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A hybrid waste stabilization pond and wastewater storage and treatment reservoir system for wastewater reuse for both restricted and unrestricted crop irrigation
PII: S0043-1354(98)00238-3
Wat. Res. Vol. 33, No. 2, pp. 591±594, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
TECHNICAL NOTE A HYBRID WASTE STABILIZATION POND AND WASTEWATER STORAGE AND TREATMENT RESERVOIR SYSTEM FOR WASTEWATER REUSE FOR BOTH RESTRICTED AND UNRESTRICTED CROP IRRIGATION M D. D. MARA1** and H. W. PEARSON2
Department of Civil Engineering, School of Civil Engineering, University of Leeds, Leeds LS2 9JT, U.K. and 2School of Biological Sciences, University of Liverpool, Liverpool L69 7ZB, U.K.
1
(First received September 1997; accepted in revised form May 1998) AbstractÐA hybrid waste stabilization pond-wastewater storage and treatment reservoir system is proposed to produce microbiologically safe euents for both restricted and unrestricted crop irrigation. The wastewater is treated in an anaerobic and facultative pond (and in a maturation pond if the number of human intestinal nematode eggs in the raw wastewater so require). During the non-irrigation season the WSP euent is used to ®ll a single WSTR, and during the irrigation season the WSP euent is used for restricted irrigation and the WSTR contents for unrestricted irrigation. A typical design example shows that the WSTR is ®nancially viable, with an internal rate of return of 58 percent. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐlagoonins, reservoirs, helminth eggs, faecal coliforms, reuse, irrigation
INTRODUCTION
Wastewater reuse for crop irrigation is becoming increasingly common, especially in arid and semiarid areas (Bartone and Arlosoro, 1987). The public health aspects of wastewater reuse are now well understood (Shuval et al., 1986), and the recommendations of the WHO (1989) (61 human intestinal nematode egg per litre and, in the case of unrestricted irrigation only, 61000 faecal coliform bacteria per 100 ml) have recently been shown to pose health risks some 10±1000 times lower than those resulting from the consumption of potable water (Shuval et al., 1997). WHO (1989) recognised that wastewater treatment in waste stabilization ponds (WSP) was an eective and low-cost way to meet its microbiological quality guidelines for wastewater reuse. However, WSP euents can only be used for crop irrigation during the irrigation season. At other times of the year they have to be discharged, essentially to waste, into surface watercourses, unless they are used for aquifer recharge or stored in a reservoir for use during the next irrigation season. Aquifer recharge is generally too complex an operation in most developing countries, and even in those countries in which it is practised, such as Israel, it is not a straightforward *Author to whom all correspondence should be addressed. 591
process (Idelovitch, 1978). The use of wastewater storage and treatment reservoirs (WSTR) is generally preferable as their operation is simpler. WSTR were developed in Israel where their contents are used mainly for restricted irrigation (Juanico and Shelef, 1991, 1994). In such cases a single WSTR preceded by an anaerobic pond (Fig. 1(a)) was sucient to ensure the removal of any intestinal nematode eggs to well below the WHO guideline value. However, the operational regime used with these systems involved the WSTR being ®lled and drawn down simultaneously during the irrigation season, and this had the disadvantage of introducing an increasingly large proportion of anaerobic pond euent into the well treated WSTR contents, such that towards the end of the irrigation season Ð i.e. closest to the time of crop harvest Ð the irrigation water was of increasingly poorer quality (Liran et al., 1994). Mara and Pearson (1992) proposed a system of three or four sequential batch-fed WSTR in parallel to produce a treated wastewater suitable for unrestricted irrigation (Fig. 1(b)): each reservoir is operated on a cycle of ®ll-rest-use, with faecal coliform die-o to below 1000 per 100 ml occurring in the ®ll and rest phases (Pearson et al., 1996). However, this system is perhaps operationally rather demanding. In this paper we present a hybrid WSP-WSTR system which is both simple to operate and ¯exible in the euent
592
Technical Note
Fig. 1. Wastewater storage and treatment reservoir options: (a) single WSTR for restricted irrigation; (b) sequential batch-fed WSTR in parallel for unrestricted irrigation; and (c) hybrid WSP-WSTR system for both restricted and unrestricted irrigation. A: anaerobic pond; F: facultative pond (a maturation pond may also be necessary in (c) Ð see text).
quality produced. Importantly it minimizes land area requirements whilst ensuring that the entire annual supply of wastewater is available for irrigation, so maximizing crop production.
