Holland in September, there were representatives there from 15 countries. The basic flowsheet of a UASB system is fairly simple. Incoming wastewater is first allowed to acidify (COD converting naturally to fatty acids) in a pre-acidification tank, then flows to the UASB reactor. Flow is evenly distributed across the reactor floor, then flows upwards through a bed of granular sludge before leaving the reactor via a system of launders at its top. Large WATER October 1992

29


GAS

EFFLUENT

INFLUENT

PRE ACIDIFICATION TANK

UASB REACTOR

Fig. 1 - Simplified UASB schematic

quantities of gas (mainly methane and carbon dioxide) are generated in the sludge bed and are collected in a system of invertedV gas hoods set below the launders . (Figure 1) Effluent from the launder system will typically have had 85-950Jo of its BOD 5 removed and can then be discharged directly to the municipal sewer. In some cases, posttreatment - usually by activated sludge is applied to treat the UASB effluent to a standard suitable for discharge to a watercourse. Various chemicals are dosed to the preacidification tank for pH control and nutrient make-up (if required), and it is common for a proportion of effluent to be re-cycled to the pre-acidification tank or just downstream to dilute the UASB influent and to recover alkalinity. Depending on the wastewater to be treated, various forms of pre-treatment may be applied - most commonly primary sedimentation and/ or DAF (dissolved air flotation). The UASB sludge granules do not react well to being contaminated with solids or grease, so efforts to remove these constituents upstream are more than worthwhile. The process of granulation is an interesting one, about which many technical papers have been written. Good UASB sludge granules can be grown from ordinary sewage sludge, but this process takes time (6-12 months) and a good deal of careful control. Most UASB plants are seeded with granular sludge from other like reactors, and in fact this sludge (which is not produced in great quantities) becomes a saleable byproduct. Gas production can either be flared off via a waste gas burner, or can be beneficially used as boiler fuel. In one plant at least in Brazil, it is pre-treated and compressed, then used as a diesel fuel replacement for the factory truck fleet. A point to watch is that extremely acidic conditions can be created at the gas/ water interface, resulting in corrosion problems if appropriate materials and protection are not used. Accordingly, the Biopaq (Paques) reactors have HDPE lining on the concrete tank inside walls, and the entire gas-hood and launder assemblies are fabricated in polypropylene in modular form . The UASB process is especially well suited to high strength biologically degradable wastes from which solids and grease have or can be removed. It has found widespread

30

WATER October 1992

Fig. 2 -

Exterior of golden circle UASB reactor

application in the pulp and paper, brewery and distillery industries, as well as in food production industries in general.

APPLICATIONS IN AUSTRALIA The first full scale application of UASB in Australia was at HP Products (now Bunge) in Altona, Victoria on a wheat starch wastewater. This plant was built in about 1984 under a direct license from CSM (later Gist Brocades) and has two large (2400m 3) reactors capable of treating a load of up to 30 T/ d COD. This plant works well, in spite of early corrosion problems from its earlygeneration aluminium alloy gas hoods, and has provided the seed sludge for all subsequent UASB plants in Australia. All four subsequent UASB plants in Australia have been built by AquatecMaxcon Pty Ltd, under license from Paques BV. The first of these was a relatively small plant (load 0.9 T/ d COD) on a paper mill effluent for APPM at Nowra, NSW. This was quickly followed by a much larger facility (design load 30 T/ d COD) for the Golden Circle Cannery in Brisbane. These two early plants have been followed by a 3.5 T/ d UASB plant on a pharmaceutical wastewater for ICI at Newcastle, NSW, and a 2.4 T/ d plant on a confectionery wastewater for Mars at Ballarat, Victoria. All four projects were won by AquatecMaxcon through competitive tendering, and scope of work has always included complete process design, as well as all. mechanical, electrical and control equipment. The Golden Circle project was total turnkey, including all civil structures too. It was commissioned in less than 40 weeks from start of contract. Load to the Golden Circle plant is seasonal and very variable, depending on the produce being canned or processed into soft dinks. As well as pineapples, it includes all manner of soft fruits and a variety of vegetables. Incoming wastewater passes through milliscreens and a grit cyclone to remove comparatively large solids, then through new primary sedimentation facilities, before pre-acidifying in a 2300 m 3 PA tank. Two UASB reactors, each of about 1000 m 3 , follow and there is a small sludge tank for surplus sludge. The UASB reactors and all tanks at Golden Circle are covered and ventilated through a compost filter to minimise oc:lours. Sludge gas is presently flared off

through a burner, but connections have been provided for its use as boiler fuel. One of the more interesting facts about granular UASB sludge is that can be stored un-fed for many months without permanent loss of activity. This allows plants like Golden Circle to adapt well to seasonal campaign loads and also allows surplus to be stored for long periods for subsequent sale to seed new UASB plants coming on line. Prior to completion of their UASB plant, the Golden Circle Cannery was paying sewer charges of about $1 million per annum. These charges have now been almost completely eliminated, giving this client a pay-back period of about 3 years on his investment.

THE FUTURE A number of industries in Australia will find it economically attractive to install their own wastewater treatment plants over the next few years. The larger ones with readily biodegradllble wastewaters will likely find UASB an attractive option . Because each industrial wastewater is distinctively different, it is desirable to undertake pilot plant testing on site as an aid to confirming process design parameters and performance guarantees . AquatecMaxcon has a sophisticated UASB pilot plant available for such testing, but also .has access to pilot and full scale performance results on a large number of potentially similar wastes treated by Paques or their licensees in some 20 countries overseas. '

AQUIFERS AT RISK February 15-17 Canberra ANU with BMR Ninth in the series: "Issues in Water Management" Information: Shirley Kral Phone: (06) 249 4580 Fax: (06) 257 3421


TECHNOLOGY

RECOVERY OF DISSOLVED METALS FROM DILUTE EFFLUENTS by C HERTLE ABSTRACT Over the past several years, it has become increasingly difficult and costly to dispose of sludges and effluents that contain heavy metals. Therefore, there has been a major effort to either reduce or eliminate the slud ge and reduce concentration in effluents. In contrast to precipitation methods which produce large volumes of toxic sludge (and cannot often meet new criteria set by authorities), and ion-exchange systems which produce eluants with concentrated heavy metals, electrolysis allows the effluent to be treated at the source to recover the metal ions as metal for onsite reuse or resale. This article describes the applications of electrolysis to recover metals present in rinse water and waste water from surface treatment shops. Electrolysis may be utilized not only for cathodic but also for anodic reactions. The simultaneous destruction of cyanides and other complexants at the anode in a system where metal is being recovered at the cathode adds a positive value to the effectiveness of this process. An example of on-site trial results are discussed from a technical and economic view point.

