Water Journal August 1989

AUGUST 1989

water

ISSN 0310-0367

Vol. 16, No. 3, August, 1989.

Official Journal AUSTRALIAN WATER AND WASTEWATER ASSOCIATION

CONTENTS My Point of View . ........ . ... .. ....... .

Association News

5

President's Report . . . . . . . . . ....... .

It Seems to Me . . . . . . . . . . . . . . . . . ... . . . . .

6 6 11

IAWPRC News . . . . . .. .. . . .. .. . . . .. .. . . . .

HAZWASTE Update ....... .. .. . . .. .

Watercomp '89 . . . . . . . .......... .

Sludge Disposal in Depth . . .. . .. . .. . .

Australian Water Technology Exhibition ... .

Effect of Controls on Water Consumption in

12

Book Reviews . .. .... . ... .. .. . ........ .

A Pilot Plant Study of the Trickling Filter Solids Contact Process at Richmond, NSW Mitchell Laginestra . .

36 38 43 43

Chichester Dam .. . . . ... . ... .

Calendar

14 16 17

Newcastle Dr M. N. Viswanathan . ........ . .

18

A New Way of Decanting Intermittent Extended Aeration Plants R. Siebert aRd R. G. Shaw ......... .

22

Enhanced Biological Phosphorous Removal P A. Cooksey and R. Cheng ... . ... . ..... .

Increasing the Capacity of Wastewater Mains by Polymer Injection F. Cozma and H. Awad . . . . . .. . . ... . . . .

Cyanobacteria Seminar

24

OUR COVER Chichester Dam in the Hunter Valley near Dungog, a main water supply source for Newcastle and the Lower Hunter, is one of the older dams in New South Wales, built between 1918 and 1926.

Story page 43.

30 35

Photograph and colour separations courtesy of the Hunter Wate r Board .

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New South Wales Mrs S. Tonki n-Hill, Sinclair Knight & Part .

1 Chandos St. , St. leonards, 2065 (02) 436 7166

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PRODUCTION EDITOR

P Abbey,

J. Grainger,

PO. Box 37283 Winnellie, NT 5789 (089) 89 7290

Applta, 191 Royal Parade, Parkville 3052 (03) 347 2377 Fax (03) 348 1206

WATER August, 1989 I

EFFECT OF CONTROLS ON WATER CONSUMPTION ¡1N NEWCASTLE

by Dr M. N. VISWANATHAN

ABSTRACT Effects of 'water restrictions ' and 'user pays water charging method' on water consumption in the Newcastle region are examined. Long term trends for consumption are established using a polynomial regression curve. The residuals obtained are corrected for rainfall effects and the effects of an ARIMA (moving average) model effects. The annual savings in water consumption due to water restrictions during the 1966-67 drought were about 140Jo and during the 1981-82 drought were about 15-20%. The water consumption dropped by about 20-29% after the introduction of 'user pays water charging method' in 1982.

Dr M. N. Viswanathan is an engineer with the Water Investigation and Planning Section of the Hunter Water Board, Newcastle West, New South Wales.

M. N. Viswanathan

INTRODUCTION Control of water demand, or demand management as 1t 1s generally called, is used by water authorities during periods of drought, when it is required to conserve dwindling water storages.

Demand management is also used when source amplification has to be postponed due to economic constraints. 1\vo of the controls generally used by water authorities are restrictions of water usage, like banning of water in gardens etc, and water pricing. To assess the effectiveness of these controls on water consumption Nvule and Maidment(1985), and Viswanathan(1981) calculated the water savings by estimating the difference between model predictions and actual water consumption during the restriction or control period.

Others (Hanke and Mehrez 1979, Anderson et al 1980, Morgan and Pelosi 1980, Morgan 1982) have employed a technique similar to the intervention analysis in which binary dummy variables representing the conservation period are included in multiple regression models of daily or monthly water consumption. Shaw and Maidment(1987) assessed the impact of restrictions by using a transfer function noise model of daily water use. Impact of price on residential water demand was studied by Howe and Lineweaver(l967) and Howe(1982) by using linear regression models who concluded that domestic demands (internal) are relatively inelastic with respect to price but domestic demands (external) are elastic with respect to price.

The present study examines the effect of restrictions and price on water consumption in Newcastle region. Newcastle region was subjected to water restrictions during the years 1966-67 and 1981-82.

Since 1982 the water charging method has been changed. Prior to 1982, cost of water for domestic use was independent of usage but after 1982, cost of water was a function of usage.

Newcastle region has a population of about 398 000 (1988 estimates) and is served by Hunter Water Board for water and sewerage requirements. The industrial and commercial component of water usage is about 35-40% of total water usage. Water restrictions during the years 1966-67 and 1980-82 included banning of fixed sprinklers, restricting garden watering to certain hours of the day etc. Details of these restrictions are given in Viswanathan(1981) . The user pays method of water charging included two part tariffs. Tariff I, which is a fixed component of the total charge, refer to the land value of the property and Tariff . II refers to actual water usage. The cost per kilolitre of water used during the period 1982-88 is shown in Table 1.

Previous studies (Bruvold and Smith, 1988), in estimating effects of controls on consumption, included developing a model for uncont~olled usage period and using the model so developed during the penod of control to estimate the consumption. The difference between the estimated and actual consumption gives the effect of controls on consumption. The major drawback of such an approach is that the assumption that the model remains unchanged over a period of time is not valid.

In the present analysis, the effect of controls on water demand is determined by estimating a long term polynomial trend of water demand. The residuals, which are the differences between the trend and the actual consumption, were further corrected for the effects of rainfall and A RIMA (Auto Regressive Ii;itegrated Moving Average) model.

P OLYNOMIAL TREND Figure 1 shows the annual water consflmption for the Newcastle region from 1942 to 1988 June 30. The drop in consumption during the years 1966-67 and 1981-82 was due to restrictions imposed on water usage, and the drop since 1983 was due to the 'user pays water charging method'.

A second degree polynomial curve was fitted for the period 1942-80, but the abnormal years 1966-67 were treated as missing observations.

300

250

200

;;;0

::i

150

,a C

.2

ci.

E

~

8

100

50

Table I Price of water (Residential) Cents per kilolitre Water only connected

Water and Sewer connected

83

40

84

45

85

50 55 60 65

60 69 75

Year ending

30 June

86 87

88 18

WATER August, 1989

45

55

60

65

70

75

80

year ending 30 June

82 90 100

so

Fig. I -

Actual and estimated consumption

85

90

The regression equation is of the form: a + b ( x+8) + c ( x+8 )'

y

(1)

where, annual water consumption ML/ Day year (last two digits only) a,b,c = coefficients (Residual Mean Square = 49.98 Multiple R Square = 0.991) The values of coefficients are given in Tobie 2. y x

FITTING OF AN ARIMA MODEL Figure 9 is a plot of autocorrelations of residuals Y2 • These plots suggest (Box and Jenkins, 1976) that an ARIMA model of the form (1, 0, 1) could be fitted to the residuals Y2 . The assumed model for Y2 is of the form:

Table 2 Value of Coefficients coefficients

standard

T value

error

8.75 - 0.84 0.04

a b C

2.3 1 0.12 0.0013

Figure 3 shows the effect of rainfall on :J,Yater demand on an annual basis. The maximum variation in annual consumption that could be explained by rainfall is about ±4ML/day. Figure 4 shows the residuals (Y 1) corrected for rainfall variation. Let the corrected residuals be denoted as Y 2 . Figure 5 shows the residual Y2 as 0/o variations of estimated water consumption.

Y2(t) Y2(t-J)

The regression curve is extended for the period 1981-88 . Figure also shows the difference (residuals) between the estimated consumption and actual consumption. Let the residuals be denoted as Y,.

a (t) a(t - 1) <I>

RAINFALL CORRECTION

e

Weather, and particularly rainfall, exerts considerable influence on domestic water demand (external use). Weeks and McMahon (1974) concluded that the number of rain days per annum is more significant in influencing water demand than total rainfall for the year. However, a study by Viswanathan (1981) showed that the level of actual rainfall is a significant parameter in influencing water demand . For example a heavy rainfall on a particular day will influence the domestic external water usage for several subsequent days. Consequently, in the present analysis, to determine the effect of rainfall on water demand, total annual rainfall instead of total rain days is used . Figure 2 shows annual rainfalls since 1942 for the Newscastle region.

Correlation between the residuals (Y,) obtained in the previous section and annual rainfall is estimated.

The correlation equation is of the form: Y1

where,

=

a1

+

e 1 a(t -

<I> 1Y2 (t - I) -

3.78 -7.00 30.90

residuals obtained from the removal of polynomial trend (ML/Day) r annual rainfall Metres/ Year values of the coefficients are: aJ 7 .23 bJ - 6.33 Standard error 5.23 T value - 1.21 Multiple R' 0.038

+

a(t)

(3)

residual at time 't' residual at time 't - l' independent random error with constant variance at time 't' independent random error with constant variance at time 't - 1' auto regressive parameter moving average parameter

3 -t-- + - - - + - - - - - f - Cr- - t - - - t - - - t -- - t - - l f l - - - t - -1

,l·rl . --,~ 1-------~- L ~~

"'

0

aE iii "' 8

:-~-4-f ~-~---4---Ad--~--,~-. ,_·

_,

.

-->-

-

V ,

"' 0

,I

l·-l'+ - 1 -1

~

· 2 -·····-·-.. -· ·----· ·-· ---·-···-· ··· - . · - - · .. - +---t---t--a111,- + - -

,

(2)

b 1 (r)

=

1)

·3 +--

Y1

.4

'

1---l->--t----<l--i -t---+--+--+--+--l

~- - - 1 - -- t - --i---t--1,1---1--,--1---t--+--+-----i

·5

40

45

50

55

60

65

70

75

80

85

90

year end ing 30 Jun e

Fig. 3 -

Rainfall effect on annual average consumption (ML/ Day)

40 , - - , - - - , - - - , - - - , - - - , - - - , - - , - - , - - , - - - ,

1.9 - ............. , , _ _,___, _ _ _ _, _ _ _ _, __

JO 4 - - ¼ - - - - - <

1.7 ···-·-··-- ···------···· -------·-- ···---- --·· ..... · -·--·1---t----+---t----1

'

~

-;::

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C

R

:J ::;

: --~;-~--Li}A-~~~-==

. -t~- ,---- , 0 .7 +-- + - -J- - f - -V,/_

'

40

45

50

55

-\-

_, _ ~ , _ _ - t - - - + - - -llt--- - t - -1

- 4 Q - 1 - - - 1 -t - - t - - -l - + - - - - t - - - + - - - tl~- · 50 4 - -- - - + - - - + - - - + - - - + - - - + - - - + - - - +~

----o-70

65

70

75

80

85

90

~- - + - - + - - -!- -+--

+-- + - - + - - -!- -

40

65

Annual rainfall (Metres/ annum)

45

50

55

60

70

75

80

85

90

yea r ending 30 J une

yea r endi ng 30 Jun e

Fig. 2 -

dillerence diff. withra inlallcorrec .

-80 · - · - - · - · · -90

60

i- 1 - _ . , _

Fig. 4 -

Differences in estimated consumption (actual-estimated) WATER August, 1989

19

Parameters <1> 1 and 0 1 are estimated using conditional least squares method. Estimates of the parameters are given in Table 3.

Figure 6 shows the effect of ARIMA (1, 0, 1) on annual water consumption. Maximum variation in consumption due to ARIMA model is limited to + 5 ML/day. Figure 7 shows the residuals (Y 3) after correcting for rainfall and assumed ARIMA model. Figure 8 shows the OJo differences after correcting for rainfall and ARIMA effects. An examination of Figure 10 suggests that the residuals appear to be random in nature suggesting that no further improvement can be made with the analysis of the residuals .

Table 4 is the summary of the results obtained with rainfall and ARIMA model corrections of the residuals. It appears that rainfall and ARIMA model have not affected the water consumption to any great extent.

The OJo savings in annual consumptions during 1966-67 drought period was about 140Jo . The OJo savings during 1981-82 drought period, due to water restrictions amounted to about 15-200Jo. The savings due to 'user pays water charging method' resulted in savings of about 20-290Jo. One major difference between restrictions during 1966-67 and user pays charging method after 1982 is that, for 1966-67, the drop

Table 3

Parameter estimates of ARIMA (1, 0, 1) model estimates - 0.0591 - 0.2580

4>1 01

Residual sum of squares Residual mean square

Year

standard error 0.8235 0.8022

=

3243.98 90.11

Table 4 Effect of controls on water consumption Nature Actual Estimated No Rainfall of cons. cons. correct. correct. control m/ day m/ day diff

1966 1967 1981 1982 1983 1984 1985 1986 1987 1988

T ratio - 0,07 - 0.32

149 143 201 221 203 194 208 220 23 1 213

restrict. restrict. restrict. restrict. user pays user pays user pays user pays

user pays user pays

167 172 250 257 263 270 276 283 289 296

- 18.0 -29.0 - 49 .0 -36.0 -60.0 -76.0 -68.0 -63.0 - 58 .0 - 83 .0

"lodiff - 10.8 - 16. 8 - 19.6 - 14.0 - 22.8 - 28.1 -24.6 -22.2 - 20. 1 -28. 0

diff

Rainfall +ARIMA correct

%diff -

- 18 .6 - 27.9 - 51.1 - 37 .2 - 59.9 -74.6 - 68. 1 -64.4 - 60.2 - 80.5

11.1 16.2 20.4 14.5 22.8 27.6 24.7 22.7 20.8 27.2

diff -

23.6 23.2 51.2 37.3 58.7 78.5 68.2 65.0 60.3 79.8

%diff - 14.1 - 13 .5 - 20.5 - 14 .5 - 22 .3 - 29.1 - 24.7 - 23 .0 - 20.9 -27. 0

40

20

30 15 20

.

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i

---

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-70

-20 ···---·-- ·----·· - - - · · < - -- t - - - + - - - + - - 1 - - i

·80

--e--

·25

% dilference % dill. with rainran correc.

-.-

______,. _..,__.._ ----

··----···

t-

-,v--ti-

--·

· - - - - - - - ' ·-wilhout correction with rainfall and arima co rreclion

-

I b

f ·90 · 100

40

45

50

55

60

65

70

75

80

85

40

90

45

50

55

year ending 30 June

Fig. 5 -

60

65

70

75

80

85

90

year ending 30 June

"lo Differences [(Difference/Estimated) x 100)

6 ~ - - - - - - - - - - ~ - - - - - --

Fig. 7 -

20~-~------..----r----,-----,---,--,----,

---~

5 +----+---+----ll---+----+~·--+----+---+--1-----1 3 +---+----+--11--+---ll'

Differences in consumption (with rainfall and ARIMA correction)

·------- ------~-+---1

15 - ....

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-15 ··------· --·······--- ·-

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------0--

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--+--

----1--1---+--+--- - \ --t---t---+----i·--

·25

·6 40

45

50

55

60

65

70

75

80

85

40

90

20

Effect of ARIMA model on consumption

WATER August, /989

45

50

55

60

65

70

75

80

85

90

year endi ng 30 June

year ending 30 June

Fig. 6 -

without correction with rain laU and arima correction

Fig. 8 -

0/o Differences in consumption (with rainfall and ARIMA correction)

in consumption disappeared once the restrictions were removed, whereas after 1982 under user pays system, the drop in consumption appears to be sustained at least until 1988.

• Once the restrictions were removed aft&r the 1966-67 drought the consumptions returned to the predicted trend line. Under user pays charging system, the drop in water consumption appears to be sustained at least until 1988.

CONCLUSIONS Effect of 'water restrictions' and 'user pays water charging method' on water consumption in Newcastle region was estimated using a polynomial regression trend curve.

• Effect of rainfall on annual water consumption was limited to ±4 ML/day .

• An ARIMA model of the structure (1, 0, 1) was used to explain further variation in water consumption and this was found to be limited to ± 5 ML/day.

