Modelling the performance of anaerobic wastewater stabilization ponds

~

Pergamon 0273-1223(95)00504-8

Wat. Sd. Tech. Vol. 31, No. 12, pp. 171-183. 1995. Copyngbt @ 1995 IAWQ Printed in Great Bnlam. All ngbU reseJVed. 0273-1223195 $9'50 + 0'00

MODELLING THE PERFORMANCE OF ANAEROBIC WASTEWATER STABILIZATION PONDS M. M. Saqqar* and M. B. Pescod**

* Jordan Environmental Study Group. P.O.Box 540/ /6. Amman. Jordan

** Depanment ofCivil En)1ineenng. University ofNewcastle upon Tyne. NE/ 7RU. UK

ABSTRACT The perfonnance of the pnmary anaerobic pond at the Alsamra Wastewater Treaunent Plant in Jordan was monitored over 48 months. Overall averages for the removal efficiencies of BODS' COD and suspended solids were 53%. 53% and 74%. respecbvely. An improvement in removal efficiency with increase in pond water temperature was demonstrated. A model. which takes IOto account the variability of raw wastewater at different locabons. has been developed to describe the perfonnance of a primary anaerobic pond in tenns of a !>Cttleabillty ratio for the raw wastewater. The model ha.'> been verified by iUustrating the high correlauon between actual and predicted pond performance.

KEYWORDS Wastewater stabilization ponds; anaerobic ponds; pond performance; settleability ratio; modelling. INTRODUCTION Anaerobic wastewater stabilization ponds are considered to be a valuable first-step treatment as they allow for settleable materials in the raw wastewater to separate out and fall to the bottom sludge zone. The important advantage in using these ponds is the reduced land requirement compared with facultative ponds. It is advisable to use anaerobic ponds whenever there is a considerable amount of settleable material in the raw wastewater. High strength domestic wastewater normally contains high concentrations of settleable materials. Reviewing the literature shows that a complete analysis of performance data and comprehensive studies on anaerobic ponds are not available. In addition. there are no easily applied relationships that can be used for the design of anaerobic ponds and a satisfactory performance model has not yet been formulated. Design of anaerobic stabilization ponds is commonly based on empirical criteria which have been developed through experience. These criteria have been based on either organic loading andlor retention time. Table I shows volumetric loadings reported in the literature for anaerobic ponds. This table indicates the wide variation in applied loading throughout the world. with no general agreement on an optimum value. It is unlik,:ly that ~ sin~Ie universal design figure can be established since perform~nce is system dependent and. therefore. vanes from system to system. Local experience may suggest a SUitable figure for that specific area. where wastewater characteristics and meteorological parameters are consistent. For the retention time approach. a range of retention times between J and 50 days has been reported for anaerobic ponds treating 171

172

M. M. SAQQAR and M. B. PEseOD

domestic wastewater (Table 2). Design using either of these two approaches provides no information on effluent quality. Vincent el al. (1963) developed an empirical formula to estimate the BODS (CBOP) removal in anaerobic ponds in tropical areas. This formula has little value for design because it has several limitations and weaknesses. Table I. Volumetric loadings reported for anaerobic ponds

VOlumetric loading (BODs glm3 .d) 42 - 283

Remarks *operating loading in Alberta province, Canada.

Reference Fisher ~.(1968)

286

189 - 236

*Maximum loading for design in South Africa. *Loading allowable in most states of the U.S.A (e.g., Iowa, Nebraska, South Dakota)

Meiring et al. (1968) White

(1970)

314

*Recommended loading for packing house wastewater, with a minimum retention time of 4 days at minimum temperature of 75"F.

Hammers and Jacobson (1970)

125

*This value is recommended in some semi-arid areas.

Gloyna (1971)

75 145

* This value is used in Australia at lS"C. * This value is used in South Africa at 25°C.

Malina and Rios (1976)

100 - 400

* This is the recommended range for design and, as an average, 250 can be used.

Mara (1976)

40 - 250

* LOading applied in six states of the U.S.A, with a tendency towards the high value.