INTRODUCTION Industries producing effluents containing heavy metals have recently been required to reconsider their water use and waste management practices. Most authorities in Australia are tightening their standards for acceptance of liquid trade waste to sewers and to water courses. A list of selected metal and other contaminant discharge criteria of significance to metal finishing industries are ' shown in Table 2. These limits are usually associated with a total discharge from the industry. Currently many industries are not capable of meeting proposed limits without amendment to existing treatment facilities. Upgrading existing treatment systems (eg. precipitation) usually includes large capital outlay and increased costs for sludge disposal. A number of high-tech methods (such as membrane and ion exchange) have found their use for some applications but there has been a reluctance by industry to incorporate these unfamiliar methods. On the other hand, electrowinning techniques have long been successfully utilised for recovery of metals from solutions containing high concentrations (e.g. electrowinning of copper and go ld from ore processing operations). However, electrochemical metal recovery techniques have for a long time been limited by the low electrochemical

efficiencies associated with recovering low concentrations of metallic ions from the treatment solutions. Progress in electrolyzer design in conjunction with new cathode materials have increased the efficiency of metal recovery for concentrations in the low ppm range (Sioda 1983, Koniak 1983). The use of modern electrolyzer design alone or in conjunction with new methods allows for maximum internal recycle of materials and attempts to close the loop (i.e. total water and metal recycle) . A simple audit of current wastewater sources usually identifies opportunities to implement electrolysis systems to recover metals, reduce water flows and thus eliminate the need for increased end of pipe treatment systems.

Chris Hertle B.E (Hons) Chemical, M.Phil is an Environmental Control Engineer with Campbell Environmental, based in Perth. He s involved in all facets of municipal and industrial wastewater and sludge disposal technologies which are offered by Campbells. One of these areas is the trialling and marketing of the RETEC heavy metals recovery system.

the hydroxide. Theory predicts that the common metals are extremely insoluble at around neutral or alkaline pH. However, since metal plating baths contain a series of organic and inorganic additives, the metal precipitation equilibriums are displaced and the metal content found in the wastewat~r after precipitation is usually higher than most legal discharge limits. For this reason a second generation of treatments has appeared. One of these involves the tr~tment of effluents with ion exchange resins (Fig. 2). The ion exchange resins play a three-fold role: • Metallic ions are concentrated on the resin. • Treated wastewater can be recycled. • Ion exchange resins can be regenerated when they are saturated with metal. When the ion exchange resins are regenerated, the eluant contains concentrated metallic ion s which must be neutralized and which produce a sizeable quantity of heavy metal hydroxide sludges. Even with this technology, the presence of organic additives can lead to a loss of resin capacity or, in extreme cases, blockage of the resin. Electrochemical processes do not usually have this problem.

EFFLUENT TREATMENT Treatment of metal finishing industry effluents has generally been based on the removal of metal from water by chemical precipitation of the metal as hydroxide, sulphide or sulphate sludges. Effluent treatment stations, which have been installed in the metal finishing industry (e.g. electroplating shops) for more than 20 years, are generally based on the concept of effluent neutralization (Fig. I). The normal option involves precipitation of the metal as Table 1 Operation

Advantages of Electrolytic Recovery of Metals lmp acl

Static Rinse

• Electrolyt ic recovery of the metal at its source • Improve rinse quality and li fe • Reduce water use and maintenance Cascade Ri nse • Improve efficiency of raising • Reduced quantity to the ion exchange resin • Less frequent regeneration • Reduce chemical and labour cost Pretreatment • Reduced handling and transportat ion Station • Reduct ion of associated costs • Reduction of sludge volume • Achieve discharge criteria

Table 2 -

Standards for Acceptance of Liquid Trade Wastes to Sewer (mg/ L)*

Cadmium Chromium Ch lorinated Hydrocarbons Cobalt Copper Cyanide Formaldehyde Lead Nickel Tin Zinc pH

Melbourne Water

Waler Board*

Malcrial

WAWA

Current (1991)

1994

5 (4) IO (8) 5 (4)

I (4) 3 (8) 2 (4)

I (0.2- 10) 3 (30- 200) 3

5 (1 - 20) 10 (23 - 420)

IO (8) 10 (4) 5 (4) 50 (40) IO (8) IO (8) 50 (40) 10 (4) 7- 10

5 (8) 5 (4) l (4) 5 (40) 2 (8) l (8) 10 (40) 5 (4) 7- 10

10 IO (100- 200) 10 50 3 (30- 200) 5 (5-250) IO 10 (200-1500) 6- 10

5 (23 - 400)

10 (21 -670) IO (21 - 420) 10 (46-560)

• Bracketed va lues show maximum allowab le, loading in grams per day (range given for different flowrates.

WATER October 1992

31


ELECTROLYSIS AS A COMPONENT OF IMPROVED WASTE WATER TREATMENT One of the first electrolysis applications that can be visualized for an electroplating shop which has a neutralization station is shown in Fig. 1 (Electrolysis Option). The metal hydroxide sludge is redissolved in sulphuric acid and the recovered metal can be recycled and the discharge of toxic products eliminated. When the pretreatment station is equipped with an ion exchange resin system, electrolysis can play a role as one of the techniques for treatment of the ion exchange eluant. In the case of ion exchange alone, the metal is concentrated and the shop must either send the metal to an outside treatment facility or neutralize on site. In both instances, the. production of sludge is inevitable as is shown in Fig. 2. But there is an advantage in combining electrolysis with ion exchange to electrolytically treat the eluant to recover the metal and to avoid the production of heavy metal hydroxides (Figure 2, Electrolysis Option). Electrolysis is however, most effective when treating more concentrated streams. Placed, for example, on a static rinse bath in an electroplating line, it can remove more than 95 % of the metal from this dilute stream. It can be used in association with ion exchange systems to remove the last traces of metal while oxidizing materials at the anode. Table 1 lists a series of practical advantages associated with the use of electrolysis for treatment of metal bearing solutions. PRETREATMENT

H o 2 <

NEUTRALIZATION

Neutral isation

>

I

1

l

Sludge M(OH)2

>

lV Disposal

ELECTROLYSIS OPTION

METAL H SO 2

4

> REDISSOLVE

ELECTROLYSIS

V H 0 2

Fig.