• The savings in water consumption during 1966-67 drought were about 140Jo. The savings in water consumption during 1981 -82 drought were about 15-20% . The savings in water consumption due to user pays water charging method were about 20-29%.

1 . 0 . . . . - - - - - - - - - - - - - -- - - - - - - - - ,

ACKNOWLEDGEMENTS The author thanks URWAA and the Hunter Water Board for the funds and facilities to undertake this project.

Continued on page 29.

1.0....-----------------------,

0.8-1- - - - - -----------------1

PLOT OF AUTOCORRELATIONS o.5 -l- - - - -- - - -- - -- - - - - - -- - -1

0.6-l- - - - - - -- - - -- - - - -- - - - - - 1

0.6- 1 - - - - - - - - - - - - - - - - - - - - - - - - t

0.4 -t------------------------1

0.4

PLOT OF AUTOCORRELATIONS

~----------------------!

0.2

0.0 ·0.2

·0.4 + - - - - - - - - - - - - - - - - - - - - - -0.6 _, ________ _ _ _ _ _ _ _ _ _ _ _ _ __

·0.8-1- - - - - -- - - - - - - - - - - - - - - 2

3

4

5

6

7

8

9

10

11

12 , 13

14

·0.4 + - - - - - - - - - - - - - - - - - - - - - - - t ·0.6

~---------------'--------1

·0.8 - 1 - - - - - - - - - - - - - -- -- - - - - - - - t

15

2

3

4

6

6

7

LAG

8

9

10

11

12

13

14

15

LAG

1.0....-----------------------,

1.0 . . . . - - - - - - - - - - - - - - - - - - - - - - - ,

0.8 + - - - - -

0.8-1- - -

PLOT OF PARTIAL AUTOCORRELATIONS

0.6 - 1 - - - - - - - - - - - - - - - - - - - - - - l 0.4

-<-----------------------<

PLOT OF PARTIAL AUTOCORRELATIONS

0.6 0.4-1- -- - - - - - - -- - - - - - -- - - - - - t 0.2 -1- - - - - - - - - - - - - - -- - - - - - - 1

0 .2

0.0 ·0.2

·0.2

-0.4

-1- - - - - - - - -- - - - - - - - - - - - - 1

·0.4 +-- - - -- - - - - - - - - - - - - - - - ~

-0 .6 -l- - - - - -- - - - -- - - - - - -- - - - 1

·0 .6

+----------------------1

-0.8

·0.8

2

3

4

5

6

7

8

9

10

11

12

13

14

15

+-- - - -- - - - - - - - - - - - - - - - - - i

2

3

Plot of autocorrelations of residuals Y2

5

6

7

8

9

10

11

12

13

14

15

LAG

LAG

Fig. 9 -

4

Fig. 10 -

Plot of autocorrelation of residuals Y3 WATER August, 1989

21

A NEW WAY OF DECANTING INTERMITTENT EXTENDED AERATION PLANTS

by R. Siebert and R. G. Shaw ABSTRACT A gas-locked syphon decanting system has been operating satisfactorily in a 4000 EP intermittent extended aeration plant at Perisher Valley for the past two years and has now been retrofitted to an older plant. The paper describes this new decanting technique in some detail.

INTRODUCTION Because of their rugged simplicity (IEA) intermittent extended aeration plants are widely used for treatment of sewage and industrial waste water for equivalent populations in the range 100 EP to 15 000 EP. A major cost component of these plants is the lowering weir arrangement used to remove the settled treated effluent from the aeration tank .

Aeration & Allied Technology have been developing a fixed submerged weir system which is considerably cheaper to build and operate and is a replacement for the lowering weir. After some years of experience in applying this technology to package plants in the size range 100 to 1200 EP they successfully tendered for the decant mechanism for Perisher No 2 4000 EP IEA plant using their "Gas Locked Syphon Decanter" system. This paper describes this system.

DEVEWPMENTAL WGIC The Public Works Department (PWD) of New South Wales largely pioneered IEA treatment in Australia and initially used Pasveer Ditches ie shallow racetrack configuration aeration tanks with a tank depth at bottom water level (BWL) of 1.5 metres and a maximum level change between top water level (TWL) and BWL of 400 mm. To reduce construction costs and land requirements these developed into simpler rectangular concrete box tanks 3 metres deep to BWL which were dubbed Bathurst boxes. More recently PWD have been using concrete-lined rectangular earth dams of a tank depth at BWL of 3.0 metres and a maximum level change of 600 mm.

Aeration & Allied Technology (AAT) started installing sewage and industrial effluent treatment plants seven years ago using IEA technology. They adopted a circular shallow aeration tank design 1.5 metres deep at BWL with a maximum level change of 500 mm.

These tanks being self stressing reduced the amount of concrete and reinforcing required for structural stability. Heney they were cheaper to build than Bathurst boxes.

With a view to becoming still more competitive in the market place they questioned the necessity of having a lowering weir when the level change in these tanks was small and chose a submerged weir fixed at BWL. Because it is difficult and costly to engineer long lowering weirs so that they remain level, PWD have tended to adopt multiple short weirs of up to 9 metres in length per unit with high weir loadings by conventional activated sludge standards.

Many PWD plants have now been installed with maximum weir loading of 21 litres/ sec/ metre of weir. This is 5-10 times the weir loadings used on conventional activated sludge secondary clarifiers. To compensate for this high weir loading greater depths of clarified effluent are required between the weir and the settling sludge blanket to prevent sludge washouts. In many cases it is not possible to start decanting effluent until the sludge blanket is well below BWL. AAT's approach has been to use weirs fixed at BWL with very conservative weir loadings so that clarified effluent can be decanted without drawing up sludge even when the sludge blanket is only 200 mm below the fixed weirs.

Conventional submerged weirs have the disadvantage that activated sludge will tend to enter and settle in them during the aeration period. A slug of sludge will come out at the start of each decant. To prevent this AAT use air trapped in the weir headers by a gas lock in the effluent line. This trapped air acts as a stop 22

WATER August, 1989

Robert Siebert graduated from RMC Duntroon with a Bachelor or Engineering degree and was employed in various postings within the Defence Force including Construction Officer in the PNG Defence Force. In 1985 he joined National Parks and Wildlife as District Engineer for Kosciusko National Park and amongst other things is responsible for operation and maintenance of municipal services in the ski resorts of New South Wales.

Robert Shaw is Technical Director of Aeration & Allied Technology Pty Ltd, a company specialising in biological waste water treatment. He graduated in Chemical Engineering at Birmingham University in the UK in 1962. He began his involvement in wastewater treatment in 1972 heading up CIG's development of oxygen applications in wastewater treatment in Australia. He formed Aeration & Allied Technology in 1982 in partnership with Ray Anderson.

R. Siebert

R. G. Shaw

valve preventing the water flow until it is vented. Submerged weirs do have the advantage that they do not have to be exactly level because it is the velocity head variation that controls the flow from each aperture, not the static head vari'!ition.

AAT have installed 80 plants using this type of decanter in the range 50-1200 equivalent population. The first large scale decanter using this principle was installed at National Parks and Wildlife No. 2 4000 EP Box at the ski-resort, Perisher Valley, in 1987. This decanter has been operating satisfactorily for two years and a small amount of operating data from this installation is included in this paper. The original Perisher No. 1 plant consists of 4 x 1000 EP 600

DECANT START

500

,.

10

20

30

40

TIME - MINUTES Fig. 1 -

Typical Decants.

50

60

Boxes and this also has been converted from lowering weirs to gas locked syphon decanters, during April 1989. Fig. 2 is a photograph of the installation, Fig. 3 a photograph of one of the decanters be.ing retro-fitted to Perisher No 1 plant.

flows backwards out of the nozzles. This gi.'es them a beneficial back flushing to remove any rags, plastic etc that may have accumulated during the decant.

DETAILED DESCRIPTION OF GAS WCKED SYPHON DECANTERS

Obviously the flow rate of water is proportional to the square root of the driving head which varies during the decant from 800 mm at TWL to 300 mm at BWL on larger installations. The flow rate at the end of the decant would be only 60% of the flow rate at the beginning of the decant. To reduce this variation in flowrates the solenoid valves are left open from TWL until the water level reaches the obvert of the nozzle arms (BWL + 200 mm). The water column is then no longer continuous and air is sucked into the vent and entrained down the u-tube. The flow rate is reduced in this manner for the first half of the decant to produce a flow which is more even through the decant and which varies less with the amount to be decanted. Further development work in the flow control area is still in progress.

FWW CONTROL

Referring to Figure 1: Each port (or flow nozzle) in the submerged weir consists of a short length of vertical or inclined pipe. Sized restrictions (not less than 50mm diameter) to balance the flow from each flow nozzle are incorporated in these vertical pipes. The short pipes are connected together at their upper ends by horizontal sub headers (nozzle arms) which in turn are connected to horizontal main headers. The main headers are connected to the down leg of a u-tube which is filled with clarified effluent. The up-leg of the u-tube is connected to the plant effluent outlet. A vent line is connected to the high point of headers to vent trapped air to initiate the decant. The vent line is shut off with solenoid operated valves.

The operation will be described starting from BWL after a decant. With the vent line shut sewage inflow raises the tank water level but trapped air prevents the water rising into the nozzle arms and headers. The horizontal cross section area of the down-leg of the u-tube is less than the combined area of the flow nozzles. As the trapped air is pressurised by the rising water in the tank, the water level is pushed down the u-tube much further than the water level rises in the flow nozzles. The difference in levels in the u-tube balances the water pressure above the flow nozzles. When the water level is above the obvert of the nozzle arms and at the correct time in the IEA cycle the trapped air can be vented using the solenoid valves and this allows the water to flow up the flow nozzles into the nozzle arms then via the headers through the u-tube and out of the plant. The solenoid valves are closed prior to the falling water level reaching the obvert of the nozzle arms and the water continues to syphon out of the plant to a point 50 mm above the bottom of the flow nozzles. At this point the solenoid valves are opened again to break the syphon and stop the water flow out of the plant.

In this way the bottom 50 mm of the flow nozzles are used as a scum bar. When the syphon breaks the water in the nozzle arms

Fig. 2 - 4000 EP Decanters at Perisher No. 2 Plant.

FWW PATTERNS AND MODELS Initially AAT designs for gas locked syphon decanters assumed a flow pattern in which water approached the nozzle from below evenly from all angles. Planes of equal potential were hemispherical around and below each nozzle. To prevent sludge washouts it was believed that the upflow vertically below the nozzle should be less than the settling velocity of the sludge. With the 1 litre per second nozzle flow adopted by AAT this occurred when there was 500 mm of clear supernatant between the flow nozzles and the sludge blanket.

Subsequent study has shown this assumed flow pattern to be inaccurate. The settling sludge blanket is substantially denser than the supernatant and also has mild Bingham Plastic properties. This enables the blanket to resist the distorting effects of upflows and hence change the flow pattern to one where the flow approaches the nozzle equally from all directions rather than only from below.

PWD have been studying flow patterns around lowering weirs for a number of years and more recently have financed a research project at the University of New South Wales Water Research Laboratory at Manly Vale (Witheridge and Wilkinson, 1988). After studying many of the PWD's IEA plants, they recognised three possible modes of failure to decant solids-free effluent using lowering weirs. ' The first mode is associated with establishing the hydraulic gradient across the tank at the beginning of the decant. A slight lifting of the sludge blanket near the decanter is required to balance the hydraulic gradient of the flowing supernatant over the sludge blanket. This mode of failure can be avoided by starting the decant flow slowly so that the blanket gradient is also established slowly. In the case of gas locked syphon decanting this slow start can be achieved by venting the trapped air at a slow controlled rate.

The second and third modes of failure are associated with high water flow rates over the top of the settling sludge blanket. These failures can occur during early settlement due to weak bonding between the floe particles or later in settlement due to the floe bonding being broken by shear stress.

The model suggested for second and third mode failures is that the sludge blanket due to its high density and Bingham Plastic properties acts as a distortable or erodable membrane across the tank with the clear supernatant flow over that membrane. Distortion or erosion of the blanket will take place when the densimetric Froude number for the flowing supernatent exceeds a critical value of around 0.3.

The dimensionless densimetric Froude No. (F) is defined

F=

Fig. 3 -

One of four 1000 EP Decanters in Perisher_No. 1 Plant.

V

(dgd) ½ V = mean supernatant velocity over the where sludge blanket d = the difference in relative density of supernatant and sludge g = gravitational constant d = depth of supernatant AAT's installation at Perisher No 2 plant consists of two 80 nozzle decanters installed in 4000 EP Bathurst box 37 .5 metres long by 12.5 metres wide. BWL is 3.650 metres and TWL is 4.150 metres.

The flow nozzles are arranged at 600 mm centres in 16 rows ten WATER August, 1989

23

deep across the tank. The array covers 130Jo of the surface of the tank.

Many PWD IEA plants of 4000 EP capacity have a design peak decant rate of 252 litres per second (21 litres per second on 12 metres of weir) . This is equivalent to 8.5 times average dry weather flow on the original 2.67 hours cycle.

Using the densimetric Froude number approach one metre of supernatant is required above the settling sludge blanket at the beginning of the decant to prevent sludge washouts due to mode 2 and 3 type failures. Local upflows immediately in front of the lowering weir are of the order of 40 metres/ hour at the blanket surface.

Using AAT's 160 nozzle array at 252 litres/ sec. decant rate the nozzles are loaded to 1.575 litres/ sec. With the same sludge and inventory AAT nozzles would be positioned half way between the surface of the water and the sludge blanket at the beginning of the decant. The local upflows vertically under the flow nozzle at the blanket surface at this loading are 1.8 metres/ hour assuming the water flows equally from all directions.

This difference in local upflow demonstrates the advantage of the AAT multipoint withdrawal system.

INSTALLATION AND COMMISSIONING AAT were engaged as contractor in 1986 to supply and install the gas locked syphon decanters on Perisher No 2 plant and after its successful operation were again contracted in 1989 to similarly modify Perisher No 1 plant. In both cases the decanters were fabricated in Sydney, and then were transported to Perisher Valley in sections. These were bolted together on site. Installation took approximately one week using a three man team.

During the commissioning of the No. 2 plant decanters it was found that insufficient air was flowing into the decanters when the solenoid valves opened to stop the water flow at the end of the decant. This was due to the high level of air entrainment in the flow as it passed from the main headers into the u-tube and the resulting suction effect holding the solenoid valves closed. The problem was solved by increasing the throat area of solenoid valves letting air in and out of the decanters and making the solenoid valves directacting rather than pilot-operated. No such problems occurred on the subsequent installation on No. 1 plant.

KOSCIUSKO WATER STUDY PROCEEDS The two-year study to survey the water quality of the lakes and streams in the Kosciusko area is now underway.

The joint study, co-ordinated by the State Pollution Control Commission, involves the regular monitoring of the area's waterways with the aim of producing a report by the end of 1991 on the quality of the water and the possible impact of future development on water quality.

The Snowy River Shire Council is responsible for monitoring water quality in the Snowy River, Mowambah (Moonbah) River, Wollondibby Creek, Little Thredbo and the Thredbo rivers.

Sampling will be carried out every month on the first three rivers and every two months on the Little Thredbo and Thredbo rivers.

The studies are scheduled to be completed by February 1991.

The Snowy Mountains Authority, which is working in another survey area in conjunction with the Department of Water Resources, has begun its water quality monitoring program.

The SMA will be monitoring the inflows to Lake Jindabyne and the outflows from the lake.

24

WATER A ugust, 1989

OPERATION AT PERISQ.ER The decanters on No. 2 plant have operated successfully at Perisher Valley producing consistent suspended solids results from a reasonable range of MLSS and SVI situations. The decanters on No. 1 plant have just been commissioned for this ski season.

The flow control method has been described earlier. The effectiveness of this control can be seen from Figure 2 which has been replotted from the plant level recorder.