Bradley and Silva (1976)

33 - 600

* Range of loading applied.

Ellis (1980)

50 - 134

* Recommended range for design in India, at low and high temperatures.

Arceivala (1981)

* Recommended range for design.

Arthur(198JT

100 - 400 300

400

* Maximum design value. * This value can be used if odour is not of concern.

WHO (1987)

In a critique of all design approaches for anaerobic ponds, WHO (1987) concluded that numerous environmental factors had not been taken into account in present design approaches and suggested that a co~.~iderable amount of investigation was still needed to achieve an acceptable rational design procedure. This conclusion is generalIy accepted by researchers.

PERFORMANCE DATA ANALYSIS Performance of the first anaerobic pond at the Alsamra Wastewater Stabilization Ponds in Jordan was jn~esti~ated. for 48 months. This paper addresses the pond's biochemicaJ performance. while details on mu;roblOloglcal ~erformance can be found in Saqqar and Pescod (l992a). Analysis of the monitoring data revealed correlations between the different measured parameters.

Modelling anaerobic wastewater stabilization ponds

173

Table 2. Retention times reported for anaerobic ponds

Remarks

Retention time (days) 2 - 5

,

*Expected BOD, reduction

~s

,

*Expected BOD, reduction

5

*The minimum for design

0.5 5 - 30 10 5 -

Reference

10

5 3 - 5

1 - 5

2

5

1 - 5

~s

,

~n

60 to 70 %.

Parker et al.

70 %.

Oswald et al.

at at at at

(1967)

South Africa. Meiring et al.

*Texas criteria, with an expected BOD, reduction of more than 50 %. *Minimum value in Montana, with an expected BOD, reduction of 70 %. *1owa criteria, with an expected BOD, reduction range from 60 to 80 %. *1llinois criteria, with an expected BOD, reduction of 60 %. *Nebraska criteria, with an expected BOD, reduction of 75 %. * 1 - 2 days * 2 - 3 days * 4 - 5 days days 5 *

(1959)

temperature 20 - 35·C. temperature 15 - 20·C. temperature 10 - 15·C. 10·C. temperature

* For summer time. * For winter time. * For temperature greater than 20·C : 1 day, expected BOD, reduction is 50%; 2.5 days, expected BOD, reduction is 60%; 5 days, expected BOD, reduction is 70%. *The above BOD, reductions should be reduced 10 to ~O% at temperature 15-20'C

(1968)

White

(1970)

Arceivala (1973)

Malina and Rios (1976) Mara

(1976)

*Oepending on the climatic conditions

Metcalf and Eddy (1979)

1

*Minimum acceptable.

Ellis (1983)

3

*Minimum recommended.

Gloyna

1

*Minimum recommended.

WHO (1987)

20 - 50

(1984)

Figures I, 2 and 3 show probability distribution plots for the efficiencies of removal of total BODS (CBOO>. COD (CCOO) and suspended solids (XSS)' respectively, throughout the study period. Mean, median.

minimum and maximum efficiencies achieved for each of these parameters are also provided on the figures. Taking into account the volume of accumulated sludge, theoretical retention time in the first anaerobic pond ranged from 4.1 to 6.5 days (average 5.1) and monthly means of water temperature (TW I) ranged from 12 to 28 ·C. Figures 4 and 5 show relationships between CBO D removal efficiency and the percentage reductions of CCOD and X ss , respectively.

174

M. M. SAQQAR and M. B.

PESCOD

99.9 mean ---;;~53-----------~ 99 median =0.54 ~ 95Iminimum=0.32 • ~ ~ 80 jmaxtmum=o. 71 'y-~.--"" j

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[BOD] removal efficiency

C

Figure I. Probability distribution plot for the removal efficiency of C BOD in the first anaerobic pond.

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0.6

Figure 2. Probablbty distribution plot for the removal efficiency of CBOD in the first anaerobic pond.

~ CIJ

~

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.........>

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O.B X [55] removal efficiency

0.9

Figure 3. Probability distribution plot for the removal effiCiency of X ss in the first anaerobic pond.