32

H 0 2

<- - - - Recycle Regeneration Elutant >

Z+

ACID

M

SEWER

<- - - - - - - ' H 0 2

DISPOSAL<- - - - - '

ELECTROLYSIS OPTION I - - --

-

- ->

H 0 2

ELECTROLYSIS

>

V

METAL Fig. 2-Schematic for Ion Exchange Pretreatment

Finally, the electrolysis operating conditions allow for the treatment of the organic additives at the anode. The best known examples are the destruction of cyanide, destruction of formaldehyde, the oxidation of metal complexing agents such as quadrol, or in general, a reduction in the oxygen demand of the electrolytically treated effluent.

Filter Press

H 0 2

RESIN

NaOH

Z+ M

Se wer <

PRETREATMENT

I -Schematic for Neutralisation Preatment

WATER October 1992

EXAMPLES OF METAL RECOVERY VIA ELECTROLYSIS The principle reasons stimulating the use of electrochemical techniques for metal recovery in the electroplating and circuit board industry are the following : • Reducing the volume of toxic waste by eliminating the heavy metal hydroxide sludges; • Recovery of metals in their noble form; • Possible recycling of the metals dependent on the quantity and the purity. • Reduced water and chemical usage. The application of electrolysis systems discussed below is based on the RETEC™ cells fabricated by ELTECH International and supplied exclusively by Campbell Environmental in Australia. A RETEC™ cell contains titanium (type DSA®) anodes which permit the cell to operate in both acidic and basic media. Their specificity for the evolution of chlorine is well known, but they also support

the evolution of oxygen in acid media or the oxidation of co~plexing agents such as cyanides in basic media. The cathodes which are utilized in these electrolyzers are reticulated metal cathodes (RME) having a high surface area as compared to the geometric surface area (3). It is the high surface area which permits these electrolyzers to be effective over a large range of concentration which extends from several grams/ litre to a fraction of a milligram per litre. These electrolysis cells are modular in design. They can contain a variable number of electrodes based on the quantity of metal to be recovered and the method of utilization: closei:I circuit or in-line. The recovery capacities of different metals for the RETEC™ systems depend on the type of metal to be recovered and the other chemicals in solution. Typical recoveries range from 1 to 6 kg/ hr for the largest commercially available unit. Examples where RETECTM has been commercially applied for metal recovery are: • Copper from sulphate, cyanide, electroless and ammonical solutions. • Zinc from cyanide and hydroxide forms. • Nickel from bright, sulphamate, sulphate and electroless solutions. • Cobalt from sulphate solutions. • Other precious metals including silver, gold, antimony, palladium and platinum. • Cadmium recovery from acid, cyanide and alkaline forms. • Lead from acid and sulphamate solutions. • Tin from acid solutions . • Separation of Cd from Ni and Co, Cu from Cd and Zn. Most applications of RETEC™ are in the metal finishing industry (electroplating, anodising and galvanising). The treatment of rinse baths decreases the amount of heavy metals which are discharged to the waste treatment system. In printed circuit board shops the etch bath or the copper plating bath can easily be treated by electrolysis to avoid the surcharges associated with effluents containing highly concentrated heavy metals. Electrolysis can also be utilized to treat static rinse baths in order to increase their useful life. The on-site recovery of gold or other precious metals and their alloys on an electrode reduces the time for recovering the metal value when compared to conventional recovery methods. The selective separation of metals such as cadmium from cobalt and nickel is possible, and is being done with the sludges produced in shops which fabricate nickel/cadmium batteries (Wiaux and Oberflache, 1988). Silver from industrial photographic fixing baths is also recovered by electrolysis (Waiux and Nguyen, 1990).

ECONOMIC CONSIDERATIONS A typical recovery system is in an electroplating shop which plates zinc using zinc cyanide solutions. The plating operation is followed by a drag out and two stage flowing rinse. The system can be set up to recycle the drag out contents through a RETEC™ cell and recover zinc as metal at the cathode and destroy cyanide at the anode.

continued on page 36


RESEARCH

RECOVERY OF MERCURY FROM INDUSTRIAL WASTEWATERS by N. S. C BECKER and R. J. ELDRIDGE SUMMARY The extreme toxicity of.mercury has led environmental authorities world-wide to impose strict limits on mercury discharged into the environment in industrial wastewaters. Limits expressed in terms of total mercury concentration are typically of the order of 50 Âľg / L or less. On-site treatment of wastewater is required to achieve these low levels. Treatment methods that are widely used to remove divalent Hg include precipitation with sulfide and adsorption on ion exchange res.ins. Sulfide treatment produces a sludge of HgS, usually contaminated witr1 other metals, which then presents severe disposal problems. Divalent mercury is readily adsorbed by a special class of ion exchanger containing thiol groups or (if complexed by anions such as Cl - ) by anion exchange resins. The biggest problem with the use of these resins is that the adsorbed mercury is not easily recovered. Thiol resins can be regenerated with concentrated hydrochloric acid but this requires costly materials of construction and results in operational difficulties. In the case of anion exchangers, the usual acid or alkali regenerants will not displace complex mercury anions from the resin. Indeed, it is common practice to discard mercury-loaded ion exchangers after use because it is not considered economic or in some cases possible to regenerate them. We have investigated some alternative ion exchange adsorption and regeneration procedures for treating wastewaters containing Hg(Il). Our aim was to develop processes that would not only remove mercury effectively from wastewater but also enable cheap and efficient recovery of mercury in a form that would make reuse possible, thus keeping it out of the environment.

ADSORPTION OF HG(II) ON A CHELATING RESIN A chelating ion exchanger with iminodiacetic acid functional groups was a promising candidate for mercury removal and recovery. Although this type of resin adsorbs most cations, published distribution coefficients (Dardenne and Rengan, 1987, and Marhol, 1982) show that at low pH there is a high degree of selectivity towards Hg 2 + over other common cations. (Mercury-containing wastewaters are typically quite acidic.) When a solution containing 40 mg/ L Hg (as HgS0 4) at pH 1.5 was passed through a bed of iminodiacetic acid resin at a flowrate of 11 mL feed / hr/ mL resin the mercury leakage, averaged over the first 180 bed volumes, was of the order of 20 Âľg / L. Zn, Cd, Pb and Fe were not significantly adsorbed at this pH. Mercury uptake can be represented by equation I:

Norman Becker graduated from RMIT in 1981 with a Bachelor of Applied Science (Applied Chemistry). He spent two years working in the Marine Chemistry laboratory in the Chemistry department at Melbourne University developing water analysis methods for the WMC company prior to joining CSIRO.