HYBRID WSP-WSTR SYSTEM
The hybrid WSP-WSTR system shown in Fig. 1(c) is a novel method of wastewater treatment and storage. The wastewater is ®rst treated in a series of WSP. During the non-irrigation season the WSP euent is discharged into a single WSTR, which has a volume equal to that of the wastewater produced during this period. During the irrigation season the WSP euent is used for either restricted or unrestricted irrigation (depending on its faecal coliform level: see design example below) and the WSTR contents for unrestricted irrigation. This strategy ensures the safety of the treated wastewater for both restricted and unrestricted irrigation as it separates the WSP euent and WSTR contents into two streams prior to reuse. It also prevents the contamination of the contents of a single WSTR with partially treated wastewater during the irrigation season, as found in Israel by Liran et al. (1994).
Design example A hybrid WSP-WSTR system is to be designed for an irrigation season of six months. Assume that a unit daily ¯ow of (Q) of 10,000 m3 of wastewater with a BOD5 (Li) of 300 mg lÿ1 is to be treated, and that the mean temperature of the coolest month of the year is 128C and that the mean temperature of the coolest month in the irrigation season is 258C when the net evaporation rate (e) is 10 mm dÿ1. The number of faecal coliforms is taken as 5 107 per 100 ml and the number of human intestinal nematode eggs as 1000 per litre. The design procedure follows that given in Mara et al. (1992) and Mara (1997). Anaerobic pond The design volumetric BOD5 loading rate for 128C is 140 g mÿ3 dÿ1. Thus, for a BOD5 of 300 mg lÿ1, the retention time is 2.1 d and, for a ¯ow of 10,000 m3 dÿ1, the volume is 21,500 m3; assuming a depth of 4 m, the mid-depth area is 5,375 m2. Using the design equation of Ayres et al. (1992) for egg removal, the percentage removal achieved by a 2.1-day pond is 85. Thus the number of eggs leaving the anaerobic pond is 150 lÿ1. At 128C BOD5 removal is 44 percent, so the BOD5 of the anaerobic pond euent is 170 mg lÿ1.
Technical Note
593
Facultative pond
Financial analysis
The design surface BOD5 loading rate (ls) for 128C is 124 kg haÿ1 dÿ1, and the area is given by:
Using discounted cash ¯ow analysis, it is straightforward to determine the ®nancial viability of the WSTR. Assume that its construction cost is US$ 2 millions and that this cost is incurred in year 1; that the irrigation water demand is 1.5 m over the ®ve month irrigation season Ð thus the WSTR contents can be used to irrigate some 116 ha; and that the net crop value (=sale value less cost of production and O&M costs of the WSTR) is US$ 10,000 per ha (based on recent ®gures from northeast Brazil for salad crops and new potatoes). Thus, for a discount rate of 10 percent and discounting over a 20year period, for example, the sum of the present values of future costs is US$2 millions and that of future bene®ts is US$9.71 millions. The bene®t/cost ratio is therefore 4.9, so the WSTR is certainly ®nancially viable. The internal rate of return (the discount rate at which the discounted costs and discounted bene®ts are equal) is, for this example, 58 percent.