ModeUing anaerobic wastewater stabilization ponds >-

u

c

-.....-... CII

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Figure 4. Variation of CBOD removal efficiency with CruD removal efficiency in the fll'St anaerobic pond.

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Figure S. Variation of CBOD removal efficiency with Xss removal efficiency in the fll'Sl anaerobic pond.

x c:i

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:~~ 500 400

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600

Figure 6. Variation ofCBOD,X with Xss •

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1000

M. M. SAQQAR and M. B. PESCOD

176

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Figure 7. Variation or CBOO removal efficiency wi1h pool! water temperalllJe.

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--l

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Figure 8. Variation or Xss removal efficiency with pond water temperature.

The resulting regression lines show that the removal efficiency of C BOD (E CBOD•1) is equal to the removal efficiency of CCOD (E cCOD •t ) and to 0.74 ofthe removal efficiency of XSS (E XSS , I)' The relationship between CBOO.x. which is equal to the total BODs (Caoe> minus the soluble BODs (5BOO)' and Xss was also examined. Data for the raw wastewater and the effluents from the first two anaerobic ponds only were used. This was to ensure that there was no contribution from algal biomass to the Xss value due to their total absence in the raw wastewater and in the fU'S! two ponds. A simple regression analysis with zero constant, illustrated in Fig. 6, shows a strong correlation (r2:0.95) between these two parameters.

Modelling anaerobic wastewater stabilization ponds

177

The resulting regression line was: CBOO.x

= CBOO - SBOO = 0.65 Xs s (no algae)

(r2 =O.95) (48 months. n "'680)

(I)

In examining anaerobic pond performance in tellDS of CBOO and X SS reductions. related to influencing parameters. it is well documented that pH influences the anaerobic treatment process. However. this parameter has no significance in the first anaerobic pond in the Alsarnra system. as well as in most anaerobic ponds receiving domestic raw wastewater. because it always ranges between 6.8 and 7.2. which is the nOllDal optimum range for anaerobic processes. Fig. 7 illustrates the vwiation of C BOD removal efficiency with pond water temperature (Tw,I)' As the theoretical retention time (9) for most months averaged 5.1 days, this figure shows that little improvement in efficiency was achieved by increasing water temperature. The resulting regression line was: EcBOO,I % = 42 + 0.54 T W (TW ranged from 12 to 28°C)

(2)

Equation (2) indicates that a 10 °C rise in water temperature increased the C BOO efficiency by only 5.4%. This value is much lower than those suggested by Mara and Pearson (1986). who indicated a 20% increase in efficiency due to a rise in temperature from 10 to 20°C. The ECBOO.I improved only slightly with increase in water temperature. indicating that temperature is not an influential factor. These findings might suggest that the relatively long theoretical retention time (9 always more than 4 days) masked the temperature influence. A second possible reason for the results might be that increasing temperature gave an increase in feedback of soluble BODS (SBOO.F) from the settled sludge to the pond water column. ECBOO.I is mainly dependant on the E xss •l • hence Fig. 8 shows the variation of EXSS.I with water temperature (Tw). for which the resulting regression line was: Exss,l % = 66 + 0.36 Tw

(Tw ranged from 12 to 28 °e)

(3)

Again. Equation (3) indicates only a slight improvement of E XSS • I with increasing temperature. It is evident that whenever 9 for anaerobic ponds is more than 4 days, the retention time factor can overshadow other influential parameters. such as temperature. In the few months during which 9 was over 6 days in the Alsarnra anaerobic ponds little improvement in terms of E XSS •I and, hence. EeBOO.! was noted. This supports the view that perfollDance improvement for the first anaerobic pond was insignificant for retention in excess of 5 days. Performance efficiency is greatly controlled by raw wastewater characteristics. specifically the amount of settleable material.