Dr Rob Eldridge is a Principal Research Scientist with CSJRO Australia. He is presently leader of the Adsorption Processes section (interested in the exchange processes for removing organic colour and recovery metal salts) part of the Water and Wastewater Treatment Program in the Division of Chemicals and Polymers, Clayton.

R-NH + (CH 2COOH) 2Cl - + HgC1 3 - R-NH + (CH 2COOH) 2HgC1 3 - + ,Cl -

[2)

Attempts to regenerate mercury-loaded iminodiacetic acid resins with m_in_eral acid were unsuccessful. Dilute H 2S0 4 caused Hg(II) to prec!p1tate as HgS0_4 .2HgO. Recoveries with HN0 3 or HCl were low, ev1de_ntl~ because m the presence ofN0 3 - or c1- Hg(Il) again forms amomc complexes and under the very acidic conditions is retained by anion exchange. We reasoned that this finding could be exploited by regenerating with a complexing agent (Cl - is the obvious choice) at a pH approaching neutrality so that the Hg on the resin would form a complex anion, which would not be readsorbed because the imino groups are not protonated at the higher pH. Mildly acidic to neutral NaCl solutions were tested and found to elute Hg(II) from an 20 , - - - - - -- - - - - - - -- - - - -~

15

0.05 M ADDED CHLORIDE

...I. j

10

j

When this resin was tested on a mercury-containing wastewater from a zinc refinery the mercury leakage, although initially low, increased steadily with the volume treated. Clearly something in the wastewater was affecting mercury uptake by the resin. One of the major components of the wastewater, apart from sulfuric acid and zinc, was chloride ion . Laboratory experiments with synthetic solutions demonstrated that chloride did indeed affect mercury uptake by the resin (Figure I). The reason is that in chloride media Hg(Il) forms a series of complexes Hgc1y-nl+ with n = 1-4. These are less readily adsorbed than the free Hg 2 + ion, which is the major species in sulfuric acid solutions. The anionic complexes can be adsorbed to some extent because at low pH the nitrogen atoms on the resin are protonated and can act as anion exchange groups according to equation 2:

~

5

-5

'-----------L---------........J

0

100

200

bed volumes

Fig. I - Leakage of Hg(II) from an iminodiacetic acid resin. Feed: 40 mg/ L Hg, pH 1.5. Flow rate: 11 bv/ hr.

WATER October 1992

33


iminodiacetic acid resin with great effectiveness (equation 3, Figure 2). Smaller vo lumes of regenerant are needed as the NaCl concentration increases and as the pH approaches neutrality. Good resu lts were obtained with 3-5 M NaCl in the pH range 5-7: R-N(CH 2COO) 2Hg + 2Na + + 3Cl - R-N(CH 2coo - Na+)2 + HgCl3-

[3]

Many other complexing agents can be used to elute Hg(Il) from an iminodiacetic acid resin. These include other halides, thiosulfate, thiourea and EDTA, although precipitation of Hg occurs with some of these. However, chloride (as NaCl) is cheap, stable, easy to handle and readily available. It forms stable, soluble anionic complexes with mercury and is clearly the regenerant of choice for mercury-loaded iminodiacetic acid resins. . At low Hg loadings ( < IOOJo of resin capacity) little Hg is recovered. It seems that a small amount of mercury is bound te naciously to a minority of resin sites, or else is red uced on the resin to Hg(!) or Hg(0). This phenomenon is observed only in the first load cycle. Recovery of Hg(Il) adsorbed in subseq uent cycles is found to be complete. ¡ Once mercury is desorbed from the resin with NaCl, the resin in the sodium form can be returned to the adsorption step. Hg(Il) desorbed in a relatively pure form in NaCl solution can be reduced with a number of reagents to calomel (Hg 2Cl 2) or elemental mercury, either of which can readily be sold as a byproduct. After separation of the red uced mercury the mother liquor can be reused as regenerant. If the reducing agent is chosen appropriately, this recycled regenerant will not be contaminated with oxidation products. For example, oxalic acid is oxidized by Hg(Il) to CO 2 and water, and thus enables the regenerant solu tion to be reused almost indefinitely. We can summarize our results with iminodiacetic acid ion exchangers by saying that they offer an attractive way of selectively removing and recovering free Hg2+ from acidic solu tion. However, species that can form comp lexes with Hg(Il) (such as chloride) are often present in wastewaters. The chelating resins then do not adsorb mercury strongly eno ugh to meet discharge limits. In such cases a different approach is necessary.

ADSORPTION OF HG(II) ON ANION EXCHANGERS Mercury present in solution as an anionic complex anion is strongly adsorbed by anion exchange resins. We have investigated both strong and weak base anion exchangers. The weak base resins contain amino gro ups that can exist in the free base form and are not protonated above about pH 7. The strong base resins contain quaternary ammonium groups, which remain charged over the whole pH range. Both types were found to adsorb Hg(Il) strongly from solutions containing chloride, resulting in very low leakages (Eldridge