For small and medium decants the flow rate is reasonably constant. A distinct slowing of the decant rate is often apparent on the plant level recorder when the solenoid valves are open at the beginning of the decant. This is due to air entrainment in the u-tube.

The flow rate can be seen to increase when the solenoid valves are shut prior to the syphoning part of the decant. Peak flow rates are usually within 200Jo of the mean decant rate. The decant rate does appear to increase significantly with larger decants indicating a possible need for more flow control.

The flow variations that exist do not appear to be detrimental to operation although peak flow rates do not appear to be approaching critical conditions as defined by the densimetric Froud No. (1).

The performance of the decanters is assessed by non-filterable residue content of effluent. Composite samples taken througl;l a decant period in 1987-88 from Perisher No. 2 gave results between 5 and 15 mg/ I but typically below 10 mg/ I. There appears to be no correlation between SVI and MLSS and hence sludge blanket levels and NFR in effluent. There is no indication that NFR level increased towards the end of the decant indicating a Mode 2 or 3 failure.

From this it can be assumed that the NFR of the effluent is not dependent on the location of the sludge blanket over the range studied. MLSS levels at or above design have been achieved with average SVI sludge. Operation of No. 1 plant decanter will be assessed during the 1989 ski season.

REFERENCES Witheridge, G. M. and Wilkinson, D. L. (January 1988). Hydraulic investigations into decanting from IEA wastewater v eatment plants.

Research Report No. 172. University of New South Wales Water Research Laboratory, Manly Vale, NSW, Australia.

NEI JOHN THOMPSON The Directors of Resources Conservation Company International (RCCI) of Seattle USA announce the appointment of NEI John Thompson (Australia) of Sydney NSW to exclusively represent them in Australia, New Zealand and selected South East Asian countries for the design and construction of Zero Liquid Discharge Plants and systems for: * Cooling tower blowdown * Process wastewater * Demineralizer regenerant waste * Ash pond/ scrubber blowdown * Cooling lake * Plant drain * Boiler blowdown * Reverse osmosis reject * Electrodialysis reject * Salty effluents * Softener waste

RCCI designed, constructed and in 1986 commissioned two 3.3 Ml/day Brine Concentrators at Bayswater Power Station for the Electricity Commission of NSW The units recover distilled water for boiler feed and cooling tower make-up from demineralisation plant regenerant wastes, cooling tower blowdown concentrate and ash system blowdown. The concentrated salts removed by the Brine Concentrator are disposed of on site.

ENHANCED BIOWGICAL PHOSPHORUS REMOVAL -Studies on a Long Sludge Age Activated Primary Tankby P.A. COOKSEY and R. CHENG ABSTRACT This report outlines pilot scale experimental work carried out by the Hunter Water Board on a long sludge age activated primary tank (LSAAPT) at Marmong Point, NSW. Preliminary results confirm CSIRO findings at Lower Plenty, in Melbourne, that such tanks when used in conjunction with suitable activated sludge configurations induce enhanced biological phosphorus removal (EBPR) in some sewages otherwise found unsuitable. Significant reduction in the volatile solids content of sludge retained in the tank has been identified, as has an apparent relationship between the solids content of the tank, orthophosphate release in a subsequent anaerobic tank, and the orthophosphate level of the final effluent from the pilot plant.

BACKGROUND Removal of nitrogen and phosphorus from sewage effluents is often required to minimise eutrophication of surface waters.

Biological removal of nitrogen is well understood, and is widely practised not only to minimise eutrophication but also because of secondary benefits with respect to oxygen consumption and alkalinity. The most common method of phosphorus removal has been chemical precipitation, but this has disadvantages in terms of operating cost, increased sludge production, and increased dissolved solids content of the effluent, so increasing attention is being given to treatment systems which remove large quantities of phosphorus without the addition of chemicals - the so-called enhanced biological phosphorus removal (EBPR) systems.

In addition to suitable plant configuration and operating conditions, successful implementation of EBPR is largely dependent on sewage characteristics, and in particular the ratio of nitrogen and phosphorus to readily assimilable COD (RACOD) . RACOD is that part of the COD which occurs in molecules small enough to be absorbed directly through the cell membranes of bacteria, and can conveniently be subdivided into two portions: (i) volatile fatty acids (VFAs), and (ii) other readily assimilable material, ie non-VFA RACOD. This latter portion is ill-defined, but is presumably made up of simple carbohydrates and substances resulting from protein breakdown. Both denitrification and EBPR itself are dependant on the availability of RACOD. Since ratios of nitrogen and phosphorus to RACOD in sewage are frequently unsuitable for EBPR, application of this system may be dependant on modification of influent characteristics. Little can be done to alter the ratio of nitrogen and phosphorus to COD, and hence modification is dependant on the conversion of slowly degradable (largely particulate) COD to RACOD. In South Africa where most of the development work on EBPR has occurred, modification systems are based on off-line fermenters and the activated primary tank proposed by Barnard (1984). This latter unit utilises accumulation and recycling of sludge within a primary sedimentation tank to increase the VFA level of the primary effluent.

In South Africa activated primary tanks are operated on a batch basis with sludge retention times of only two to three days, since longer retentions are said to result in the consumption of the VFAs by methane producing bacteria before they can be utilised for denitrification and phosphorus removal.

Despite predictions that long solids retentions in activated primary tanks would reuslt in failure of the process, a research group in Melbourne headed by Mr Bill Raper of the CSIRO extended the concept by operation of an activated primary tank on a continuous basis, with sludge retained in the tank for many days. The term long sludge age activated primary tank (LSAAPT) is used herein to distinguish this operating system from the one used in South Africa. CSIRO experience in operation of their pilot scale LSAAPT at Lower Plenty, in the suburbs of Melbourne, has shown the system capable (except in very wet weather) of reliable. removal of 26

WATER August, 1989

Both authors are engineers in the Wastewater Investigation and Planning Section of the Hunter Water Board. R Cheng has been directly responsible for day to day operation of the Marmong Point pilot plant, white P Cooksey has been responsible for technical direction and assessment.

Peter Cooksey, BE is a civil engineer who has specialised for several years in the technical aspects of wastewater transportation and treatment. In addition to his present involvement in nutrient removal studies, he is responsible for revision of the Board's sewage design and sewerage reticulation design standards.

Ray Cheng B E (Hons), Dip Comp Sci is a civil engineer with many years experience in the water and wastewater field and who has special interest in engineering computing.

P. A. Cooksey

R. Cheng

phosphorus to very low levels of effluent orthophosphate with a sewage which has otherwise been found incapable of producing more than normal biological removal. T,his plant has not exhibited any appreciable increase in VFAs in the sewage passed through it, but a significant increase in soluble COD has been observed (Bayly et al, 1989). The LSAAPT is currently the subject of a patent application by the CSIRO.

INITIATION OF STUDIES AT MARMONG POINT It was against this background that the Hunter Water Board (then the Hunter District Water Board) initiated pilot scale trials of a LSAAPT at Marmong Point because, like many sewerage Authorities, it has been coming under increasing pressure to remove phosphorus from sewage effluents. While such action is obviously necessary in some circumstances, and should then be carried out even at high cost, the Board perceived in the LSAAPT the possibility of a low cost option which could be applied even where the need for phosphorus removal had not been fully established.

Because the CSIRO group was being funded to examine the performance of various EBPR configurations under Australian conditions, it was limited in the amount of attention it could devote to it's LSAAPT at that time. Operation of a second unit by the Board offered advantages over additional funding (or the secondment of staff) to the CSIRO group in that it would prove the LSAAPT on a second (although similar) "domestic" sewage, would enable the Board to evaluate influent parameters for a sewage within its own area of operations, would provide experience in this field for several Board's employees, and would enable the Board to minimise "additional" costs (ie those other than budgeted salaries and wages). Marmong Point WWTW, although a biological filter works, was chosen as the site for the pilot plant as the influent is considered to be "typical" domestic, the plant is reasonably convenient to the Boards's head office and laboratories from which staff would operate, and it is in an area where new plants discharging to Lake Macquarie were at that time proposed, the effluent from such plants almost certain to be subjected to a total phosphorus limit of 1 mg P/ L.

It is significant that while the object of studies at Marmong Point was the elucidation of mechanisms operating within a LSAAPT it was decided that lack of understanding of these mechanism; necessitated the operation of an EBPR removal plant in series with the LSAAPT so as to identify those periods when the LSAAPT was inducing phosphorus removal. While this has considerably increased operating problems and costs it has allowed identification of the relatively short period during which the pilot plant has so far achieved its objective, and has thus allowed comparison of conditions within the LSAAPT at that time to conditions at other times.

For the pilot plant proper the Board had available a 4 m' Bardenpho unit. During previous use it had been found necessary to install a run-down type milliscreen ahead of this plant to minimse chokages in the small diameter pipework, and inadequacy of the clarifier had also been identified. Two additional sedimentation tanks were therfore constructed prior to commencement of the trials at Marmong Point, one to replace the existing pilot plant clarifier and the other to be used as a LSAAPT. Intermittent mixing of the primary tank was adopted for transfer of products from the sludge to the tank effluent rather than sludge recycle in order to avoid the problems experienced by the CSIRO in pumping the somewhat pulpy sludge developed in their LSAAPT. It now appears likely that such a procedure could also have significant advantages in the operation of full-scale units, particularly if intermittent use of the primary effluent is desired, eg for use in conjunction with intermittent aeration systems.

Sl.OOGERECYClE

l,IJXEOUOUORRET\.lm

"""'

CLAR• S.4m

1· Fig. 1 -

Pilot Plant Configuration.

Table 1 TANK DETAILS Volume

TANK

Detention (hrs)•

(L)

Primary Sed. Anaerobic Primary Anoxic Primary Aerobic Secondary Anoxic Secondary Aerobic Secondary Sed

476 345 1145 2175 800 345 476

1.4 4.8 9.1 3.3 1.4

*Based on inflow rate

.

Table 2 PLANT OPERATING PARAMETERS Normal FLOW RATES (L/min) Influent Mixed Liquor Recycle Sludge Return

Recent

4

4

16

16

2

2.5

SEWAGE STRENGTH (mg/ L) COD NFR TKN Ammonia Ortho P Total P TKN/ COD RACOD/ COD*

500 175 45 30 8 9 0.10 0.08

300 75 30 25 4 4 No data No data

OPERATING PARAMETERS MLSS F/ M (COD basis)

3500 0.20

3500 0.10

*Limited data only

Pllfil PLANT OPERATIONAL DETAILS The pilot plant in use at Marmong Point is shown diagrammatically in Figure 1, while important dimensions operating parameters, etc are given in Tables 1 and 2. Raw sewage: pumped from the influent area of the Marmong Point WWTW, 1s passed through the milliscreen and gravitated into the primary tank of the pilot plant at the required rate, excess flow being bypassed to waste. Inflow to the primary tank is maintained at a steady rate except when mixing of the solids is required, at which times the feed pump is switched off in order to minimise solids loss during mixing, As discussed later, a delay period is required between the termination of pumping and the commencement of mixing. It should be noted at this stage, however, that because of these problems the primary tank cannot be considered as having operated in the long sludge age mode until at least 1st September 1988, and hence most analysis of performance is restricted to data obtained after that date, although earlier monitoring is also considered for comparative purposes.

Effluent overflows from the primary tank and gravitates to the feed tank of the pilot plant proper before being pumped into the anaerobic tank at a steady rate. Inflow to the feed tank is interrupted intermittently while the primary tank is mixed, but this change in influent rate is accommodated by a change of level in the feed tank.

Inclusion of the feed tank also allows for by-pass due to overflow if flows from the primary tank should become excessive. For many months after commissioning, the feed tank was stirred continuously, but this operation is now carried out intermittently as it was felt that the stirrers might be introducing too much oxygen into the influent and hence adversely affecting performance of the anaerobic unit.

From the anaerobic tank flow passes successively (by gravity) through the primary anoxic, primary aerobic, secondary anoxic, secondary aerobic and secondary clarification tanks. Mixed liquor is returned from the primary aerobic to the primary anoxic tank by pumping, while return sludge is pumped from the clarifier to the anaerobic tank.

Close attention has been paid to D.O. control ever since commissioning to ensure both good nitrification and denitrification in the pilot plant, since the adverse effect of nitrate on EBPR is well known. The two aerobic tanks are aerated by separate air compressors, and the amount · of air entering the two tanks is regulated by bleeding off excess air. on levels in the aerobic tanks are recorded continuously, while operation of the compressor is controlled .automatically to maintain D.O. between preset limits.

Ammonia and nitrate are tested on site using HACH test kits, and the results of these tests are used in conjunction with the aeration patterns to adjust the D.O. settings, and thus the amount of air entering the two tanks, to improve either nitrification or denitrification. The aim is to match aeration with oxygen demand by achieving a smooth aeration pattern, while also minimising the amount of air passing into the two anoxic tanks. This technique of D.O. control has been found to be effective in achieving good nitrification and denitrification in the pilot plant.

To further ensure complete denitrification, floating styrofoam covers are placed in the anoxic tanks to prevent air transfer from the atmosphere. Baffle boxes are also installed in the inlets to the anoxic tanks to minimise backflow from the aerobic tanks.

OPERATING PROBLEMS As discussed in Section 5, obtaining accumulation of solids in the primary tank has been a major problem during these investigations. While weak influent due to extended rainfall has been a major factor, solids loss during intermittent mixing has also been critical, and certain other operational features have aggravated the situation as indicated below.

Solid Loss Due to Intermittent Mixing: Flow to the primary tank is not continuous due to the need for periodic mixing of that tank, so operation is controlled by timers which switch the feed pump off when the mixer is on. In August 1988, due to concerns that the solids concentration in the primary tank was not increasing, operation of the tank was examined in detail and it was discovered that influent remaining in the feed pipe was switched off. This caused overflow from the primary tank while the mixer was operating and hence significant loss of solids. A mixer delay timer was immediately installed, and as can be seen from Figure 2, solids then began to accumulate in the tank .

WATER August, 1989

27

MLSS %Vol 10000 1 0 0 - - - - - - - - - . . . - - ~ - - - - - - - - - ,

8000

W -- -- -- --- - - - --- - -,_

% Volatiles

Fully O'--_ _ _ _ _ _ _ _ ___.____ _ _MLSS __ _Mixed _ _ _____, 1 Jan 89

1 Sep88

Fig. 2 -

1 May 89

Primary Tank MLSS (mg/I) and "lo Volatiles.

Chokage of Feed Pump: The pilot plant is lcoated at an operational wastewater treatment works which is normally manned during working hours, 5 days per week. Staff is therefore only available during those house to check the pilot plant for problems such as pumps chokage, screen blockage, mechanical failure or electrical interruption. When something goes wrong after hours it can be a considerable time before the problem is discovered and rectified. Chokages of the feed pump have been common, and while the total feed time lost has probably not been great this would certainly have had some effect on accumulation of solids in the primary tank.

Run-down Screen: As with most pilot plant investigations, chokage of small diameter pipes and valves is very difficult to avoid.

As mentioned earlier, a run-down type milliscreen has been installed in order to minimise these problems. While satisfactory from this point of view, it has had the disadvantage of reducing the feed NFR concentration to the primary tank. Analysis of treatment work influent and screened raw sewage data indicates an NFR difference of about 150Jo, although the effect may be even greater than this, since observations on-site indicate solids build-up on the screen which would reduce screened raw sewage NFR even further (samples for analysis being taken after the screen has been cleaned).

solids are shown against time in Figure 2. Figure 3 shows orthophosphate levels in the primary effluent, the anaerobic tank and the plant effluent. Screened raw sewage orthophosphate levels are similar to, but slightly lower than those in the primary effluent, but the latter have been shown because more data points are available. From Figure 3 it can be seen that while orthophosphate levels in the primary effluent and the plant effluent have generally been similar, plant effluent levels suddenly dropped to levels of the order typically associated with EBPR early in February 1989. This occured durirtg a period of high solids levels in the primary tank, and following a period of several weeks during which orthophosphate release in the anaerobic tank gradually increased.