MODEL DEVELOPMENT Figures 7 and 8 provide a general indication of the average performance of the first anaerobic pond at Alsarnra and can be used as a design base in locations which have raw wastewater characteristics similar to the Alsamra influent wastewater. However. a rational approach to describing the perfollDance of a primary anaerobic pond should take into account the vwiability of raw wastewaters at different locations in the world. This principle will be adopted in developing the following model. A mass balance for the first anaerobic pond. shown in Fig. 9. gives the following form: CBOO.SM

=CBOO,X.O· • =[(CBOO,o·

=(CBOO,o

CBOO,x,1 SBOO,O) - (CaOO,! - Saoo) C BOD•I ) • (SBOO,O - Saoo.I)]

(4)

(5)

Equation (5) requires a knOWledge of four parameters to compute the C BOO reduction due to the settleable ma~rial removed (CBOO.SM)' Knowing that SBOO,I CBOO.I - CBOO,x,1 and utilizing equation (I). from whIch CBOO,x,1 = 0.65 XSS,I. equation 5 can be rewritten as:

=

CaOD.SM = CBOD•O • SBOO.O - 0.65 XSS,I

(6)

M. M. SAQQAR and M. B. PESCOD

178

SO.D.I]-

C a • o ••

e. olf ••••

INLET ~'r--

__

c.L sJ~""'/ 1

C ooa • 1

OUTLET ;n'~

, _SLUDGE _- - - - - - I

Figure 9. Schematic of CaoD and SaOD components for lbe first anaerobic pond. This equation was used to calculate CBOD,SM for each month throughout the study period. Since it is more convenient to deal with CBOD,SM as a fraction of the CBOD,O value, the ratio RSM (CBOD.SM I CBOD.O> was calculated for the whole of the study period. As the ratio RS M is a measure of the raw wastewater settleability, it will be called the settleability ratio for raw wastewater. The higher the settleability ratio, the higher the fmetion of CBOD,O removed by the sedimentation process. The value of the ratio ranges theoretically from zero, when there is no settleable material, to one, when the wastewater contains only organic settleable materials (5BOo=O). With typical domestic wastewater, this ratio will nonnally fall between 0.4 and 0.6. According to equation (I) and (4), an equation can be established to describe RSM as: RSM

=0.65 (XSS •O- XSS,I)/ (CBOD,O)

(7)

It will be noticed in equation (7) that the value of RSM is improved by an ~crease in s~s~nded solids -X ). It is known that suspended soltds removal IS unproved. to reduction as expressed in terms of (X SS,O SS,I T ) As . . . th retenUon tlme 10 e a certain extent, by an increase in retention time or pond water temperature ( w· first anaerobic pond of the Alsamra system averaged around 5.1 days for most mon~s (actually from 4.1 to 6.5 days), the R was examined versus T w, as illustrated in Fig. 10. Accordmg to Sto~es Law, the SM settleability of the settleable material is improved by increasing water temp<:ra~re; Fig: 10, which shows the with T w. exhibits a slight increase of RSM values With JDcreasmg temperature. The variation of R SM resulting regression line is: RSM % = 37 + 0.67 T w (Tw ranges from 12 to 28°C)

(8)

The RSM time series exhibited no obvious seasonal movement and a cross correlation factor of only 0.2 was found with the water temperature time series. This only indicates that there is a positive correlation. It seems that the temperature influence was masked by the relatively long retention time provided. For the few months when 9 was over 6 days, a slight improvement in settleability ratio (R SM ) was achieved. It can be concluded from the above findings that whenever 9 is more than or about 4 days and T w is more than 12°C. the variation of R SM is small and, hence, it can be described by a narrow range for a particular system and centralized around the arithmetic average. RSM values for the Alsamra system were found to range from 0.41 to 0.64 b~t val~es centralized around the arithmetic average, 0.51, which was equal to the median and the mode. This vanable, which is a feature of the raw wastewater, can be determined for each trelltment plant. RSM for the raw wllstewater can be approximated. using an Imhoff cone, by the ratio between the

Modelling anaerobic wastewater stabilization ponds

179

C BOD for the settled wastewater (after two hours) and the C BOD for the raw wastewater. It is expected that there will be a variation in the average R SM value for different domestic wastewaters. According to equation (5) and utilizing the RS M definition. the C BOD leaving the first anaerobic pond (C BOD,I) can be described as:

r.. . - ' - ,

(9)

CBOO,I = (l-R SM ) CBOD,O - (SBOO,O - SBOD,I)

0.75

-'-.'