and Becker, 1992). Strong base resins were more effective, with mercury leakages < 20 J.lg/ L from a sold'tion containing 40 mg/ L Hg(ll), 350 mg/ L so/ - and 1080 mg/ L c 1- at pH 1.5. Strong base resins were also fo und to have higher Hg capacities than the weak base resins tested. Figure 3 shows typical breakthrough curves for both strong and weak base resins at a relatively high Hg(II) concentration (400 mg / L) chloride solution. As noted above, a major problem with the use of anion exchangers is that mercury is very difficult to desorb from such resins. Treating strong or weak base resins with the usual acid and alka li regenerants results in negligible mercury recovery, even though the weak base resins are fully deprotonated at high pH. The exact nature of the binding in this case is not clear, but Hg(II) appears to form a very stable comp lex with the amino groups on the resin. Again we reasoned that mercury could be removed from an ion exchangers by elu ting with an agent that forms a more stable complex with mercury than does the resin. Chloride was not successfu l at any pH, but partial recovery was obtained with 2 M ammonia solution. A precipitate (believed to be diamminemercury(Il) chl or id e Hg(NH 3) 2C l2) formed in t he regeneration effl uent. C lear ly, adsorbed mercury can be complexed by nitrogen ligands in the eluant. We decided to try organic complexing agents containing more than one amino group, a nd found complete recovery of mercury with 1-2 M aqueous 1,2-diaminoethane (ethylenediamine) (Eldridge and Becker, 1992). Figure 4 shows this reagent to elu te Hg(II) very efficiently from a strong base resin. Mercury is desorbed with similar efficiency from weak base res ins. Other a,w-diamines were a lso tested, but Hg recovery in a given number of bed volumes decreased with increasing carbon number (with the exception of C3), reflecting the decreased stability of the complex formed. Na 4EDTA at 1 M will also elute Hg(II), but not nearly so well as ethylenediamine. A lthough ethylenediamine is a very effective eluant, we looked for an inorganic reagent that might allow more economical Hg recovery. Hg(Il) forms a very stable complex with sulfi te, and when I M Na 2SO 3 was passed thro ugh a mercury-loaded resin (strong or weak base), desorption was complete. Figure 4 shows that sodium sulfite is not so effective as ethylenediamin~ but is still ab le to reach IO0OJo desorption in an acceptable number of bed vo lumes. Elution with complexing agents was not successful with thiol resins.

RECOVERY OF MERCURY FROM ANION EXCHANGE REGENERATION EFFLUENTS When ethylenediamine (weak base resins only) or sodium su lfite (both strong and weak base resins) are used to elu te Hg(Il) from an ion exchangers, a slow-growing crystalline precipitate forms in fractions of regeneration effluent containing high Hg concentrations 20

. ..

20

~

15

= '0

J f

I.

r--------------------

WB

j

15

1 tft

a:

10

10

0 o~-~~~-~~~-~~~-~~~-~_._,

0

10

15

20

L-~c:!<c==-===<a~::......--=.:!'.:...~---"-~~~_._J 0

Fig. 2 - Elution of a mercury-loaded iminodiacetic acid resin with 3M NaCl at pH 5. Flow rate: 14 bv/ hr.

WATER October 1992

200

300

bed volumes

bed volumes

34

100

25

Fig. 3 -

Lea kage of Hg(II) from strong and weak base resins. Feed: 400 mg/ L Hg, 0.05 M chloride, pH 1.5.

400


precipitate provides a highly concentrated and reasonably pure form of mercury that can be further treated to yield a desired end product. After the precipitate is separated, the mother liquor can be recycled (with a small charge of fresh regenerant) to recover more mercury from loaded resin. This operation has been demonstrated on a laboratory scale. Typical mercury concentrations in I M sodium sulfite regenerant were around 7000 mg/ L. This decreased to 4000 mg/ L after crystallization. Before recycling, the regenerant was topped up with 5% by volume of 2 M sodium sulfite. Hg(Il) in sulfite solution is not completely stable. Munthe et al (1991) report that divalent mercury at low concentrations in 10 - 4 M sulfite solution is reduced to the metal. The rate of reduction decreases with increasing sulfite concentration. We have observed traces of elemental mercury on the glass frits of columns containing resin eluted with 1 M sulfite, but the effect was only slight, with most of the mercury coming through as fhe divalent ion . Isolated solid Na 2 Hg(SO 3) 2 has been kept in the laboratory for over a year with no sign of greying which would indicate reduction of H g(Il) .

CONCLUSIONS

bed volumes

Fig. 4 -

Elution of mercury-loaded strong base resin with I M ethylenediamine and sodium sulfite solutions.

(typically > 4000 mg/ L) . These precipitates have been identified by microanalysis as Hg(en)Cl 2 and Na 2Hg(SO 3\ respectively (en = ethylenediamine). No precipitate was observed when strong base resins were regenerated with ethylenediamine, presumably because [Cl - 1 is lower since these resins need to retain so me Cl counterions. Equations 4 and 5 represent two of the many possible reactions involved in these regeneration processes. R-N + (CH) 3 HgCl3 - + en R- N + (CH 3) 3CL - + Hgen 2 + + 2Cl -

[4)

R-N + (CH 3) 2H HgCl3 - + 3SO/ - R- N(CH 3) 2 + H g(SO 3) / - + 3Cl - + HSO 3 -

[5]

The formation of these precipitates provides a useful means of isolating mercury from regeneration effluents (Eldridge and Becker, 1992). In the case of sodium sulfite elution of strong base resin, crystallization was usually complete 3 to 6 hours after elution . The precipitate begins to appear shortly after the regeneration effluent leaves the ion exchange column , but in extensive trials column blockages caused by the precipitate were rare due to its slow formation, even at very high effluent mercury concentrations. The

A major problem with ion exchange treatment of mercurycontaining wastewaters has been desorption of mercury from the resin. We have shown regeneration with complexing agents to be effective with both chelating resins and anion exchangers, and capable of forming part of an economically attractive process for recovering mercury from wastewater. Thiol resins, which bind mercury tenaciously, could not be regenerated with complexing agents but residual mercury concentrations approaching those attainable with a.thiol resin can be achieved with an appropriatelychosen chelating resin or anion exchanger. Chelating resins adsorb H g + 2 selectively at low pH in the absence of complexing agents, with the mercury efficiently recoverable by neutral sodium chloride solution. Anionic complexes can be removed on an anion exchange resin (especially a strong base resin) and can be eluted with aqueous ethylenediamine or sodium sulfite. The desorbed mercury can be further treated to yield a commercially valuable product, keeping it out of the environment. The depleted regenerant can be recycled, and there is no mercury-containing waste stream.

REFERENCES Y. Dardenne and K. Rengan , 198 7 J Radioanaly t. and N ucl. Chem. , Arlie/es, 116, 355 . R. J . E ldridge and N . S. C . Becker, 1992 Internation a l Patent A pplica ti o n PCT/AU92/00091. M. Marhol, 1982 in 'Wi lson and Wi lson's Co mprehensive Analytica l C hemistry', ed. G. Svehla, Volum e X IV, Elsevier, Am sterdam , p. 549. J. Mu the, Z. F. Xiao a nd 0. Lindqvist, 1991 Wate1; Air and Soil Pollution , 56, 62 1.