Af 10LiQ=ls 10 170 10,000=124 137,000 m2 :
1
The retention time is given by: yf Af D=Q
2
where D = facultative pond depth, m (taken here as 1.75 m).Thus: yf 137,000 1:75=10,000 24 d: The corresponding egg removal is 99.96 percent, so the number of eggs in the facultative pond euent is (0.0004 150), i.e. <<1 lÿ1 and so that WSP euent is suitable for restricted irrigation. During the irrigation season the euent ¯ow, allowing for evaporation, would be 8,630 m3 dÿ1 (i.e., an evaporative loss of 14 percent). Wastewater storage and treatment reservoir The WSTR must store the facultative pond euent for six months, so its volume is (8630 365/2), i.e. 1,575,000 m3. Assuming a depth of 10 m, its area is 157,500 m2. Allowing a minimum rest period at 258C of one month (Mara et al., 1996), the WSTR contents are available for unrestricted irrigation for ®ve months. Design summary The design comprises an anaerobic and a facultative pond and a single WSTR, each with the following mid-depth areas, depths and retention times: Anaerobic pond: 0.54 ha, 4 m, 2.1 d Facultative pond: 13.70 ha, 1.75 m, 24 d WSTR: 15.75 ha, 10 m, 183 d. Inclusion of a maturation pond The faecal coliform count of the facultative pond in the above design example can be readily determined, using Marais' (Marais, 1974) equations, as 23,000 per 100 ml at 258C, which is the design temperature for the irrigation season. To produce an euent suitable for unrestricted irrigation (61000 faecal coliforms per 100 ml) would require a 3.6day maturation pond with a mid-depth area of 20,700 m2 and a depth of 1.5 m; the size of the maturation pond is minimised since its euent is required to have 61000 faecal coliforms only during the irrigation season (in winter, when the euent has >1000 per 100 ml, it is used to ®ll the WSTR). Thus, at relatively little additional cost, both the WSP euent and the WSTR contents could be safely used for unrestricted irrigation. Local agricultural requirements will normally determine whether the maturation pond should in fact be included.
CONCLUSIONS
(1) The hybrid WSP-WSTR system can be successfully used for the production of microbiologically safe euents for both restricted and unrestricted irrigation. The WSP euent is used to ®ll the WSTR in the non-irrigation season and for restricted irrigation in the irrigation season, when the WSTR contents are used for unrestricted irrigation. If the WSP system includes a maturation pond (or ponds), then the whole year's wastewater can be treated for use for unrestricted irrigation. (2) The area of land irrigated, and hence the quantity of crops produced, are greatly increased. Wastewater is conserved for microbiologically safe reuse, rather than being discharged to the environment. (3) The ®nancial viability of the WSTR is amply demonstrated in the typical design example used. Its internal rate of return was calculated as 58 percent. (4) The system can be readily adapted to the storage and treatment of conventional secondary euents: during the non-irrigation season the conventional euent can be used to ®ll the WSTR and during the irrigation season it can be treated in a maturation pond designed to achieve the required degree of helminth egg removal to ensure its safety for restricted irrigation. AcknowledgementÐThe ®nancial support of the former Overseas Development Administration (Research Scheme R5676) is gratefully acknowledged. REFERENCES
Ayres R. M., Alabaster G. P., Mara D. D. and Lee D. L. (1992) A design equation for human intestinal nema-
594
Technical Note
tode egg removal in waste stabilization ponds. Water Research 26(6), 863±865. Bartone C. R. and Arlosoro S. (1987) Reuse of pond euents in developing countries. Water Science and Technology 19(12), 289±297. Idelovitch E. (1978) Wastewater reuse by biological±chemical treatment and groundwater recharge. Journal of the Water Pollution Control Federation 50, 2723±2740. Juanico M. and Shelef G. (1991) The performance of stabilization reservoirs as a function of design and operation parameters. Water Science and Technology 23(7± 9), 1509±1516. Juanico M. and Shelef G. (1994) Design operation and performance of stabilization reservoirs for wastewater irrigation in Israel. Water Research 28, 175±186. Liran A., Juanico M. and Shelef G. (1994) Coliform removal in a stabilization reservoir for wastewater irrigation in Israel. Water Research 28, 1305±1314. Mara D. D. (1997) Design Manual for Waste Stabilization Ponds in India. Lagoon Technology International Ltd., Leeds. Mara D. D. and Pearson H. W. (1992) Sequential batchfed euent storage reservoirs: A new concept of wastewater treatment prior to unrestricted crop irrigation. Water Science and Technology 26(7/8), 1459±1464. Mara D. D., Alabaster G. P., Pearson H. W. and Mills S. W. (1992) Waste Stabilization Ponds: A Design