-. - -T"--

r- -""1

--

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.

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.-

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10

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15

_t..

_I

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20

•

water temperature C

30

25

Figure 10. Variation ofRsM with pond water temperature.

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25

Figure II. Variation ofRsB with pond water temperature.

1

180

M. M. SAQQAR and M. B. PESCOO

To investigate the second part of this equation. a mass balance for the soluble BODS components. shown in Fig. 9. gives the following relationship: (10)

SBOO,O - SBOO,I = SBOO,B - SBOO,F = SBOO,r

SBOO,r here represents the net soluble BODs (SBOO) removed in or added to the first anaerobic pond and is equal to the difference between the soluble BODs broken down in the pond (SBOO,B) and the feedback of soluble BODs (SBOO,F) from the bottom sludge. Data analysis showed that SBOO,r ranged (after rejecting 3 outlier values) from +58 to -18 mg/l. As it is more convenient to deal with SBOO,r as related to the SBOO,O value. RSB will be defmed as (SBOO r I SBOO 0>. The higher the RSB ratio. the better pond performance will be in terms of C BOO removal efficiency, It is interesting to note that values of both SBOD,B and SBOD,F increase simultaneously as water temperature (Tw) increases. Equation 9 can be presented in terms of RSM and RSB as:

or

CBOO,I = (J -R SM ) CBOD,O • (RSB ) SBOD,O CBOO,I I CBOD,O

(II)

=(1 - RSM) • RSB (SBOO,O I CBOO,a>

0.8

'0 0

0

1!!.

0.7

u ......

0.6

8 ~

O~

u

0.4

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0.7

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......

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~

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0.4

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n ~~"'l

O.!!!!

0.6

Figure 12. Values of (CBOD,IJCBOD.o) in lCnns of RSM and RSB for SBOO,O equal 10 0.3, 0.3S and 0.4 of CBOD,o-

Modelling anaerobic wastew8Iel' stabilization ponds

181

Values of RS B were found normally distributed and ranged from +0.21 to -0.08. with a mean and median equal to 0.10. The RSB values exhibited no obvious seasonal movement and the cross correlation factor between time series of the RSB and the T w was found to be almost zero (-0.035). Plotting RSB versus T w, in Fig. II, conf"ums the above finding by exhibiting a wide scatter of points but, surprisingly. with negative slope for the best fit line. of which the equation was: RSB %= 12 - 0.21 T w (Tw ranged from 12 to 28°C). This can be simply explained as at higher temperature. SBOD,F exerted from sludge decomposition increased at a higher rate than SBOD,B' Sludge decomposition occurs at lower rates in ponds during cold months. when the pond acts mainly as a settling basin. Low winter temperatures retard bacterial activity, hence anaerobic reactions in the benthal layer are negligible. It should be emphasized that Fig. II is no more than a simplified diagram to describe the variable RSB' 0.6

0:

8

~

u

*

..... ~

8

EE.

•

0.55

0.5

•

•

*

u

"

i

0.45

E

045 "...dicled

I

0.5 C [BOO.1J

0.55

I c [BOO,OJ I

0.6

Figure 13. Predicted and measured values for (CBOo.•ICBOo.O>.