C. HERTLE Continued from page 32 The subsequent flowing rinse can be ·reduced in flowrate without affecting work quality and reduces the treatment required for precipitating Zn and destroying cyanide. Typical savings and operating costs using a RETEC™ system are shown below in Table 3. Typically the savings generated by installing a RETEC™ system offer a payback period of 1 to 3 years. Table 3 -

Savings Versus Costs for RETECTM Operation

RET ECTM Co sts

Savings

Capital Cathode Replacement Electricity Labour

Metal Recovery Cyanide destruction chemicals Zinc precipitation chemicals Neutralisation chemicals Reduced sludge management Reduced water usage Precipitation plant expansion not required

36

WATER October 1992

CONCLUSIONS Modern techniques for recycling and recovery of metal-bearing effluents offer the advantage of minimizing pretreatment costs for electroplating shop wastes. Electrolysis allows for treatment of the entrained metal at its point source without modifying the shop production scheme. It is also possible to treat effluent at ()ther points on their way to the treatment station. Effluent treatment will be analyzed in the future with the objective of maximizing recycling in the form most acceptable not only from the economic viewpoint, but also from the standpoint of minimizing the necessity to handle, produce, stock and transport a minimum of toxic waste. The various aspects of the problem must be considered according to a principle relatively new in the area of environmental protection: risk minimization .

The RETEC™ system will enable most of the metal finishing industry to minimise discharge of metals in their waste stream, and provide a payback by recovery of noble metal. This fullfills the criteria of most environmental authorities and aims for the criteria outlined in the AWWA (1992) position paper on waste minimisation and recycling.

REFERENCES Australian Water and Wastewater Association. "Position Papers on Waste Minimisation and Recycling", p 13, June 1992. Sioda et al, (1983). "Flow - through Porous Electrodes", C hemical Engineering, Feb 21, pp 57- 67. Konicek M.G., Platek G ., (1983). "Reticulated E lectrodes Cell Removes H eavy Metals from Rinse Waters", New Materials a nd Processes, Vol 2 pp 232- 233. Wiaux J.P. , (July 1988), Oberflache - Surface, No. 7, 21 - 26 . Wiaux J. P. a nd Ng uye n T., (1990). " Recovered Value from E lectroplating Indu str y Wast e", Metal Finishing, pp 85- 92, June.


TECHNOLOGY

APPROPRIATE CONTROL METHOD FOR CHWRINE DISINFECTION by D. W. BRADEN ABSTRACT In dealing with something as dynamic as chlorination of a potable water supply with respect to variable flow rates, mixing kinetics and physicochemical water quality, it is most important that the objective of maintaining a free chlorine residual of 0.2 to 0.5 mg / Lat a point where water and chlorine have been in contact for 30 minutes is not lost in the design of control equipment which regulate chlorine dose rate. Control techniques set out to regulate chlorine dosage such that a predetermined chlorine residual level is consistently achieved . The control techniques when implemented, however, often do not achieve the desired result. In most cases, the manner in which the plant performance is monitored mistakenly leads to the conclusion that the process is effective whereas in reality, chlorine underdosing or overdosing is occurring. This paper highlights the inability of commonly practiced chlorine dosing techniques to achieve 0.2 to 0.5mg/ L free chlorine at the 30 minute detention point, and provides a method for consideration as a preferred standard practice, which would greatly enhance that of the 'Flow paced Control' method.

INTRODUCTION There are four basic methods of chlorine dosing control practiced. These are: 'MANUAL CONTROL'; 'RESIDUAL CONTROL'; 'COMBINED CONTROL'; 'FLOW PACED CONTROL'. When considering that the primary aim is the maintenance of a free chlorine residual of 0.2 to 0.5 mg/ L at the 30 minute contact point (WHO, 1984) each type of control mechanism is hindered by the fact that in almost every situation, the point where the water has been in contact with chlorine for 30 minutes is not static but dynamic. As a consequence, control of the chlorination process by monitoring devices which are in fixed locations is difficult. They reflect information that varies with time. This is particularly significant where chlorine is added directly into a pipeline supplying consumers and assumed flowrate variation can be in the order of 30:1.

'FIXED' 30 MINUTE SAMPLING POINT vs. A 30 MINUTE CONTACT POINT The term 'fixed' 30 min. sampling point, (also known as Detention Point), refers to a sampling location on the supply main, where at design flowrates, the time taken to reach this point from the dosage point is intended to be 30 minutes. It is important to realise, that unless the flowrate is the same as the design flowrate there will be either more or less than 30 min. contact and, if the flowrate is varying this will change the contact time again. If a residual is measured at the fixed 30 min. sampling point, it should not be assumed that this value will represent the actual 30 min. contact value. The actual flowrate must equal the design flowrate, or, the water being treated must be of premium quality, having no demand for chlorine. In practice however, this situation is rarely found. Coagulated and filtered water still can have a significant chlorine demand. One cannot expect consistent residual values at a fixed location, where chlorine demand is severe or changeable, (ie. water quality is poor or variable), and where flowrates vary. Some control methods practised can often lead to the incorrect assumption that a consistent residual level at the disinfection plant will provide a consistent level at the fixed sampling point. The only time where this is correct is where both flowrates and water quality are constant.

Derek Braden is Senior Technical Officer Water Quality, S.E. Region Melbourne Water and has 19 years experience in the water & wastewater industry, including II years in chlorination and process instrumentation, and three years as Chlorination Officer with the Rural Water Commission.

MANUAL CONTROL The 'Manual' control method, is where the chlorine feedrate is set manually. This is best suited to a fixed known water flowrate. Taking for example a pipeline, the location of a 30 minute sample point can be precisely made. However, if dosing is applied at the inlet to a tank from which water is drawn at variable rates, the 30 minute contact location may be dispersed somewhere within the tank. Depending upon the residence time within the tank, mixing, and the rate of chlorine 'decay', free residuals monitored at the outlet of the tank will be variable. If both the 'd~cay' and residence time is great, free chlorine residuals may be reduced to zero at the tank outlet, even though there may have been 0.2 to 0.5 mg/ L present after 30 minutes of contact. The lack of free residual at ~his location however, is often interpreted as 'under' chlorination and increases are made to the chlorine feedrate. This will cause higher than desired levels to occur during shorter residence time periods, brought about by the incidence of higher flowrates.