The apparent relationship between the primary tank solids level, the anaerobic tank orthophosphate level and the final effluent orthophsophate level over the next five weeks is quite striking, although further investigation is required before it can be confirmed that this relationship is significant.

Early in March 1989 the period of good phosphorus removal suddenly ended. It appears that this was probably due to a significant loss of solids from the primary tank which occurred over a period of about two weeks, but as there was a power failure for about 14 hours part way through the period of solids loss, the change in performance may have been due to a combination of factors. Since that time it has not been found possible to re-establish good phosphorus removal, nor has it been possible to achieve any significant increase in the solids level in the primary tank. While this latter feature may be the result of a decision to move tlie pump feeding the plant to a location where chokages would not be as frequent, it is more probably due to extremely wet weather which has resulted in unusually low raw sewage NFR levels. Figure 5 shows

60 Primary Effluent Aaerobic Tank

t

28

WATER August, 1989

I\

'

I

'',, ,,

Final Ettluent

,,

40

PRELIMINARY RESULTS Although the Marmong Point pilot plant was commissioned in September 1987, the problem of solids loss from the primary tank at the beginning of the mixing cycle was not eliminated until September 1988, and it cannot be considered that the system incorporated a LSAAPT until shortly before the end of 1988. Even since recognition of problems with the mixing cycle, accumulation of solids in the primary tank has been slow, partially due to the effect of the milliscreen, but mainly because of weak sewage resulting from an extended period of rainfall - flow through Marmong Point WWTW for the last six months has been approximately 300Jo higher than normal, while for most of March and April 1989 it has been about 600Jo higher than normal.

While it had always been suspected that solids levels in the primary tank were not as high as they should be, it was not until after solids actually began to accumulate that sufficient information became available to estimate the rate at which such accumulation could be expected. Even the existence of a similar LSAAPT in Melbourne was not of great assistance in this respect since that tank is never completely mixed, and hence the quantity of solids in it is indeterminate. Attention during the earlier months of operation was therefore centred on building up solids levels in the activated sludge part of the pilot plant, and on establishing efficient nitrification and denitrification, rather than on trying to determine why solids were not accumulating in the primary tank.

At about the time that the cause of solids loss from the primary tank was finally identified, regular monitoring of the solids level in the tank and periodic determination of the volatile proportion of those solids was initiated. As can be seen from Figure 2, the volatile proportion of the suspended solids in the primary tank when first monitored was about 800Jo, which is very close to the value that has been determined for screened raw sewage NFR at Marmong Point, thus confirming the suspicion that the tank was not operating as a LSAAPT at that time. Over the ensuing weeks, however, the percent volatiles has decreased steadily despite an increase in total solid so that at the time of writing slightly less than 600Jo of the suspended solids in the tank are volatile. Both the MLSS level (during mixing) and the volatiles proportion of these suspended

J

30

..J

,,,,

,,,,

50

,4,J:

20

/ V

': n

~

10 0

~-----------'-----'--=C---='-'-------'

1 Sep88

1 Jan 89

Fig. 3 -

1 May 89

Filtered Orthophosphate (mg P /L).

1----------.+----r+--------1

30

Primary Effluent

Screen~ Raw Sewage

1 Jan 89

Fig. 4 -

1 May 89

Sulfides (mg/L).

200

.,/""

150

1...------------

100

--J -----------

. 50

/

L.----'

---------

-100 0

50

100

150

200

250

300

350

Screened Raw Sewage (mg/L)

Fig. 5 -

Primary Sedimentation NFR Removal (mg/ L).

400

350

i

200 150 l-+--Hl-14l--'----'+-+-----+,i+-ij----Jl/--+,'----l,H-I--.I---H'-+-,r+ff,__-+---

--I

OL-_ _ _ _ _ _ _ _ _ _L __ _ _ _ _ _ _ _ ___, 1 Sep88

Fig. 6 -

1 Jan 89

1 May 89

Screened Raw Sewerage NFR (mg/ L).

the relationship between NFR removal and the screened raw sewage NFR level, determined by polynomial regression over more than 200 data pairs, while Figure 6 shows screened raw sewage NFR levels since 1st September 1988. As can be seen from these, recent raw sewage NFR levels would not be expected to lead to accumulation of solids in the primary tank.

One other feature of the work at Marmong Point merits special mention, and this is the data shown in Figure 4 regarding sulfide production in the primary tank. As can be seen from Figure 4, good phosphorus removal was achieved during a period in which sulfide levels in the primary effluent were high. This is contrary to some suggestions in the literature that the presence of sulfide interferes with EBPR. sulfide production in the LSAAPT may however have serious consequences in terms of odour, since although problems have not been observed in the pilot scale operation at Marmong Point, levels generated are such that problems could be anticipated in a full scale plant.

Covering of LSAAPTs may therefore be required in practice unless suitable mixing sequences can be determined which reduce sulfide generation while not having any adverse affect on phosphorus removal. Minimisation of sulfide production could in fact be beneficial to phosphorus removal since the organisms responsible for sulfide production presumably use some of the RACOD as substrate. Careful study of mixing cycles is in any case warranted once more data is available on REDOX potentials, etc, since cycle selection at this stage has been fairly arbitrary.

FUTURE WORK Progress at this stage is being hampered by the low influent NFR levels resulting from continued wet weather. The opportunity is being taken, therefore, to monitor some parameters across and within the primary tank which have previously been somewhat neglected due to lack of time. REDOX values and changes in RACOD level are foremost amongst these.

A second, larger primary tank is also being brought into service (although at present off-line from the activated sludge unit) by artificial feeding from the primary tank of the biological filter works. This should allow retention of greater volumes of sludge

W. N. VISWANATHAN

Continued from page 21.

REFERENCES Anderson , R. L., Miller, T. A. and Washburn, M. C . , (1980) Water savi_ngs from lawn watering restrictions during a drought year, Fort Collins, Colorado Water Resources Bulletin, 16(4):642-645 .

Box, G . E . and Jenkins, G . M. (1976) Time series analysis, Forecasting and Control. Holden Day, California.

Bruvold, W . H. and Smith, B. R. (1988) Developing and assessing a mo?el of residential water conservation, Water Resources Bulletm, 24(3):661-669.

Hanke, S. H. and Mehrez, A . (1979) The relation between water use restrictions and water use . Water Supply and Management, 3:315-321.

p:

without approaching concentrations at -.vhich washout has previously become a problem.

Analysis is also being carried out to determine the rate of volatile solids reduction in the LSAAPT, and hence the theoretical rate of RACOD production. This will be compared with monitored values when improved performance is again observed, and should also allow determination of whether such increases in RACOD are sufficient to explain the improved performance of the system as a whole.

Two other aspects which need to be investigated are modifications to the mixing cycle (alluded to ab9ve), and studies regarding properties of the sludges generated - particularly the degree of additional stabilisation required for satisfactory disposal.

CONCLUSIONS Although EBPR has only been achieved at Marmong Point for a short period at this stage, it is considered that such performance is confirmation of the results obtained by the CSIRO with a similar LSAAPT at Lower Plenty.

Operation of a LSAAPT with periodic mixing instead of sludge recycle is seen to have advantages in terms of simplicity of operation and ease of process control. It also enables a more thorough study of several important parameters than is otherwise possible.

Combining the results from these trials (with respect to volatile solids reductions, and the apparent influence of the quantity of solids in the LSAAPT) with the CSIRO data (with respect to increases in soluble COD and the absence of any significant increase in VFAs), leads the authors to the conclusion that the probable function of the LSAAPT is hydrolysis rather than prefermentation.

This could have major implications in terms of the products resulting from sludge retention in a LSAAPT, and in determining why such products are not consumed in that tank, but are available to subsequent stages of the process.

At this stage of development, and at least until parameters which would indicate that a LSAAPT is functioning properly have been identified, it is considered essential that such tanks be operated in conjunction with systems which can be used to define when EBPR is being achieved.

ACKNOWLEDGEMENT The authors would like to acknoi ledge the valuable advice provided by Mr Bill Raper of the CSIRO, Melbourne, without whose willing assistance, this project would not have been possible.

The diligent application to the project of Sharon Butler, Greg Short and the operators at Marmong Point Wastewater Treatment Works is likewise gratefully acknowledged.

REFERENCES BARNARD, J. L. (1984). Activated primary tanks for phosphate removal ,' Water SA, 10: 121-126.

BAYLY, R . C., DUNCAN, A., MAY, J. W., PILKING10N, N. H., RAPER, W. G . C. and VASILIADIS, G. E . (1989). The effect of primary fermentation on biological nutrient removal. AWWA 13th Federal Convention, Canberra, 6-10 March, 162-166.

Howe, C. W . and Linaweaver F. P. (1967) The impact of price on residential water demand and its relation to system design and price structure, Water Resources Research , 3(1): 13-32.

Howe, C. W. (1982) The impact of price on residential water demand: Some new insights. Water Resources Research, 18(4):713-716.

Morgan , W. D. and Pelosi, P . (1980) The effects of water conservation kits on water use. J. AWWA, 72(3):131 - 133 .

Morgan, W. D. (1982) Water conservation kits : A time series analysis for a conservation policy, Water Resources Bulletin, 18(6) : 1039-1042.

Nvule , D. N. and Maidment D. R. (1985) Water use forecasting and evaluating the effects of conservation measures. In : Computer application in water resources . Proc. Spec. Conf., Water Resources and Planning and Management Div., ASCE, Buffalo, New York. 401 -411.

Shaw, D. T. and Maidment D . R . (1987) Intervention analysis of water restrictions, Austin, Texas . Water Resources Bulletin, 23(6):1027-1046.

Viswanathan, M . N . (1981) Determination of effectiveness of water consumption restrictions: Methodology and Application. ME Thesis, University of Newcastle, 77p .

Weeks, C. R. and McMahon T . A . (1974) Urban water use in Australia , Civil engineering transactions, The Institution of Engineers Australia.

58-66 .

WATER August, 1989

29

INCREASING THE CAPACITY OF WASTEWATER MAINS BY POLYMER INJECTION

by F. COZMA and H. AWAD

ABSTRACT It may be economically possible in some circumstances to increase the capacity of a sewer by adding certain polymers to reduce the friction coefficient of the fluid flowing.

The literature relating to this effect was reviewed. In the laboratory it has been possible to nearly double the flowrate of water. Overseas experiments with sewage have reported flow increases of the order of _200Jo. Three field trials were operated at Gosford and Port Macquarie, NSW, exploring the effect of polymer concentrations from 1 to 33 mg/ L, in 100 mm and 200 mm diameter rising mains carrying domestic sewage. Significant increases in velocity were observed, from 25% up to 800Jo. Modifications to the polymer injection technique may yield even better results. Some limited experiments in gravity mains showed similar increases in flow, but a transient pressure wave occurred as the faster-flowing dosed sewage impacted on the previous flow. This could lead to surcharging.

The economics of polymer injection have to be determined for individual sites by in-situ testing. Injection may be useful for short term flow increases such as holiday loadings, or where site considerations make immediate augmentation impracticable.

Fred Cozma, is a Graduate Chemical Engineer involved in a number of research investigations being undertaken by Trade Wastes and Research Section.

Hamman Awad, is the Inspecting Engineer, Trade Wastes and Research.

The two authors are officers in Wastewater Engineering Branch, Public Works Department, NSW.

INTRODUCTION The traditional method of upgrading sewer rising mains usually involves construction of additional mains and therefore considerable capital works expenditure.

A possible alternative to capital expenditure is to dose the wastewater transportation system with friction-reducing additives.

Such a method could be applicable to mains subjected to short term hydraulic overloads either on a daily or seasonal basis, or where site (or budget) considerations make immediate augmentation impracticable.

A survey of the literature indicated that substantial work has been undertaken under laboratory conditions for rough and smooth pipes carrying water but published work on the dosing of sewers is very limited.

To further the knowledge in this area the NSW Public Works Department carried out a series of field trials in gravity and rising mains using a polyacrylamide powder polymer.

LITERATURE REVIEW Background

Considerable information is available on the subject of friction or drag reduction in pipes by the addition of polymers. Most of the detailed information has become available in the last 15-20 years as interest in friction reduction is developing due presumably to rising energy and material upgrading costs. (Toms, B.A. 1948; Virk, P.S. et al 1967; Basilevitch, V.A., Shabrin, A.N. 1972; Virk, 1971) Work in the area of theoretical models has also been done. These models account for drag reduction as a result of the polymer macromolecules inhibiting secondary instabilities and initiation of turbulence. This results from the large increases in elongational viscosity produced by the extended polymer molecules being orientated in the mean flow direction. These models generally aid in understanding the mechanism of drag reduction but are still a long way from providing predictions for engineering applications.

In the first instance investigations involved laboratory tests with relatively small pipe sizes using, in most cases, water as fluid. Only recently have tests in sewerage systems been conducted, however these generally have been limited in nature (Sellin R. et al 1978, 1980, 1982).

30

WATER August, 1989

Effect of Polymer Type

It is considered that to be effective, drag-reducing polymers must have the following characteristic: (Sellin, 1982).

• Very high molecular weight (betw1Jen 0.5-5 million).

• Linear polymer or copolymer links with no side branches.

• Little or no ionic charge.

• Solubility in the fluid.

• Limited degradation in use.

One of the best drag reducing polymers found yet is polyethyleneoxide (PEO) . However very high molecular weight PEO polymers are easily damaged; the higher the molecular weight, the easier damage occurs. A smaller polymer, while being more robust, generally requires a higher dose to achieve the same drag reduction.

It has been shown that polyacrylamides (PAMs) are more robust than PEO, but require a higher dosage for equivalent drag reduction.

Tests have shown that polymer degradation occurs when high shear stresses are present, eg high velocities in small diameter pipes, abrupt changes in pipe diameter, turbulent entry of sidestreams, intensive pumping, particularly in centrifugal pumps.

Degradation will not pose a serious problem as long as the solution is not exposed to prolonged pumping and provided that mean flow velocities are less than about 2 mis (Sellin et al 1982).

Diminution in efficiency due to adsorption onto the pipe wall, suspended solids or ionic particles, is afso a factor to be considered.

Laboratory Results Due to practical limitations, laboratory investigations have been performed in relatively smooth pipes with sizes ranging from 3 mm to 50 mm diameter; water has been used as the fluid in all reported works.

Sellin's data, when plotted on the conventional friction factor diagram, reveal the existence of five general zones. This is shown in Figure 1. The five zones represent the effect of increasing Reynolds number on the friction coefficient for a given pipe size and type. The description of the zones are:Zone A: Laminar flow region: the fluid obeys Poiseuilles law and there is no drag reduction observed when polymer is introduced.

I

-

ONSET OF OllAG REOUCTI

_.

-. .._ --:::- UNOOS

FLUID

120

''

0

POLYMERIC ZCM

-

0

1

= <t

V>

MAXIMUM ORACi

RHlJCTION ASYMPlOlf

ZONE 8

ZO'lE (

5

....,_', ~

'=

80

0

ZONE 0 0 0

10

JO

REYNOLDS

UMBER

IL(Xj SCALEI !AFTER SELLIN 19821

x

70

10 3

0

0

Figure 1

•

40

0

p

Zone B: Laminar to turbulent transition: no apparent difference when polymer is introduced; no delay of transition zone.

Zone C: Turbulent flow without drag reduction: drag reduction onset has not occurred. No drag reduction is observed at any polymer concentration. The fluid obeys the same friction factor relationship as the Prandtl-Karman Jaw for smooth pipes and Nikuradse's sand modification for rough pipes.

Zone D: Turbulent flow with drag reduction:(a) the onset of drag reduction (Point A) occurs at a well defined wall shear stress which is dependent on the polymer - fluid system but is independent of polymer concentration. (b) following onset, drag reduction is induced by a (as yet unknown) complex function of flow, polymer type and pipe roughness. This zone is called the polymeric zone.