Equation (11) explains the contradictory reports on the performance efficiency of primary anaerobic ponds. The difference in RSM values for different raw wastewaters and, to a lesser extent, the variation of RSB can lead to different results. Although this variation is nonnally slight it can be significant in particular cases. A lower removal efficiency is generally expected where the SBOD fraction (of the total CBOO> is high. and vice versa. At Alsamra, the C BOD leaving the first anaerobic pond nonnally ranged from 0.6 to 0.3 of the influent CBOD. This is equivalent to a reduction of 0.4 to 0.7 in tenns of CBOD,()o For design purposes, and since the improvements obtained for the RSM by temperature increase were more than the losses in tenns of RSB' the efficiency corresponding to the lowest expected water temperature should still be used. The removal efficiency of CBOD is mainly dependent on suspended solids reduction. It was noticed that XSS,I was influenced significantly by a high jet velocity of the incoming wastewater during rainy days. Although XSS removal efficiency was around 74% in the first pond of the Alsarnra system. this figure could fall to 54% during days of heavy rainfall. A high jet velocity of the incoming wastewater can create turbulent conditions not favourable for the settling process and can also agitate the accumulated sludge and cause scouring. This suggestion was supported by the shape of the accumulated sludge deposit in the first anaerobic pond. To facilitate the use of equation (II). charts are provided in Fig, 12 which can be used to detennine (C BOD I I CBOD,O) in tenns of RSM and RSB for S800,O = 0.30 CBOD,Oo SBOD,O = 0.35 SBOD,O and for SBOD,O = 0.40 CBOD,Oo respectively. MODEL VERIFICATION A.~ a diagnostic check for the model developed to describe the perfonnance of the first anaerobic pond (equation (II» and based on water temperature for each month, both RSM and RSB were obtained from the regression lines shown in Fig. 10 and II. respectively. Then the CBOD,I was estimated using equation (II).

M. M. SAQQAR and M. B. PESCOD

182

Figure 13 shows predicted and measured values for the fraction of CBOD leaving the ftrst anaerobic pond (C BOD,1 I CBOD,tV throughout the study period and demonstrates the strong correlation between the two values. Figure 14, which illustrates model residuals, shows that the residuals were normally distributed around the zero value (mean median mode 0.00) and, hence, can be assumed as random white noise. Both of these figures indicate the model's suitability.

=

=

=

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_ _~ _ ~ ~ ~ _ ~ ~ u 0 1 0.2

model reSiduals (measured - predICted)

Figure 14. Probability distribution plot for model residuals. CONCLUSIONS Analysis of 48 months of data on the Alsamra Wastewater Stabilization Ponds in Jordan resulted in the following major conclusions. There was a strong correlation between the BOD associated with solids in the influent wastewater (C BOD • SBOD) and the suspended solids concentration (X ss ). The The percentage BOD removal efficiency in teh first anaerobic pond of the system (ECBOO,I) was equal to the COD removal efficiency CECOO,I) and to 0.74 of the suspended solids removal efficiency (Exss I)' Very little improvement in BOD removal efficiency (EcBOD,I) or suspended solids removal efficiency (ExSS,I) occurred in the first anaerobic pond of the system as a result of increasing water temperature over the range 12-28 cC. With a retention time in excess of four days, it was evident that the effect of retention time overshadowed the influence of temperature on the performance of the anaerobic ponds at Alsamra. The perfo"?~ce efficiency of the Alsamra anaerobic ponds was primarily controlled by the raw wastewater ch:uacte~sucs. The higher the raw wastewater solids settleability (R SM )' the higher the BOD removal effiCiency m the anaerobic ponds at Alsamra. At higher temperatures, the soluble BOD released as a result of anaerobic decomposition of benthic sludge was greater than at low temperatures, when sludge decomposition became negligible. The BOD removal efficiency in the anaerobic ponds at Alsamra was mainly dependent on suspended solids removal. Heavy rainfall, causing high jet velocity in the Alsamra ponds, significantly reduced suspended solids removal efficiency in the ftrst two anaerobic ponds compared with that during normal flow conditions. REFERENCES Arceiva\a, S. J. (1973). Si"'P/~ Waslt Tr~almelll M~/lwds: A~rat~d LagOOrlS. Oxidation Ditch~s. Slabilization Ponds ill Warm Temperature Climates. Ankara, Turkey. Arceiva\a, S. J. (1981). Was/~Waltr Trtalm~1Il and Disposal: Engine~ri"g and Ecology in Pollulion Colllrol. Marcel Dekker, Base~ New York, USA.