RESIDUAL CONTROL The 'Residual' control method is one not used very widely due to the total reliance on continuous accuracy of monitoring equipment, and its ability to perform consistently without breakdown or faults occurring. This method will only be successful where both flowrates and water quality are reasonably constant, or vary at a rate within the capacity of the equipment to adjust, ¡a situation not often found in town water supply systems.

COMBINED CONTROL The combination of Flowpacing control with Residual control is what is referred to as, 'Combined' control. The 'Combined' control method has long been considered an appropriate form of control where the purpose of maintaining a desired value of free chlorine residual at the point of treatment is required. This method has a proven capability of achieving control of a value within an acceptable tolerance. In practice, if the chlorine 'dosage' is recorded, it can be seen that much higher doses than normal are required if the flowrate is low. This is due to the increased 'decay' of chlorine residual as sample return time (or 'Process Time') lengthens on its way to the analyser equipment. On analysis of this sample, the dose of chlorine increases in order to maintain the desired level, thereby compensating for the 'decay'. In reality, over-correction occurs despite the consistency of a residual level shown on plant monitoring equipment. The quality of the water will obviously have a bearing on the degree of dosage increase or decrease, however, even the cleanest of water will cause decay to residual to some degree.

WATER October 1992

37


In Fig.I , plant disinfec tion dosage levels are seen to rise as the flowrate falls. 'Combined Control' maintains a residual returning to plant monitoring equipment at a.desi red level of 2.0 mg/ L. ~ his has been achieved by increasing the dosage as the process time lengthens. Note that the dosage more than doubles during the low flowrate period . When comparing this res ult with Fig.2, which shows the 'Process time' over the same range of flows, one can see the period for chlorine to decay increases, from 4 minutes at 5.5 ML/ day, to 18 minutes at 0.7 ML/day. For Combined Control, this is necessary since the increased process time allows greater chlorine decay to occur, and, if left to dose at the same value, the residual would fall away from the desired level as shown in Fig.3. The process time can be anything up to and beyond 30 minutes. Typically, the desired residual level at the plant generally referred to as the 'Set-Point' is higher than 0.2 to 0.5 mg/ L (2.0 in the example shown in Fig.1),and will result in overdosing water at low flowrates even though the residual chart res ult shows a consistent level being maintained at the disinfection plant. Low water flowrates with high chlorine do sag<;! predominantly occur overnight. With earl y morning increases in wate r demand , consumers will receive this 'overnight' water during early to midmorning periods. Conseq uentl y, complaints may arise as a res ult. Typical consumer comments suggest that the water has a metallic or bitter taste, often with the presence of strong chlorine odours, a nd improves late r in the day.

----

ML/D (or) mg/L

10 ~ - -- - -- - - - - - - - - - - - - , FLOW ML/D RESID'L

DOSAGE

-a-

Q LLaLLLL~~LLLL~~~LLLL~~LLLL~.U.ULLLL~~

7 8 HOUR (p.m.)

Fig. l -

Combined control method showing do sage required to maintain a desired residual at the plant (site: Pakenham)

FLOW (ML/DAY)

6 ,-'...-_:___ _ _ _ _ _ _ _ _ _ _ __

1

PROCESS TI ME

FWW PACING CO~TROL The flow pacing control method sets out to dose water at a predetermined rate proportional to flow. This rate does not alter unless by manual intervention. This process is heavily relia nt on a high degree of accuracy from the flowm eter, and assumes little if any variation in water quality. Where water quality variation does occur using this method , free chlorine residuals in samples from a fixed location will vary unpredictably. Normally, however, these variations in residuals are as a result of different decay periods determined by the flowra te entering the system . Most flowmeters, whilst they show a high degree of accuracy at 'mid-range' flowrates, are not always capable of providing accurate flow signals over the range of demands from urban consumers, and particularly at the lower end of the scale, as may occur overnight. It is important that flowm eters used for fl owpacing chlorinators are scaled to accommodate the anticipated low flows. Figure 3 illustrates that the free chlorine residual level monitored at the disinfection plant reflects a trend similar to that of the flow. This shows the 'decay' of chlorine as process time increases, since the dosage rem ains the same throughout the treatment period . As stated above, water quality will determine to what extent the decay of residual will be reduced. There can be situations where the residual measures only 0.2 to 0.5 mg/ L, but has taken 30 minutes to return to the monitoring equipment. This suggests that the water has in fact been adequately disinfected (according to the stated objective) despite the low value shown . Even so, this low value is often incorrectly interpreted to be a res ult of 'under chlorination', and dosage levels are unnecessarily increased as a result.

SUMMARY OF CURRENT CHWRINE CONTROL MECHANISMS Physicochemical water quality has a significant impact on the ability to consistently achieve free chlorine residuals of 0.2 to 0.5 mg / Lat the 30 minute contact point, and in situations where water quality is poor, higher levels than 0. 5 mg/ Lor longer contact periods may be necessary to achieve compliance (WHO, 1984). It is clear that interpretation of chlorine residuals ta ken at designated fixed locations must take into consideration effe cts of sample transport time to monitoring equipment, and rates of chlorine 'decay'. Where this information is used for feedback control of chlorinators, care must be taken not to cause an unneeessary increase in chlorine dosage in order to overcome variations in chlorine 'decay' that are due to variations in sampling time, if it can be shown that a sufficient residual level existed at the desired 30 minutes. This may require the monitoring analyser to sample from a treated source of water which has a 'fixed' residence time totalling 30 minu tes. A desired level could then be compared with this res ult before being used as control feedback information . This modification could be incorporated into existing 'Combined Control' systems, using a small separate holding ta nk fill ed with a sample and retained for a set 30 minutes from the time it was dosed. An appropriate controlling device would also be necessary.

PROPOSED METHOD -

MINUTES

F'ig. 2 ML /D 3.5 or ,-::__ mg/L _

Flowrate vs. process time for (fig. I)

_ __ _ _ _ _ _ _ _ _ _ _ _- ,

h -+-- - - - - - - - - - - - - - - ,

19

20

21

22

23

24

1

2

-FLOW

R~

3

TIME (Hr.)