Zone E: Turbulent flow with drag reduction independent of polymeric parameters: in both smooth and rough pipes the miximum drag reduction is limited by an asymptote which is independent of polymeric parameters. This line is known as the maximum drag reduction asymptote (MDRA). Generally the optimum polymer concentration for maximum drag reduction increases with increasing pipe diameter and pipe roughness. The existence of the MDRA has been demonstrated in smooth pipes up to 50 mm diameter for a range of polymer types, but not so clearly in larger diameter pipes.

Fig. 1 (Zones D and E) show that for a given pipe characteristic with a constant polymer dose as the velocity increases, friction reduction increase.

For gravity sewers and rising mains the area of most interest lies in zones D and E since in most cases sewage velocities and pipe diameters seldom give Reynolds numbers within Zones A, B or C.

•

00

0

•0

• • •

•

a

a

a

•

0

•

0

0 40

20

80

100

200

400

POLYMER CONCENTRATION , mg/ L. !LOG SCALEI (AFTER SELLIN 19821

Figure 2

One of the most interesting findings was a pressure peak for some minutes after commencement of dosing, and a pressure drop after cessation of dosing. Figure 3 shows the rise and fall of the free surface in a gravity sewer consequent on int(,rmittent dosing. This is probably due to the transient conditions established when the faster flow of polymer-dosed sewage impacts onto the preceding flow (and vice-versa on cessation).

Rising Mains

Only one documented case was found for polymer injection into a sewage rising main (Sellin, 1980). The main, at Bath, UK, was 8 km long and 760mm in diameter. A high molecular weight copolymer emulsion was injected after the main centrifugal pumps by means of a mono pump. (These pumps are ideal for injection of polymers since low shear stresses are incurred.) Using a dosage of 10-12 mg/ L, a friction reduction of the order of 200Jo was attained.

POLYMER

0 010

WSR·l01

Gravity Sewers

Sellin's investigations are summarised in Figure 2. For dry weather sewage flows, velocity increases of up to 1200Jo (ie, more than double) could be attained with 60-100 mg/ L dose of PEO powder, although most results showed Jess than 80% increase. In these tests, PAM polymer was less effective than PEO. However, the polymers were added as freshly-mixed suspensions in water. Although PEO dissolves fairly rapidly, PAM solutions may take up to an hour to reach full effectiveness. Since the test section was on five minutes downstream from the dosing point, different results may have been obtained with an improved dosing system.

Results from actual storm events gave lower velocity increases than were expected, especially at the higher polymer concentrations.

They were typically 15-400Jo when using powder polymer at 10-70 mg/ L dosage. Preparations of polymer emulsions gave lower increases. No explanation was advanced by Sellin for the reduced performance during storm surcharge conditions. A possible reason may well be that at the higher velocities, polymer dissolution time was reduced even further.

NE

0 01 5

;

<t

1

;

0 010

X

3

~

0 oo s

10

10

JO

,o

10

60

70

TIME !MIN I !AFTER SELLIN 19811

Figure 3 WATER August, 1989

31

One finding in this test was the "onset" point appeared to be at a much higher Reynolds Number than for the laboratory results described in Figure 1. Sellin was of the opinion that the "onset"· value increases with pipe diameter, and that with very large pipes there may be problems in achieving onset conditions. In a practical application, this would mean that a high drag reduction may not be attainable. Sellin also observed that friction reductions reached their final values only when the main was entirely full of the polymer-dosed sewage.

Summary of Literature Review

Reports of laboratory experiments indicate that friction may readily be reduced by up to 200Jo, and in favourable conditions, by up to 800Jo. However, these results were obtained on small diameter pipes.

Reports of experiments on actual sewers are limited, but significant friction reductions have been reported, despite reservations on the systems used for injection . The most effective polymer for drag-reduction seems to be high molecular weight polyethyleneoxide, (PEO) but this polymer degrades when subject to high shear stresses. Polyacrylamides (PAM) are more robust, but were found to be less effective at similar concentrations. This may have been due to the limited solution time available in the test procedures, since it is known that polymers, especially PAM , may require up to an hour to reach their maximum effectiveness.

Drag reduction in both smooth and rough pipes has been shown to occur in fully developed turbulent flow conditions only when an onset criterion is satisfied . This is believed to be a critical wall shear stress that must be satisfied. This value is dependent upon the polymer-solvent system under consideration but not on the polymer concentration. Little is known about the exact nature of the onset point and its requirements. However, there appears to be little problem in meeting the requirements.

The major parameters that have been identified as affecting the magnitude of drag reduction are Reynolds number (ie pipe diameter, fluid velocity, polymer type and molecular weight), polymer concentration, pipe roughness and injection technique. There is also a question about the effect of wet weather flows when compared to test results in dry weather flows. It is difficult to isolate one prrameter and study it individually as these parameters are very closely related, changing one invariably affects another. The outcome is that it may be more beneficial to limit study of these parameters to a set of selected values instead of a broad range as many investigators have attempted . This may eliminate interaction between parameters. Nevertheless, until predictive equations can be formulated, results can only be obtained from on-site tests .

FIELD TRIALS IN NSW The objective of these tests was primarily to determine in practical conditions whether a significant flow increase could be obtained in a sewage rising main by means of a polymer additive.

Rising Mains Rising Mains Gosford CS and CU

For the trials, a 2.50Jo solution of a PAM polymer (Polymer Powder 1011 , Allied Colloids Pty Ltd) was prepared and allowed to age for an hour, before being dosed into the feedwell of the pump station. Flows were monitored by cutting off the inflows, and measuring the rate of fall of the liquid surface in the feedwell.

At these test sites the pump operating head could not be measured due to the pressure tapping lines becoming blocked . The manufacturer's pump curves were used to estimate the operating pressure and this may have introduced some errors into the friction reduction calculation. With this reservation the friction reduction results from a limited number of tests are shown in Figure 4 and summarised in Table 2.

The friction reduction in these mains was considerable, between 55-71070, with velocity increases between 25-30%. The polymer doses for these increases were between 16-32 mgl L. Higher doses did not give higher velocity increases, in fact, the opposite occurred. It is possible that high polymer doses (more than about 45 mgl L) affected the performance of the pump itself.

These tests showed that polymer could be used to increase the velocity in rising mains, although these appeared to be an optimum concentration for the dosing configuration chosen .

The most interesting results to arise from these tests 'were: More than 750Jo of the rising main length had to be full of the polymer mixture before significant friction reduction occurred, and !OOOJo full for the ultimate value to be attained.

RM-Cll showed somewhat lower friction reduction than RMC8. It is possible that polymer degradation may have occurred as a result of the higher velocities (3.4 mi s, compared to 1.9

mi s) .

As the polymer dose was increased beyond 45 mgl L the velocity started to decrease. This was presumably due to the polymer affecting the pump's characteristics (This might be resolved by injecting the polymer into the pressure side of the pump) .

Rising Main 6, Port Macquarie

Similar trials were conducted at Port Macquarie. The details of this main are summarised in Table 3 and the same procedure was employed as at Gosford. The results are summarised in Table 4, and also shown in Figure 4.

The friction reduction in this main w~s 15-84% for a dosage range of 1-32 mgl L. It was interesting that a polymer dose of only 1-2 mgl L gave a 150Jo friction reduction. This appears to be consistent with information found in literature. The velocity increases measured ranged from 6 to 780Jo representing significant extra capacity.

It was found that high polymer concentrations (between 53-69 mgl L) changed the pumps performance characteristics. After the introduction of an instantaneous 69 mgl L polymer dose the delivery head and the flow rate decreased presumably caused by the impeller slipping against the fluid . The low friction reduction values at high polymer doses are consistent with similar results from Gosford .

The details of these two mains are summarised in Tables la and lb.

"''°

FRl( T

REOOC Tl().I ,•;.

Table la Gosford Rising Main Cl 1 Details Main

Pump

Length Diameter Static Head (av.)

Friction Head

154 m 100 mm 4.3 m

Type - Forrers 4S10/ 3 Impeller 229 Power 4.5 kW

D ~

0

STAT

6

80

70

PU~P STAT(t~ ( 8 Pll"P STATW(ll

60

---=

----------1

© so

8m

,o NOTE

SIZE OF BARS IN 01CM

RAMll

lO

Of EXP £RH-'E~.T/ll IH.SUUS

Table lb

-----

20

Gosford Rising Main CS Details 10

Main Length Diameter Static Head (Av.) Friction Head 32

WATER August, 1989

Pump 464 m 146 mm

8.6 m 8.5 m

Type - Forrers 4SI0/ 3 Impeller 250 mm Power 6.5 KW

10 POLYr-'ER (Qf;(£NTRATl)N liflglll

Figure 4

]0

-

Table 2: Port Macquarie Rising Main 6 Details Pump

Main Length Diameter Static Head (Av.) Friction Head

820 m 200 mm 7.7 m 7

Pump detail s not known

m

Gravity Sewers - Port Macquarie

Limited tests were performed on gravity sewers at Port Macquarie in dry weather flow conditions. A 2.50Jo solution of PAM was made up, allowed to age for an hour, then dosed into a manhole, and sewage heights monitored at manholes 70m and 480m downstream.

In such a system the friction reduction was difficult to calculate due to the small depth of water in the pipe and the normal variations in flow rate. However, the following observations were made.

Positive and negative disturbances, ie transient pressure peaks and reductions, accompanied the arrival and departure respectively of the dosed sewage at the downstream manholes.

The pressure peak appeared suddenly, and took only 1-2 minutes to .reach peak height. The depth of flow in the pipe increased significantly with part full pipe conditions. Consequently, in times . of storm flows, when the polymer dosing is most likely to be required, surcharging at shallow manholes may well take place. The negative disturbance took a longer time to dissipate. Typically it took 5 to 15 minutes to regain the normal level of flow.

CONCLUSIONS The results of this investigation confirm that high molecular weight polyacrylamide polymers can reduce friction in pipes of 100 and 200 mm diameter carrying wastewater. The magnitude of the friction reduction is similar to that reported in literature for smaller diameter pipes carrying water. A maximum friction reduction of 84% was measured with velocity increases of 78% attained without a change in either pump or pipe configuration or characteristics.

In rising mains there is an optimum polymer concentration with the dosing configuration used in these trials. This was in the vicinity of 30-32 mg/ L and definitely less than 45 mg/ L. Above the latter concentration the velocity increase (as measured) decreased due presumably to the pump impeller slipping against the fluid. The use of direct polymer injection after the pump may solve this problem; however, there might then be some polymer degradation due to turbulence at the sidestream entry. On the other hand, velocity increases for a specific polymer dose ·are dependent upon the rising main friction curve and its relationship with the pump characteristic curve and generally differ for each site.

The observations made on the gravity sewer trials indicate that it is not unreasonable to expect similar friction reduction values to those obtained in rising mains. Higher polymer concentrations could be used since there would be no pump to affect. However, the possibility of transient surcharging at downstream manholes must not be overlooked.

It should be noted in both cases that significant velocity increases are not achieved until the sewer in question is almost totally filled with the dosed sewage, so applications are limited to fairly short sewers, or bottlenecks in the system. The time delay necessary to prepare and "age" a polymer solution and ensure that it is distributed along the length of the sewer may also be a complication in emergency situations.

The mechanism of friction reduction is not understood, but could follow the hypothesis of conversion of the flow to near "laminar" flow conditions by polymer addition. This idea, however, has been

AWWA

3rd National Conference MANAGEMENT OF HAZARDOUS WATER Melbourne University 19-21 November, 1989.

Registrations are now being accepted.

Further details: Conference Secretariat, P.O. Box 29, Parkville Vic 3052 Telephone: (03) 387 3120.

34

WATER August, 1989

discounted in literature, but may be of value a~ a simple model to describe the effect.

The results in these trials have not provided sufficient information to formulate relationships between the parameters that are suspected of dominating friction reduction (ie pipe diameter, velocity, polymer type, injection method and the effect of wet weather flows). As predictions of friction reductions for other locations cannot be made, experiments must 'be undertaken to evaluate performance at sites where the use of polymers is under consideration.

In conclusion, it can be said that polymer additon is applicable to wastewater systems which are subject to short-term flow demands due to either wet weather or holiday loading. Addition of polymers may also be useful where site or budget considerations make augmentation difficult. The economics of using polymer injection to increase pipe capacity is most definitely site-dependent and must be determined for each possible application.

ACKNOWLEDGEMENTS The co-operation of Allied Colloids in providing some equipment and materials for these trials is gratefully acknowledged.

Table 3. Results from Gosford Rising Mains CS, Cll Polymer Dose* (mg/ L)

RM - Cl!

32 55

RM-C3

15-20 16-24

Velocity (m/ s)

Velocity Increase ('lo)

Red. ('lo)

Fricllon

3.3 2. 7 1.8 1.9

29. 7

55%

5.5

25.2 30.1

63 % 71%

• Range of polymer concentration in pipe.

.

Table 4. Results from Port Macquarie Rising Main 6 Polymer Dose (mg/ L)

0.8-1.3 1.5

6-8 6-8 22-25 32-33

Velocity (mi s)

Velocity t

Increase ('lo)

Friction Red. ( "lo)

6.2 9.1 34.9

14 15 56

45 .5 60

64

1.29 1.44 1.74 1.77 1.94 2.16

78

71 84

• Range of polymer concentration in pipe.

Reference velocity changed over testing period.

t

REFERENCES

BASILEVICH, V. A. and SHABRIN, A. N. (1972) "Use of Polymer Additives for Reducing Hydraulic Resistance in Pipes", Fluid Mech. Soviet Research 1, No. 5.

SELLIN, R. H . J., HOYT, J. W. and SCRIVENER, 0., (1982) "The Effect of Drag-Reducing Additives on Fluid Flows and Their Industrial Applications, Part I: Basic Aspects", J. Hyd. Res. 20, No. I, pp 2968.

SELLIN, R. H. J. et al (1982) "The Effect of Drag-Reducing Additives on Fluid Flows and Their· Industrial Applications, Part 2: Present Applications and Future Proposals", J. Hyd. Res. 20, No. 3, pp 235-292.

SELLIN, R. H . J. (1978) "Drag Reduction in Sewers: First Results from a Permanent Installation", J. Hyd. Res. 16, No. 4, pp 357-370 SELLIN, R. H. J. and OLLIS, M. (1980) "Polymer Drag Reduction in Large Pipes and Sewers: Results of Recent Field Trails", J. Reol., 24, 5, pp 667-684.

TOMS, B. A. (1948) "Some Observations on the Flow of Linear Polymer Solutions Through Straight Tubes at Large Reynolds Numbers", PROC. 1st Int. Rheological Con. (The Netherlands).

VIRK, P. S., MERRILL, E. W., MICKLEY, H . S. and SMITH, K. A. (1967) "The Toms Phenomenon: Turbulent Pipe Flow of Dilute Polymer Solutions", J. Fluid Mech. 30, Part 2, pp 305-328.

V!RK , P. S. (1971) "Drag Reduction in Rough Pipes", J. Fluid Mech. 45, Part 2, pp 225-246.

VIRK, P. S. (1971) "An Elastic Sublayer Model for Drag Reduction by Dilute Solutions of Linear Macromolecules", J. Fluid Mech. 45, Part 3, pp 417-440.

WAC

USE OF CONSTRUCTED WETLANDS IN WATER POLLUTION CONTROL Cambridge UK 24-28 September, 1990.

Call for Papers ...

Abstracts Required by 1st October, 1989.

Further details: T. Davies, Chisholm Institute of Technology.

Telephone : (03) 573 2168.

Cyanobacteria (Blue-Green Algae) ACWTWQR SEMINAR, ADELAIDE, APRIL 1989 A Report by Michael Burch A half-day seminar on the toxicity, ecology and significance of Cyanobacteria for water supply was convened as a joint meeting of the South Australian branch of the Australian Water and Wastewater Association and the Australian Centre for Water Treatment and Water Quality Research (ACWTWQR) on 5 April, 1989. The seminar was held to take advantage of a visit to Adelaide by Professor Ian Falconer of the University of New England, Armidale, an internationally recognised expert on toxic cyanobacteria, who is collaborating with the Centre on a UWRAA research project.