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Artbur, 1. P. (1983). NOIU on the Design and Operation of Waste Stabilization Ponds in Warm Climates ofDeveloping Countries. World Bank, Technical Paper, No.7. Bradley, R. M. and Silva, S. A. (1976). Stabilization lagoons mcluding experience in Brazil. Effluent a1ld Water Treatment Jour., 16(12),619-625. Ellis, K. V. (1980). Stabilization ponds - water quality, preliminary treatment and pre-treatment. In: Waste Stabilization Ponds. Design and Operation. WHOIEMRO. Technical Publication No.3, pp. 181-206. Ellis, K. V. (1983). Stabilization ponds: design and operation. CRC Critical Reviews in Environmental Control, 13(2), 69-102. Fisber, C. P.• Drynan, W. L. and Van Fleet, G. L. (1968). Waste stabilization pond practices in Canada. In: Advances in Water Quality Improvement. Water Resources Symposium No. I. Gloyna, E. F. and Bekenfelder, W. W. (eds). University of Texas, Austin. TX, USA. G1oyna, E. F. (1971). Waste Stabilization Ponds. WHO. Monograph, Series 60. Geneva, Switzerland. GIoyna, E. F. () 984). Waste Stabilization Pond Design Considerations. Public lecture, Amman.lordan. Hammer, M.l. and Jacobson. D. C. (1970). Anaerobic lagoon treatment of packingbouse wastewater. In: Proc. ofthe Sec01ld Int. Symposium on Waste Treatment Lagoons. University of Kansas. USA, pp. 347-354. Malina, 1. F. and RillS, R. A. ()976). Anaerobic ponds. In: Ponds as a Wastewater Treatment Allernative, Gloyna, E. F.• Malina, J. F. and Davis, E. M. (eds). Water Resources Symposium Number Nine, Center for Research in Water ResourceS, The University of Texas, Austin, USA. Mara, D. D. (1976). Sewage Treatment in Hot Climates. John Wiley and Sons, Chichester. UK. Mara, D. D. and Pearson. H. ()986). Artificial freshwater environment: Waste stabilization ponds. In: Biotechnology - A Comprehensive Treatise, Rehm, H. and Reed, R. (ed.). Vol. 8. 177-206. Meiring. P. G.• Drews, R. J .• Van Bek. H. and Stander. G. J. (1968). A Guide to the Use of P01ld Systems in South Africa for the Purification ofRaw a1ld Partiolly Treated Sewage. Report WAT 34. National Institute for Water Research, South Aliica. Metcalf & Eddy. Inc. ()979). Wastewater Engineering: Treatment, Disposal a1ld Reuse, 2nd edn.• McGraw Hill, New York, USA. Oswald, W. J.• Golueke. C. G. and Tyler. R. W. (1967). Integrated pond systems for subdivisions. Jour. WPCF.39(8), 1289. Parker, C. D.• Jones, H. L. and Greene. N. G. () 959). Performance of large sewage lagoons at Melbourne, Australia. Sewage a1ld I1Idustriai Wastes, 31(2). 133. Saqqar. M. M. and Pescod, M. B. (l992a). Modelling coliform reduction in wastewater stabilization ponds. Water Science 4<

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Saqqar, M. M. and Pescod, M. B. 0992b). Modelling nematode egg elimination in wastewater stabilization ponds. Water Science & Technology, 26(7-8), 1659-1665. Vincent, J. L.• Algie, W. E. and Marais. G. V. (1963). A system of sanitation for low cost high density housing. In: Proc of a Symposium on Hygiene a1ld Sanitation in Relation to Housing. Commission for Technical Cooperation, South of !be Sahara, Publication No. 84. 135-172. White, 1. E. (1970). Current design criteria for anaerobic lagoons. In: 211d International Symposium/or Waste Treatment Lagoons. University of Kansas, USA. pp. 360-363. WHO (1987). Wastewater Stabilization Ponds: Principles ofPlanning a1ld Practice. WHOIEMRO Technical Publication No. 10.