Fig. 3 -

38

Flowpaced co ntrol at fixed dosage (site: Garfield)

WATER October 1992

'30 MINUTE TRIM'

This method is designed to establish a constant 30 minute contact time before residual analysis. The result is then used as control fee dback in fo rmation . The measured residual is compared to a desired value or 'Set-point' (typically 0.2 to 0.5 mg/L). The difference between the set-point and measured value is then used to 'trim' the dose of chlorine. The method provides precisely the ri ght amount of chlorine to be applied, to produce a consistent residua l level afte~ a 30 minute period wherever this point may be in the supply mam. Trials of the method were conducted at Garfield, Victoria. The trials commenced with manual adjustments to the dosage. Results obtained were sufficientl y encouraging to pursue an automatic method . The resul ts are given in Fig.4, and show a consistent free chlorine residual of 0.3 to 0.5 mg/ L over the trial duration. Bacteriological samples were not taken on this occasion, the intention being focused on ac hieving a consistent residual res ult. Fig. 5 is a schematic diagra m showing a proposed automated method. Water being treated flows in the pipeline from left to right at a flowrate which determines the time (t ) variable, to cover the distance between the injection point of chlorine, and the sample off-


--

ML Dor mg/L 5 ------------------,

~-r-,o-w-~ PLANT RESIDUAL

-&DOSAGE

--a---

30 MIN RESULT

---A-

20

21

22

23

24

I

2

3

4

TIME (Hr.)

Fig. 4 -

'30 minute trim' method, dosage adjusted manually at - 30 min. intervals (site: Garfield)

-

Flow T • Preparation Tank. A • Ch lorine Analyser. O • Chlorine Dispenser . C • Controller. f' • floYJ11eter

tl • Time between injection point and sample off-take point . t2 • Time between sample off-take point and preparation tank ,

Fig. 5 -

Schematic of proposed method

take point. A sample of treated water is returned via a sample return pipeline, (t) constant, to a chlorine residual analyser (A). In the sample return pipeline, a tee junction allows sample water to enter a small PVC tank (T), when two in-line valves permit. The valve operation is controlled by a programable electronic controller (C), opening after a total detention time (t 1 + t2 +T) has reached the desired 30 minute criteria. When the time period has been reached, supply to the analyser is provided by the tank. Residual is measured and compared to a target value known as the 'Set-point', whereby discrepancies are applied to adjust the chlorine dispenser dosage (D). Valves actuation at tank (T) is governed by the controller (C), and will vary as time (t 1) changes. The flowrate of treated water is measured by the flowmeter (F), and provides the basis for the timing required . A series of bacteriological water samples taken from tank (T) after 30 min. cycles, would establish optimum 'Set-point' control levels to be maintained. Regular samples taken would indicate the possible need to modify the level should there be a water quality change. However, the cleanliness of the tank used for this purpose is paramount and must be maintained . During periods where water is being held in tan.k (T), sample water is allowed to flow through to the analyser normally, but has no influence on chlorinator control. Control during these periods is 'Flow pacing' using a fixed dosage (kg/ ML) as trimmed when the 'Set-point' was last reviewed, ie: period between trimming of dosage is 30 minutes. High and low residual alarms can be provided by the controller, and can initiate operator attention as necessary. A default control process will automatically overide the normal operation and adopt preset values in the event of flowmeter failure. Recording of analyser residuals will indicate levels maintained after the 30 minute contact periods, as well as peak residuals reached at the treatment plant. The difference reflects 'true' water quality variations. Other controller provisions could include a direct readout of time (in minutes), for water being treated at the current flowrate to arrive at a predetermined fixed pipeline location, such as the Detention Point. In add ition, the controller could automatically initiate 'Residual Control' on any failure of the flowmeter, as well as providing continuous monitoring of plant dosage level to a recording device. (For the trials described above, a Fischer and Porter 53MC2000 programmable microprocessor was used).

CONCLUSIONS Disinfection efficiency will always be impeded where chlorine is used in waters of inferior physicochemical quality. Disinfection

40

WATER October 1992

efficiency is greatly dependent upon opti~izing such treatment that will provide direct contact between the disinfectant and any pathogens in the water. The processes of chemical coagulation, sedimentation and filtration are best suited to ach ieve this aim . Decay of chlorine residual in waters which are not so treated make control of the disinfectant difficult. The extent and rate of decay and its effect on reducing the disinfectant should not be underestimated. If the effect of chlorine decay in sample-water returning to control equipment is not given adequate consideration, inappropriate dosage may occur. Both underdosing and overdosing are to be avoided, the latter because of increased formation of potentially dangerous byproducts, as well as customer complaints. Current established methods do not anticipate adequately the significance of chlorine decay in sample transmission lines and give rise to over- and under-dosing of chlorine particularly during low flow conditions. Current process control methods are inadequate in providing feedback control information based on the result of sample water being analysed after treatment with chlorine. The Combined Control method does not highlight excessive dosage excursions that take place during certain flow and water quality conditions. This leads to incorrect conclusions when interpreting microbiological results and some customer complaints. With the application of current computer technology by way of programmable micro processors, the ability to customize the control process is available. A method which controls the dosage such that a predetermined free chlorine residual value will exist after 30 minutes should be adopted to provide reliable and consistent disinfection.

REFERENCES W.H.O. (1984) 'Guidelines for drin king-water quali ty' Vol.I Recommendations. (also) 'Gu idelines for Drinking Water Quality in Australia' (1987) Section 3.3 p. 7 (N.H.& M.R.C. ).

D. PARIS AND A. JELL Continued from page 28 Table 2 Basis of Design

NH4-N CN SC N Phenols

ss

BOD 5 COD pH Visually C lear

Planl Results (4 week period)

Req ' mcnt

Inf lu ent Compo nent

Performance Data

Effluent

mg/ L

mg/ L

330 93 184 333 71 610 2200 8. 5

25 0. 5 0.5 0.1 50 25 50 6- 9

M in mg/ L

Mas mg/ L

mg/ L

I < 0.1 < 0.1 0.014 3 9 7.9

20 < 0.1 < 0.15 0.09 JO 3 44 8.3

8.3 < 0.1 0.10 0.047 6 2 25 8. 1

Yes

Yes

Yes

I

Average

wastewaters. This plant demonstrates the wide range of industrial wastewaters to which biological treatment can now be successfully applied and the high standards achievable. Coke Ovens By-Products wastewater ranks with petrochemical and refinery wastewater as some of the most difficult to biological! treat. This plant highlights the advances the industry has achieved in utilising and en hancing biological processes.

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Water Journal October 1992  

Water Journal October 1992