Over 100 local members and interstate visitors attended, reflecting the growing interest in this topic. Those attending had a variety of backgrounds, including water quality and water supply management, biology, microbiology and chemistry, public health, and health surveying, and water resources planning.

widely held theory is that certain nutrient conditions favour Cyanobacteria, in particular low ratios of supply of nitrogen to phosphorus (low TN:TP ratio). Sufficient data is now available to show that this is not a reliable predictor of the incidence or dominance of Cyanobacteria in the phytoplankton.

A second belief is that Cyanobacteria only develop under conditions of thermal stratification in deep lakes. Cyanobacteria can in fact proliferate under both stratified and non-stratified conditions, but a pre-condition that allows them to develop massive bloom populations in deep reservoirs is ongoing stability in a water column. Dr Ganf then discussed the effect of intermittent circulation (artificial or natural) and how this could be used as a factor to control the growth of Cyanobacteria.

TOXICITY

Mr Colin Heath, Senior Biologist from the Water Board, Sydney, gave an account of the practical management of cyanobacterial blooms in the Nepean-Hawkesbury River. Mr Heath described the monitoring program used to predict early development of blooms in the river and the action taken at the North Richmond Water Treatment Works to control odour problems from Anabaena blooms in particular. The North Richmond works is a modern dissolved air flotation (DAF) plant with recently installed granular activated carbon (GAC) filter beds. It can successfully overcome any potential odour and/ or toxicity problems from Cyanobacteria. Some novel ideas of using booms and surface spray jets to keep floating Anabaena away from the plant intake were described .

Mr Heath's critical dissection of the genesis of the algal problem at Richmond promoted considerable discussion of the economics of control measures. Options to control eutrophication are either 'front-end' nutrient control at the source or 'back-end' engineering solutions such as a sophisticated treatment works. The discussion that followed indicated that both solutions had their merits and role, and with both measures in ,place, emphasis on nutrient source control may considerably reduce operating costs in a water treatment plant.

The two remaining speakers were from the State Water Laboratory and were involved in the UWRAA Toxic Algae Project. In complementary talks they outlined their investigations into the significance of toxic cyanobacteria to water supply. Mr Michael Burch, Biologist, State Water Laboratory described the incidence of toxic Cyanobacteria in South Australia. Field studies have shown that there may be a link between visible morphological changes in Microcystis colonies and toxicity. These are simple observations to make and may be useful for water supply biologists to predict when a bloom should be monitored more closely or even treated with an algicide.

Ms Dianne Flett, a chemist with the Australian Centre for Water Treatment and Water Quality Research spoke on the development of High Performance Liquid Chromatographic (HPLC) techniques for detection of Microcystis toxins. Currently detection and measurement is by mouse bioassay - a test requiring relatively large samples and which is only available in specialised veterinary laboratories. The HPLC techniques being developed, although also rather specialised, show promise of having routine application, and are being used to investigate the mechanisms of toxin production by Cyanobacteria in field situations. Current deficiencies in knowledge for the water industry are firstly, clear identification of the toxic organisms and secondly, the extent of toxicity. Both these issues are being addressed with development of a sensitive analytical technique.

The meeting was concluded with an open discussion chaired by Dr Dennis Steffensen, Senior Biologist, Engineering and Water Supply Department.

Further information on studies of toxic Cyanobacteria at the ACWTWQR is available from Dr Dennis Steffensen (08) 259 0326 or Mr Michael Burch (08) 259 0336; State Water Laboratory, Private Bag, Salisbury 5108. Information on toxic Cyanobacteria from others in the Water Industry would be appreciated.

The seminar was opened by Tony Glatz, Vice-President, South Australian Branch. He pointed out that Cyanobacteria are recognized as one of the major biological-water quality problems in reservoirs subject to nutrient enrichment - no more so than in South Australia.

KEYNOfE SPEAKERS In his broad-ranging keynote address Professor Falconer outlined the historical perspective of toxicity associated with Cyanobacteria (Blue-Green Algae). The first record of algal toxicity was from South Australia in 1878 when George Francis described a 'Poisonous Australian Lake'. The lake was Lake Alexandrina at the Murray Mouth and the alga was Nodularia spumigena. The organism caused ''A conferva that is indigenous and confined to the lakes and has been produced in excessive quantities, so much so as to render the water unwholesome". (Nature, May 2, 1878).

Professor Falconer pointed out that the main toxic 'culprits' in Australian waters are the well known and widely distributed Microcystis aeruginosa, which produces a peptide, hepato- or liver toxin, and Anabaena cylindrica which produces the alkaloid, neurotoxin.

Both these organisms produce floating surface scums and taste and odour problems in water supplies. Both organisms have also been associated with stock deaths in rural farm dam situations, but Microcystis is likely to be more important in domestic water supply because of the association between human ingestion of Microcystis-laden water and gastro-enteritis and liver damage.

Professor Falconer's landmark studies in Armidale, NSW demonstrated this connection, and in describing this work he related some anecdotes about the trials and tribulations of conducting epidemiological studies - an interesting insight into methods used in a field unfamiliar to the majority of the audience.

Professor Falconer went on to describe their studies to identify th!! pathological effects of these compounds on laboratory animals. The chemical structure of the toxins has only recently been determined and the existence of slight variants in structure has implications for the detection methods being developed.

Toxin removal from water supply has been successfully achieved only with the use of activated carbon. Conventional alum floculation and sand filtration is ineffective. Provessor Falconer described his pilot plant studies on toxin removal using a variety of well-known brands and types (powdered and granular) of activated carbon.

ECOWGY The second speaker was Dr George Ganf of the University of Adelaide, a respected Australian expert in the area of ecology of phytoplankton, especially Cyanobacteria. Dr Ganf's talk covered the ecology of nuisance Cyanobacteria - what causes or allows them to grow and why they become a problem in water supply reservoirs and lakes.

Dr Ganf summarised the present state of knowledge and dispelled some commonly held assumptions about physical and chemical conditions which precede the dominance of Cyanobacteria. One

A WATER SUPPLY PERSPECTIVE

WATER August, /989

35

[~---"-'--"-'--"----B_O_O_;_K_R_E_Vl_E_W:_S_ _--'--'-~)

ALTERNATIVE WASTE TREATMENT SYSTEMS Elsevier Applied Science Publishers Ltd, UK. Ed: Rao Bhamidimarri 249 pp, 6 chapters, 22 separate papers ÂŁ34.

The book presents the Proceedings of the International Conference on Alternative Waste Treatment Systems held at Massey University, Palmerston North, New Zealand, May 26-27, 1988. The Conference was co-sponsored by the International Association on Water Pollution Research and Control, Massey University and the Department of Health, New Zealand.

The book is divided into six chapters with the following headings: (1) Septic Tank Systems (3 papers), (2) Biomass Production (4 papers), (3) Soil and Sub-Soil Treatment (3 papers), (4) Composting Technology (3 papers) (5) Anaerobic Systems (2 papers) and (6) Poster Papers (7 papers). Ten of the papers are from New Zealand, four from the USA, two each from Australia and Japan and one each from Norway, India, Singapore and Thailand.

The papers on septic tank systems in Chapter 1 commence with a review of the state-of-the-art of design, operation and maintenance practices. The second paper describes a segregated on-site system developed in the USA which removes phosphorus and nitrogen and comprises septic tanks, a sand filter and a drain field.

The chapter concludes with a review of alternative domestic systems suitable for the Mt Lofty Ranges in South Australia where septic tank systems are inappropriate.

Chapter 2 on biomass production contains papers on wetlands treatment of municipal secondary effluent, pasture irrigation of meat processing primary treated effluent, spray irrigation of pulp and paper mill treated effluent, and flood irrigation of forest soil.

The third chapter on soil and sub-soil treatment contains a mix of subjects. It deals firstly with sub-soil injection of agricultural liquid waste, then proceeds to design criteria for household wastewater infiitration systems. The last paper in the chapter deals with new technology for biological detoxification of hazardous waste involving the use of white rot fungi for treating organically contaminated water and soil.

Two of the papers in Chapter 4 deal with aerobic composting, one of primary meat waste solids from a slaughterhouse, the other of anaerobically digested municipal sewage sludge. The effects of applying the composting product on crop yield are presented in both cases. The third paper in Chapter 4 deals with vermicomposting (using worms) and mushroom cultivation.

Chapter 5 on anaerobic systems contains two papers. The first discusses treatment of poultry wastes using a two stage semi-batch anaerobic system with full recycle. The second paper discusses a municipal sewage sludge teatment system incorporating heat 36

WATER August, 1989

treatment to increase filterability of the sludge, dewatering using a filter press, and anaerobic treatment of the filtrate using a fixed bed reactor.

The poster papers discuss a variety of subjects, including composting of slaughterhouse waste solids, reuse of wastewater in cement manufacture, evapotranspiration of domestic wastewater, microbiological treatment of brewery waste by fungus cultivation, pH control for faecal coliform bacterial destruction, rotating biological contactors, and finally a review of alternative domestic wastewater treatment systems applicable to the environmentally sensitive and remote areas in New Zealand.

In general, the papers deal with technologies and concepts that have been known for some time, rather than with alternative or innovative ones. Nevertheless, through the presentation of several case studies, valuable data on design approaches and operational aspects are given. The book makes a worthwhile contribution to literature on approaches to waste management.

DR G. J. SEWARDS

RIVER ENGINEERING Part II STRUCTURES AND COASTAL DEFENCE WORKS Edited by Thomas W. Brandon Compiled and published by The Institution of Water and Environmental Management London, England. ÂŁ33 incl. surface mail to Australia.

This is the latest book from the Institution of Water and Environmental Management, the 8th, in their water resources engineering series . It complements volume 7; River Engineering Part I - Design Principles.

Volumes 7 and 8 together are best described as an encyclopedia of coastal and river engin(' ring . They fill the niche 0 between the purely theoretical text and the operations manual and as a result provide a good practical starting point for the student and inexperienced engineer.

In this eighth volume the opening chapters deal with marine parameters and the design of coastal defence structures. The discussion is wide ranging and topics covered include sea levels, recording gauges, waves, meteorology, currents, modelling, statistics, geology and coastal processes.

Also included is a brief discussion on present and future technical achievements and the pros and cons of various coastal management options. Some key formulae are provided which should enable the reader to grasp the fundamentals and hence narrow the choice of options when confronted with a particular problem. In a publication of a little over 300 pages and

covering such a wide subject it is not practical to cover the detailed design of the many structures but it does highlight the areas requiring investigation by experienced professionals.

However the greater part of the volume has been directed towards river engineering and a discussion of conservation issues relevant to both coastal and riverine environments.

In dealing with river structures nearly every known type of installation is mentioned with the list ranging from gauging stations and measuring devices to spillways, flood retarding basins, culverts, debris screens, navigation locks and fish passages.

Further chapters cover construction and maintenance aspects including health and safety issues and legal requirements as they apply in the UK. Reference is also made to quality assurance techniques and special considerations needed with respect to contract documentation.

In these days when there is an ever increasing awareness for conservation and the irreparable damage that can result from ill-advised stream stabilisation works some of the most valuable information is to be found in the chapters dealing with river bank protection, stream maintenance and conservation practices. Commonly used methods of protecting river banks and the relative merits of various materials used are widely discussed. Although aimed specifically for the U. K. where the use of willows, for example, is quite appropriate the principles expounded are none the less relevant to Australian conditions and worthy of consideration. It is disappointing, however, that in an otherwise excellent coverage little reference is made to stream bed stabilisation and energy dissipation. A few comments can be found on bed scour, deposition, riffles and dredging but no specific reference to grade control or energy dissipation structures was found and yet, in many cases, these provide the river engineer with a valuable means of stabilising a stream.

The final chapter concerns conservation and contains valuable hints on how to ensure the continued existence of native fauna from both design and operational maintenance aspects.

In summary, despite a few misgivings I found it a valuable and practical addition to the limited literature available in this specialised area of civil engineering. It contains only a few basic formulae and should not be regarded as a design manual.

It serves to introduce the reader to the problems encountered in coastal and river engineering and the common solutions adopted. One should not be discouraged by its UK flavour because the principles used can apply anywhere. The bibliographies at the end of each chapter are excellent and well worth following through.

The publication provides a generally good overview of the subject matter and is worthy of serious consideration by anyone interested in coastal and river engineering.

JOHN R. TILLEY

A PILOT PLANT STUDY OF THE TRICKLING FILTER SOLIDS CONTACT PROCESS AT RICHMOND, NSW

by MITCHELL LAGINESTRA

ABSTRACT The trickling filter process for sewage treatment is generally regarded as highly stable, simple and inexpensive to operate, but largely outdated (although it is gaining renewed popularity with the advent of plastic media). The older trickling filter cannot usually meet today's more stringent effluent licence conditions.

The trickling filter solids contact process (TF/ SC) was developed in the USA, and found to lead to a marked improvement in the quality of secondary effluent (at a much lower cost than full plant upgrading). The process involves a short period of aeration of the trickling filter underflow together with sludge recycle, prior to final sedimentation.

At Richmond STP, an old trickling filter plant, a pilot plant, consisting of an aeration tank, with a detention time typically less than 80 minutes, and clarifier, was set up to enable a direct comparison of the TF/ SC process with the existing trickling filter plant secondary effluent. The pilot plant results showed there was a significant improvement over the existing plant effluent. In addition it was shown that both a tertiary standard of sewage treatment (less than 15 mg/ L NFR) and nitrification was capable of being achieved using the TF/ SC process.

INTRODUCTION Many trickling filters still in use today were constructed well over 25 years ago. In the majority of cases they do not achieve an effluent to meet the current stricter discharge criteria. These plants typically produce a slightly turbid effluent containing very fine particles. As a result, trickling filters are now not so much in favour, instead activated sludge plants have become more popular. In many overseas countries and in Australia, many trickling filter plants are being considered for replacement. Activated sludge-plants are known to produce effluents of great clarity but have high energy usage and can be subject to various process problems.

Trickling filters are simpler and less expensive to operate than activated sludge plants and, because of the feature of attached biomass, have a proven ability to cope with high organic and hydraulic loads (which can cause washout in activated sludge systems). In addition, they have the ability to recover faster from toxic shocks. However, consistently achieving a high quality effluent from trickling filters is a problem. One reason for the relatively high suspended solids concentration in trickling filter plant effluents is that the underflow (effluent) from the filter usually contains less than 100 mg/ L suspended solids or non-filterable residue, which is much less than that contained in the overflow of an equivalent aeration tank. Therefore, there is little opportunity for contact and agglomeration of the small particles, which cannot settle out and so appear in the effluent (Barnes, 1986).

THE TF/SC PROCESS The trickling filter-solids contact process involves a flocculation stage prior to secondary sedimentation (clarification), which provides small particles an opportunity to become incorporated into larger floes (Barnes, 1986). This stage typically consists of an aerated channel to 'bump' particles together, which then settle out in a clarifier. Sludge collected from the clarifier is returned to the aerated channel. Air input must not be so great that the floes suffer from shear forces (Matasci et al, 1986).

Initial development of the TF/ SC process occurred in the USA, at Corvallis, Oregon, in 1979. The Corvallis plant was a conventional trickling plant producing an effluent with BODS and non-filterable residue (NFR) in the range 20 to 40 mg/ L (Matasci et al, 1986). Implementation of the TF/ SC process resulted in an effluent containing less than 10 mg/ L NFR and BOD,.

38 WATER A ugust, 1989

Mitchell Laginestra, BE (Sydney) is a Chemical Engineer with the Water Board at North Head Sewage Treatment Plant.

He is currently doing a Masters of Environmental Studies at Macquarie University and has over 7 years experience in operation and investigation of sewage treatment processes.

.

M. Laginestra

It is now generally regarded that there are 3 modes of operation of the TF/ SC process. The difference in the modes is based on aeration of the trickling filter underflow (Mode I), return sludge (Mode II) or both (Mode III).

Some typical features of the TF/ SC process include: • aerated solids contact tank which has a retention time of between 15 minutes and 2 hours (based on trickling filter underflow plus ' return sludge flow rate); • mixing of trickling filter underflow with return sludge solids; • low sludge age in the solids contact stage, usually less than 2 days; • specialised clarifier design, which incorporates a flocculator centre well (detention time of about 20 minutes) (Matasci et al, 1986).

Overseas results have demonstrated the effectiveness of the TF/ SC process in reducing NFR and associated BOD. It can reduce carbonaceous BOD significantly, given adequate contact time, although the main organic removal unit is usually the trickling filter.

However, design trade-offs can be made between sizing of trickling filters and solids contact tanks (Matasci et al, 1986). Implementation of the process has resulted in major savings in capital expenditure as well as operating and maintenance costs.

TRIAL AT RICHMOND STP Objectives The potential of the TF/ SC process for old and overloaded trickling filter plants was realised and a trial was established by the Sydney Water Board at Richmond Sewage Treatment Plant (STP). The STP is a slightly overloaded trickling filter plant, with a small oxidation pond to 'polish' the effluent, and is required to meet State Pollution Control Commission (SPCC) standards of 20/ 30 mg/ L for BOD,/NFR. As with many old trickling filter plants fine particulate solids are generally noticeable in the humus tank effluent, which typically contains 30-40 mg/ L NFR. Hence the objectives of the trial were seen as: • determining the extent of improvement over the existing trickling filter plant effluent.

• testing the practicality of achieving a high quality secondary effluent, which could be equivalent to a tertiary effluent; • testing the theory of solids contact, using aeration only to 'bump' particles together (ie no recycle of sludge); • determining the optimum conditions for the solids contact process (with sludge recycle) by varying mixed liquor concentration, aeration tank detention time and other parameters; • ascertaining the microbial population that develops in the solids contact tank for possible process control implications; • determining the feasibility of the process to remove nutrients.

Richmond STP Richmond STP is a conventional trickling filter plant with coarse screening, primary sedimentation and fine screening a?ead of_ the trickling filters, followed by humus tanks for sed1mentat10n.

Performance data for the plant are summarised in Tobie 1.

For both phases of the trial, waste sludge was discharged periodically (every few days) from the clarifier cone onto the disused drying beds at Richmond STP via gravity. During the second phase of the trial sludge was continuously recycled to the aeration tank at a largely constant rate. Wasting of sludge was used to control the mixed liquor suspended solids in the aeration tank.

TF/SC Pilot Plant

Monitoring

An activated sludge pilot plant, consiting of aeration tank, secondary clarifier and associated equipment was slightly modified for use as a solids contact plant and installed at Richmond STP in 1987. Feed to the pilot plant (10-40 L/minute) was trickling filter underflow from humus dosing chamber No. 1. The pilot plant was operated for 12 months and was essentially run in parallel to humus tank No. 1 to enable direct comparison of the effluents.

Phase 1 of the trial involved a study of improvement in settleability (if any) of trickling filter underflow solids _after ~eration only (ie, no recycle of sludge). Phase 2 was the more mtens1ve part of the trial and involved operation of the TF/SC Mode I process, ie aeration in a solids contact tank of trickling filter underflow alid return sludge. Figure 1 is a schematic diagram of the pilot plant.

Grab and composited samples of trickling filter underflow, humus effluent, and pilot plant sludge, mixed liquor and effluent w~re taken 2-4 times per week and analysed. Dried sludge from the drymg bed, as well as supernatant/ underflow from the drying beds immediately after sludge application, were also sampled and analysed - although less frequently.

In additon a Partech NFR monitoring system (consisting of a sensor and indicator) with chart recorder, was installed to provide continuous measurement of suspended solids (or non-filterable residue) of pilot plant effluent. Laboratory analyses were used to check monitor results and inconsistent monitor results were disregarded.

Mixed liquor flow was continuously measured (Manu rotapulse) and flow to the pilot plant was obtained by subtracting the return sludge flow rate.

Operation Influent flows to the pilot plant and aeration tank detention time were varied to check performance of the process under different conditions.

The level of the adjustable weir in the pilot plant aeration tank was changed to vary detention time. However, detention times less than about forty minutes were not possible because the differential head between the aeration tank and clarifier was very low.

Table 1 Richmond STP Data Parameter

Data

Average Dry Weather Flow (ADWF) ML/day Equivalent Population (EP) Nominal Design Capacity Typical Plant Performance: NFR Removal - Primary/ Secondary Plant BOD , Removal - Primary/ Secondary Plant Nitrification over Trickling Filters Oxidation Pond Detention Time (days) Typical Plant Effluent: NFR mg/ L BOD, mg/ L NH, ;- N mg/ L NO, - N mg/ L

2.6 9 650 8 000

83% 88%

8% 4 .3

22 26 24 0 .7

mmm 3 11, 4 1 1 \ta!ltd, mtr1J lt"I

"

'" '"

10¡ 10

RESULTS AND DISCUSSION Phase 1 This phase of the trial involved aeration of trickling filter underflow (TFU) without sludge recycle. Results of humus tank effluent (HE) and pilot plant effluent (PPE) quality (obtained during similar secondary sedimentation tank detention times) were compared, which are shown in Tobie 2.

A significant improvement in the quality of the pilot plant effluent over that of the humus effluent was found (the statistical 't' distribution theory was used).

Data was grouped for different detention time categories for Phase 1 of the trial to determine the effect. if any, of the detention time in the aeration tank. Results are shown in Tobie 3. There was a significant improvement in PPE when aeration detention time was greater than 2 hours.

Settleability tests (using Imhoif cones) were carried out on trickling filter underflow and aeration tank contents (ie, aerated TFU) and showed -that settleability was markedly improved. TFU ranged from 0.3-2.3 mL sludge/L (average 1.2 mL/L). A considerable improvement in clarity of PPE and in the supernatant of the aerated TFU was also evident during the settleability tests.

The improved settleability is also demonstrated by the high solids concentration in the pilot plant sludge (average 2.1%).

Unfortunately it was not possible to compare this to the humus sludge concentration at Richmond because of the continual recycle.

However, humus sludge is known to be typically about 0.5%.

Phase 2 mu lo..-.uer

II II - 21 LP! t0O11d1t1O1

'"' rtl ~r1

Phase 2 involved the trial of the TF/ SC process (Mode I) and results obtained, when the humus tank and PP clarifier had similar detention times, are compared in Table 4.

Results show a significant improvement in PPE quality over the humus effluent for NFR, BOD,, and ammonia nitrogen. In addition there was a significant increase in oxidised nitrogen and a marginal Table 2 Effluent Quality Pilot Plant vs Humus Tank (Phase 1 - No Sludge Recycle)

t).. -<><,..__~

sl1t9,pn1p

mm201l yl ciu t ucordtr llot pli l

TEU

Characteristic

' - - - - - - - -=C7'.'11ud pu p

Range

NFR, mg/ L BOD ,, mg/ L

ri ma r eff u e n t

H E T F U

to

HE NFR , mg/ L

pond

Range

Pilot Plant for Trickling Filter Solids Contact Trial at Richmond.

74 62

Range

Average

16- 56 5-130

44

37

PPE Range Average

6-60 6-50

26 27

"lo

lmpronment (PPE vs HE)

30 39

Table 3 Effect of Aeration Tank Detention Time on Pilot Plant Operation

oxidation

Fig. 1 -

20-160 17-175

HE

Average

33-55 24-56 18-51 23 -47

Average

46 39 32 36

Aeration Tank Detention time Hours

< 1.5 1.5-2.0 2.1-3.0 > 3.0

. PPE NFR, mg/ L

Range

Average

35-60 15-42 6-46 6-35

46 29 21 18

0/oNFR Reduction

0 260/o 340/o

50%

WATER August, 1989 39

Table 4 Effluent Quality Pilot Plant vs Humus Tank (Phase 2 .:_ TF/ SC) Characteristic mg/ L

Range

NFR BOD, (Inhibited) NH, - N NO, - N Phosporus pH

28-147 32-140 11 -65 0.1 -5.5 4.7-12.8 7. 0-7. 6

HE

TFU

Average

71 70 33 1.4 9.2 7.3

Range

10-75 2-120 13 .6-52 0.2-5.0 5.8-13.1 7.0-7.6

reduction in pH. The pH decrease might be expected since some nitrification was found to occur. There was no significant difference in phosphorus concentration. The slight reduction could be expected since the solids contact process involves an additional biological stage, and the microorganisms would utilise nutrients (including phsophorus) during normal metabolic activity.

Almost half of the pilot plant results were equivalent to a tertiary standard of treatment. This was over a wide range of operating conditions (clarifier detention times of 0.6-2.4 hours, and overflow rates of 15-50 M'/m'.d) .

Little correlation was found between NFR of the mixed liquor (usually operated in the range 500-2500 mg/ L) and pilot plant effluent quality, which tends to agree with Matasci et al (1986).

Results were also considered for comparable overflow rates for humus tank and the pilot plant clarifier. Pilot plant results for similar overflow rates were shown to be similar to those above (ie, those of comparable clarifier detention times). This confirms information from the USA, which claims that TF/ SC operation is insensitive to overflow rate (Matasci et al, 1986). See figure 2.

Effect of Aeration Tank Detention Time An attempt was made to find a correlation between pilot plant effluent (PPE) quality and detention time in the pilot plant aeration tank. Results were compared to the corresponding humus tank effluent (HE), and are shown in Table 5 below. These show there was a significant difference between the quality of HE and PPE when detention time was greater than 0.9 hours.

PPE NFR (mg/L)

40

30

20

10

a l-------'-----'-----'----__,__ _ __.___ __ 10

20

30

40

50

60

OVERFLOW RATE cu.m/m.sq.day Fig. 2 - Relationship of Effluent Quality (NFR) to Overflow Rate.

PPE NFR (mg/L)

70

60

33 40 30.0 1.5 7.9 7.4

4-62 1-78 4.8-37.0 0.7-19.9 4. 7- 9. 7 6.9- 7.4

0/o Reduction

Average

(PPE vs HE)

21 23 18. 6 7.5 6.8 7.2

36 43 38 14

Table 5 Effect of Aeration Tank Detention Time (Phase 2 - TF /SC) HE

Characleristlc

NFR

PP E

Aeration Tank Del.Time

mg/ L Range

Average

Hours

20- 55

"lo Reduction

Average

Range

31

1.4-1.9

18

420Jo

NFR BOD (1) NH,-N NO,-N

13- 75 2- 120 13.6- 52 0.2- 3.4

33 40 29.8 I.I

1.0-1.3

4-60 1-78 4.8-37 1.3-19.9

18 19 18.0 7.5

460Jo 530Jo 40 0Jo

NFR BOD(!)

NH,-N NO,-N

10 ¡ 23 ¡ 18.90. 2-

34 39 30.0 3.1

0.7-0.9

6-62 5-5 1 11-31.5 0. 7- 15

27 28 19.8 7. 5

210Jo 280Jo 340Jo

58 75 48 5.0

8-46

Table 6 Effect of Sludge Age on TF /SC Process HE

Characteristic

Range

Average

13-55 NFR NH,-N 25.7-52 NO,-N 0.2-4.4 "To Nitrification

30 31.7 1.3

NFR 10-75 NH,-N 19.3-48 NO, - N 0.2-5.0 "To Nitrification

35 31.3 1.6

PP E

Sludge Age

'lo

Group

Range

>4.5 days

4-22 8.8-37 0.8-2.2 5.4-74.5

19.4 16.0 40.3

6-62 13 .9-31.5 0 .7- 14.5 0-65

29 21.3 6.3 32.8

<4.5 days

Average

Reduction

11

630Jo 380Jo

170Jo 320Jo

Nutrification was shown to be possible with' the solids contact process at various aeration tank detention times (even less than one hour) .

Pilot plant effluent data was grouped according to the pilot plant sludge age and compared to the corresponding humus tank effluent results.

Results are shown in Tobie 6 below. The relationship between pilot plant effluent NFR and sludge age is shown graphically in Figure 3.

Statistics showed there was a significant reduction in ammonia nitrogen in both categories, but a significant NFR improvement in PPE over HE was found only when sludge ages were above 4.5 days (ie, conventional activated sludge mode of operation). There was found to be a definite difference in pilot plant performance between the two groups, with the categories of results corresponding to a sludge age less than 4.5 days resulting in significantly poorer PPE (NFR) quality.

An increase in removal of ammonia nitrogen with increasing sludge age was also found. This is a documented trend with activated sludge plants .

Overall, operation at higher sludge age resulted in a consistently better effluent quality. Hence, sludge age appears to be a very important factor in the performance of the TF/ SC process.

Sludge From the TF/SC Process

50 40 30

. . ..

20 10

. ..

. ..

0 0

2

4

6

8

10

Sludge Age (days)

Fig. 3 40

Range

Sludge Age

50

0

PP E Average

Effect of Sludge Age of Effluent Quality (NFR).

WATER August, 1989

12

Flocculation of solids for the TF/SC pilot plant was visually noticeable in the Imhoff cone settleability tests. The floes were generally large and settled well. The clarity of the pilot plant effluent and the supernatant of the mixed liquor was striking. The photograph in Figure 4 shows the improvement of the PPE and ML supernatant over the turbid humus effluent and TFU. The extent of the flocculation of the solids in the TF/SC process was also demonstrated by the drainage from the drying beds. The beds are disused at Richmond and are not in prime condition (there is some collapse of the suport drains and most of the liquid flows to one end of the beds). However, the flow stream was of reasonable quality (average of 54 mg/ L NFR and 74 mg/L BOD). Return

activated sludge systems. However, some. filamentous bacteria, notable Microthrix parvicel/a, (which generally result in poor settlement of sludge), were also found. A few nematodes were also present in the samples. These occur very infrequently in activated sludge plants (Poole, 1984), and are more commonly found in trickling filters (Tuft, 1982) and hence probably common in the TF/ SC process. However, in activated sludge systems, nematodes may occur when there is a lot of food available. They are known to feed on filaments not floes (Tuft, 1982), which may be seen as an added advantage of the TF/ SC process.

Both low D. 0 . and high F/M ratios experienced during the 2 weeks could have contributed to the presence of filamentous organism. However, the pilot plant effluent results during the microbial assay show that despite the presence of filaments, the TF/ SC process performed very well - producing an effluent of excellent quality (averaging 8 mg/ L, with all results well below the typical 15 mg/ L limit associated with tertiary treatment) . During this period the return sludge was not particularly compact (average NFR was only 2500 mg/ L) . The filaments were probably caught up with the floes and not free floating - which could be the reason why the effluent was not affected.

In a full scale operational plant, process stability and consistently high F/ M ratios should not present problems, as long as the design is adequate and the plant is operated correctly.

General

.t'ig. 4 -

Settleability Tests, from left to right: HE/ PPE/ TFU/ PP Mixed Liquor (note clarity of PPE).

sludge (same as waste sludge) NFR ranged from 1500-6000 mg/ L, (average of 3200 mg/ L) .

The underflow from the sludge drying bed was found to be low in phosphorus (similar to sewage effluent), high in oxidised nitrogen, (average of 85 mg/ L) and relatively low in ammonia nitrogen (15 mg/ L) . Hence, this flow stream could be discharged directly to tertiary treatment stage; or, for a nutrient removal sewage treatment plant, it could be returned prior to the denitrification stage.

Microbiological Assessment of the Mixed Liquor A microscopic inspection was carried out on several pilot plant mixed liquor samples, collected over a two week period. Some Protozoa and amoeba were found, which are desirable for healthy

The pilot plant was by no means the ideal design for a solids contact plant. The pilot plant consisted of a circular aeration tank with coarse bubble aeration, there was no long detention time flocculating chamber in the clarifier and no clarifier scum baffle or collection system. Nevertheless the plant performed very well, and it is reasonable to assume that with a better design (including rectangular aeration channel, fine bubble diffusion, clarifier scum collection) the solids contact process could perform even better than that achieved by the pilot plant at Richmond .

The results from the trial seem to indicate that the presence of a flocculating chamber after the aeration tank may not be necessary as long as additional aeration time is prdvided in the contact (aeration) tank. This tends to support results at Medford, Oregon.

Hence, this may not have been a problem with the pilot plant.

However, other experience overseas (Parker, Stenquist, 1986; Matasci et al, 1986) indicate that better results'are obtained with this feature.

A comparison of overseas TF/SC plants with Richmond pilot plant is shown below in Table 7.

At Corvallis and Morro Bay, trickling filter underflow BOD values are very low, which permits the use of minimal aerated contact detention time (Norris et al, 1982). Medford and Richmond TFU BOD concentrations are higher and so warrant a higher detention time in the aerated contact tank .

COST COMPARISONS Implementation of the solids contact process to existing trickling filter plants would be a very inexpensive method of upgrading. For many plants this would simply involve installing an aeration tank along with associated pipework and blowers. However, for some TF plants upgrading might be more expensive, involving installation

Table 7 Comparison of TF /SC Plants including Richmond Plan!

Co rvallis

C haracteri stic

OREGON

A.D.W.F . (ML/day) Trickling Filter Media Type TF BOD, loading (kg/ m'.d)

+ 36.8 rock 0.38

Aeration Tank Detention time, mins NFR of Mixed Liquor, mg/ L Flocculating Chamber Det. Time, mins.

NFR results prior to TF/ SC, mg/ L NFR results with TF/ SC, mg/ L % Improvement

Medford OREGON

+

Morro Bay CALIFORNIA

RJchmond N.S .W.

+

68.2 plastic l. 84

3.7 rock 0.48

2.6 rock 0.44 PILOT PLANT

2( + 9 mins for ret urn sludge) 3130

39 1620

3( + 9 mins for return sludge) 1140

40 - 85 (Avg. 68*) 1000 - 2000 (Avg. 1550)

25

5

19

1.5

(Clarifier inlet well) 31

28

30

9 71%

14 50%

11 â&#x20AC;˘ 63%

+ from Matasci et al, 1986/ Norris et at, 1982/ Matasci et al, 1988. â&#x20AC;˘ Average conditions for sludge age >4.5 days .

WATER August, 1989 41

of aeration tanks and construction of new clarifiers. However, one possibility to reduce these costs might be to convert existing humus tanks to aeration tanks and build new clarifiers (if this provides . sufficient aeration detention time, then flocculating chambers of the new clarifiers need not be installed). Humus tanks could be _converted to an aeration tank by concreting the bottom to form a flat section and so provide a base for a submersible aerator.

At Richmond STP, conversion to a Trickling Filter/Solids Contact Plant is estimated to cost about $0.9M * - based on converting one humus tank to an aeration tank and constructimg new clarifiers to supplement the existing humus tank. A complete upgrading of the plant to provide full scale conventional treatment would cost $1.5M* (additional trickling filtration; sedimentation and polishing) not including the cost of additiona_l land required.

Consideration should also be given to constructing new plants involving the TF/SC process, since overseas information (Fedotoff et al 1982) suggests that a TF/SC plant would involve less capital expenditure and be cheaper to operate and maintain than an activated sludge plant. It is believed that the TF/SC process will produce an effluent of consistently high quality, similar to activated sludge, and better than extended aeration processes (including intermittently decanted aerated lagoons - !DAL). There is also the added advantage of the inherent stability of the trickling filter.

It is believed that because of the low aeration tank detention times involved, the TF/SC process will involve markedly lower operating costs.

A TF/SC plant, consisting of preliminary treatment, primary sedimentation, sludge digestion, trickling filters (with plastic media, so no concrete retaining wall is required), aeration tank (0.5 hours detention) and secondary sedimentation is estimated to cost approximately 58% of an activated sludge plant (preliminary • treatment, primary sedimentation, sludge digestion, aeration tanks, secondary sedimentation tanks, and tertiary sand filters) and 92% of an extended aeration plant (preliminary treatment, !DAL, flow balancing tank, tertiary sand filters).

Overall, the pilot plant performed well despite the lack of proper control of the process, which perhaps demonstrates the ease of operation of the TF/SC process. Some problems with denitrification in the clarifier occurred, although this could normally be avoided by reducing sludge age, reducing aeration tank detention time, decreasing clarifier detention time or increasing sludge removal flow rate (Parker and Stenquist, 1986); or implementing a proper denitrification stage if nitrogen removal is required.

The best effluent results during the trial were obtained with sludge ages of greater than 4.4 days. The design of the aeration tank will directly depend on the existing and expected ultimate trickling filter organic loading.

It appears that higher detention times in the aeration tank (ie, greater than half an hour) can compensate for the lack of flocculating chamber feature in the secondary clarifier.

The extent of flocculation of sludge with the·TF/SC process was demonstrated by the excellent dewatering on the disused drying beds.

The quality of the drying bed underflow was susch (low NFR and BOD) that it could be returned to the flow prior to the oxidation ponds . However, should nutrient removal be necessary this flow should be returned to the head of the works prior to a denitrification stage.

Upgrading of trickling filter plants by implementing the TF/SC process appears a very cost effective option. Consideration should also be given to construction of new TF/SC plants on the basis of lower capital expenditure and lower operating costs.

ACKNOWLEGEMENTS I thank the Urban Water Research Association of Australia for funding the research project and to many people within the Water Board who contributed in many ways, especially Branko Dimitrijevic, Barry Smith, Sean Gilchrist, Duncan McLuckie, Paul Elkington, and laboratory personnel at West Ryde and Quakers Hill.

REFERENCES

SUMMARY AND CONCLUSIONS Phase 1 -

Aeration of TFU

The aeration of trickling filter underflow greatly improves the settlement of humus sludge. However, to make a significant improvement in effluent quality (in terms of NFR) it appears that a detention time of greater than 2 hours in the aeration tank is required.

The sludge generated is much thicker than humus sludge and is of sufficiently high solids concentration to discharge directly to anaerobic digestion (or drying beds or lagoons) which should take a considerable load off the primary sedimentation stage/ digester.

However, much better effluent quality was obtained during the sludge recycle phase of the experiment; ie, the operation of TF/ SC Mode I.

Phase 2 -

TF/SC Trial

Some excellent results were obtained during the operation of the pilot plant in TF/SC Mode I. The results obtained, demonstrated that this process gave a much better effluent than could be achieved by the trickling filter/humus tanks alone, with many results better than, or equal to, the tertiary standard of treatment in terms of NFR and BOD. The ability of the process to significantly nitrify (average of 430/o during the trial) was also demonstrated, even during winter.

• Using Sydney Water Board order of cost estimates, Trickling filter costs were based on Noosa Council's cost of co nstruction of new plastic media filters (commissioned 1988).

Barnes, D. and Fitzgerald, P. A . (1986). Recent developments in water and wastewater treatment. Civil Engineering Transactions, 19-20.

Bliss, P. J. (Editor) (1983). 'Municipal Wastewater Treatment. A five day course'. School of Civil Engineering, The University of New South Wales, 4-8 July, 1983.

Fedotoff, R. C ., Merrill, D. T., O'Malley, D. l'-jl., Owen, M. C. R. and Parker, D. S. (1982). Design for trickling filter/ solids contact Process applications.

55th Annual Conference of the Water Pollution Control Federation, St Louis, Missouri, October 7.

Joint Committee of the WATER POLLUTION CONTROL FEDERATIO and the AMERICAN SOCIETY OF CIVIL ENGINEERS (1977).

'Wastewater Treatment Plant Design, a manual of practice'. (Lancaster Press Inc., USA).

Laginestra, M. (1989). 'A Pilot Plant Study of the Trickling Filter Solids Contact Process at Richmond, NSW.' Urban Water Research Association of Australia. Report No. 9 / 1.

Matasci, R. N ., Clark, D. L., Heidman, J. A ., Parker, D. S., Petrik, B., and Richards, D. (1988). Trickling filter/ solids contact performance with rock filters at high organic loadings. J. Water Pollution Control Federation, 60, 68-76.

Matasci, R. N., Kaempfer, C . and Heidman, J. A. (1986). Full-scale studies of the trickling filter/ solids contact process. J. Water Pollution Control Federation, 58, 1043-1049.

Norris, D. P., Parker, D. S., Daniels, M. L. and Owens, E. L. (1982) . High quality trickling filter effluent without tertiary treatment. J. Water Pollution Control Federation, 54, 1087-1098.

Parker, D. and Stenquist R. (1986). Flocculator - clarifier performance.

J. Water Pollution Control Federation, 58, 214-219 .

Poole, J.E. P. (1984) . A study of the relationship between the mixed liqu or fauna and plant performance for a variety of activated sludge sewage treatment works . Water Res. 18, No. 3, 281 -287 .

Tuft, R. (1982) . ' Microbiology of Sewage Treatment', 2 day Water Board course, July, 1982.

A.W.W.A. 13th Federal Convention 1989 PROCEEDINGS - 132 PAPERS Management /Administration : Distribution Technology : Treatment Technology : Science and Environment : Public Health 42

WATER August, 1989

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iCALENDAR

I

Chichester Dam

1989 September 11-15, Brazil .

IAWPRC Technology Transfer Seminar and Course on Waste Stabilisation Ponds September 12-14, Birmingham, UK IWEM 89 - Technology Transfer in Water and Environmental Management

September 24-29, Sydney, Australia Fifth National Conference on Local Government Engineering September 25-27, Istanbul, Turkey IAWPRC International Symposium on Waste Management Problems in Agro-Industries October 13-15, Bowral, NSW AWWA NSW Branch Regional Conference October 15-19, San Francisco, USA ¡ WPCF 62nd Annual Conference October 22-25, Hawaii WPCF Asia/ Pacific Conference on Water Pollution Control October 25-28, Nairobi, Kenya Industrial Wastewaters '89 Nairobi The First lAWPRC Eastern Africa regional Conference on Industrial Wastewaters October 29-November 2, Nagoya, Japan Water Nagoya '89 - 7th Regional ASPACIWSA Conference November 14-16, Adelaide, SA Australian Instrumentation and Measurement Conference November 14-16, Bangkok, Thailand lAWPRC Tec_hnology Transfer Seminars Mathematical Modelling of Biological Wastewater 1teatment Processes November 15-16, Albury, Australia Pollution from Intensive Agricultural Industries AWWA/ CASANZ November 19-21, Melbourne, Australia AWWA 3rd National Conference on the Management of Hazardous Wastes December 4-7, Adelaide, SA Combined Conference on Coastal, Ocean Engineering & Marine Science

1990 February 12-16, Melbourne, Vic. AWWA 1990 Summer School Improving Efficiency February 13-16, Wollongong University, NSW Water Research Foundation of Australia (Illawarra Branch). Effluent Revised April 24-26, Jonkoping, Sweden IAWPRC - IWSA Joint Specialist Group on Coagulation, Flocculation, Filtration and Sedimentation June 17-21, Cincinnati, Ohio, USA AWWA Annual Conference and Exhibition July JO-August 3, Kyoto, Japan 15th WPRC Biennial '90 International Conference on Water Pollution Research and Control September 10-11, Glasgow, UK Fourth Annual IWEM Conference and Exhibition

September 11-14, Belgrade, Yugoslavia Fifteenth Symposium of the !AHR.

Chichester Dam is the Hunter Water Board's second biggest water source.

Situated near Dungog, just below the junction of the Chichester and Wangat Rivers, it was constructed between 1918 and 1926 and upgraded between 1980 and 1985.

The catchment covers an area of 197 square kilometres, bounded on the north and east by the Great Dividing Range, which separates it from the Gloucester and Karuah rivers.

The area is considered one of the most favourable catchments in the state. The extensive virgin forests, together with high ranges at the head of the rivers, ensure a large rainfall and minimun evaporation and consequently a high runoff.

The dam wall is 254 metres long, including 112 metres of spillway. Its greatest height above the foundation is 43 metres. The wall is curved with a radius of 368 metres.

The storage lake has an available capacity of 20 300 megalitres, and at full supply level covers 180 hectares to a maximum depth of 37 metres.

The dam wall was built by the then Department of Public Works, using hundreds of interlocking units of concrete, each of 230 cubic metres. When one block was finished the formwork was raised ready for the next.

A terrace was excavated in the hill above the dam site for concrete-making plants and engines. A nearby sawmill supplied timber, which was hauled on wooden tramlines, and a quarry supplied stone and gravel. Two steam-driven cableways, each with a 335 metre span across the gorge, delivered concrete and other materials to the workforce.

A lack of sand in the area forced a decision to transport sand from Newcastle in steam-powered trucks. At the peak of construction the Department of Public Works used more than 20 motor lorries and many more horse-drawn vehicles to carry materials and pipes.

About 1000 men, women and children lived in a nearby works settlement during construction, which proceeded in two shifts, five days a week.

The original 915 mm gravitation main from the dam was 85 kilometres long. The first 14.5 kilometres of the line was made from woodstave, cut from brush box in the timber mill. Each section was made up of 22 wooden staves, tongued and grooved, and held together with steel bands.

The rest of the delivery system was steel locking-bar pipes and lap-welded steel pipes.

Water was first delivered from Chichester in November 1923, but construction of the dam was not fully completed until 1926.

The wood stave section of the pipeline had been replaced by 1944 and the main was further amplified in the years 1952-59 and during 1974-77.

In 1960 it was discovered that dams built in the same period as Chichester might not comply with modern design criteria. Investigations undertaken by the Board showed

the original design ?lid not take into account hydraulic uplift.

Consultants confirmed that significant uplift pressure existed under the dam and that modifications were required to the spillway to increase discharge capacity. This second point was brought home dramatically in March 1963, when almost three metres of water flowed over the spillway and parapet. The flood was almost twice the design capacity of the spillway.

In 1966 the Board approved the lowering of the spillway by 2.7 metres, so reducing the hydrostatic and uplift forces on the dam. Drainage holes were also drilled at the rock-concrete interface.

In the following years the Board considered the related problems of how to increase water storage capacity in the region and how to render Chichester Dam safe from uplift pressures at its base.

The solution devised by the Board's staff was to anchor the dam wall to bedrock using restressable steel tendons, relocate the spillway towards the centre of the dam and restore it to its original height, thus bringing the storage back to its former capacity.

The remedial works were approved in September 1980 and the dam was oficially recommissioned in May 1985.

Longstanding problems with water quality after heavy rain in the catchment were eliminated in February 1988 with the commissioning of a new water tratment plant at Dungog.

Further remedial works may be necessary because of revised peak flood projections, but planning has been delayed until the Bureau of Meteorology is able to ¡deliver a final, site specific estimate of probable maximum rainfaJI. Chichester's important role in the Lower Hunter water supply system is being greatly extended by the Board's ability to put off building a planned new dam on the Williams River near Dungog.

One of the major reasons the delay has been possible is the so-called " user-pays" pricing system introduced by the Board ip 1982.

The tariff, made up of a fixed charge component and a charge per kilolitre of water consumed, was intended as a more equitable rating system, but with the added purpose of reducing demand .

Since its introduction there has been a significant and sustained drop in demand, well below the previous trend.

The resulting savings have been greater than expected and have allowed capital intensive major water source augmentation to be deferred, at least until the year 2002. The Board's customers have also benefited from the delay and proposals to put back the new dam's start date further are being investigated.

An unexpected spinoff from user-pays has been a significant reduction in peak day demand. This is an important parameter in water system design and has enabled the deferment of amplification works on treatment plants, pumping stations and trunk watermains.

The savings in this area are considered even more significant than in the deferment of the new dam.

WATER August, 1989

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