Manual for Eastern Africa. Lagoon Technology International Ltd., Leeds. Mara D. D., Pearson H. W., Oragui J. I., Cawley L. R., de Oliveira R. and Silva S. A. (1996) Wastewater Storage and Treatment Reservoirs in Northeast Brazil. TPHE Research Monograph No. 12. University of Leeds (Department of Civil Engineering), Leeds. Marais G.v.R. (1974) Faecal bacterial kinetics in waste stabilization ponds. Journal of the Environmental Engineering Division, American Society of Civil Engineers 100(EE1), 119±139. Pearson H. W., Mara D. D., Arridge H. and Cawley L. R. (1996) Pathogen removal in experimental deep euent storage reservoirs. Water Science and Technology 33(7), 251±260. Shuval H. I., Adin A., Fattal B., Rawitz E. and Yekutiel P. (1986) Wastewater Irrigation in Developing Countries: Health Eects and Technical Solutions. Technical Paper No. 51. The World Bank, Washington, DC. Shuval H., Lampert Y. and Fattal B. (1997) Development of a risk assessment approach for evaluating wastewater reuse standards. Water Science and Technology 35(11± 12), 15±20. WHO (1989) Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture. Technical Report Series No. 778. World Health Organization, Geneva.
Wat. Res. Vol. 33, No. 2, pp. 591±594, 1999 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00
TECHNICAL NOTE A HYBRID WASTE STABILIZATION POND AND WASTEWATER STORAGE AND TREATMENT RESERVOIR SYSTEM FOR WASTEWATER REUSE FOR BOTH RESTRICTED AND UNRESTRICTED CROP IRRIGATION M D. D. MARA1** and H. W. PEARSON2
Department of Civil Engineering, School of Civil Engineering, University of Leeds, Leeds LS2 9JT, U.K. and 2School of Biological Sciences, University of Liverpool, Liverpool L69 7ZB, U.K.
1
(First received September 1997; accepted in revised form May 1998) AbstractÐA hybrid waste stabilization pond-wastewater storage and treatment reservoir system is proposed to produce microbiologically safe euents for both restricted and unrestricted crop irrigation. The wastewater is treated in an anaerobic and facultative pond (and in a maturation pond if the number of human intestinal nematode eggs in the raw wastewater so require). During the non-irrigation season the WSP euent is used to ®ll a single WSTR, and during the irrigation season the WSP euent is used for restricted irrigation and the WSTR contents for unrestricted irrigation. A typical design example shows that the WSTR is ®nancially viable, with an internal rate of return of 58 percent. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐlagoonins, reservoirs, helminth eggs, faecal coliforms, reuse, irrigation
INTRODUCTION
Wastewater reuse for crop irrigation is becoming increasingly common, especially in arid and semiarid areas (Bartone and Arlosoro, 1987). The public health aspects of wastewater reuse are now well understood (Shuval et al., 1986), and the recommendations of the WHO (1989) (61 human intestinal nematode egg per litre and, in the case of unrestricted irrigation only, 61000 faecal coliform bacteria per 100 ml) have recently been shown to pose health risks some 10±1000 times lower than those resulting from the consumption of potable water (Shuval et al., 1997). WHO (1989) recognised that wastewater treatment in waste stabilization ponds (WSP) was an eective and low-cost way to meet its microbiological quality guidelines for wastewater reuse. However, WSP euents can only be used for crop irrigation during the irrigation season. At other times of the year they have to be discharged, essentially to waste, into surface watercourses, unless they are used for aquifer recharge or stored in a reservoir for use during the next irrigation season. Aquifer recharge is generally too complex an operation in most developing countries, and even in those countries in which it is practised, such as Israel, it is not a straightforward *Author to whom all correspondence should be addressed. 591
process (Idelovitch, 1978). The use of wastewater storage and treatment reservoirs (WSTR) is generally preferable as their operation is simpler. WSTR were developed in Israel where their contents are used mainly for restricted irrigation (Juanico and Shelef, 1991, 1994). In such cases a single WSTR preceded by an anaerobic pond (Fig. 1(a)) was sucient to ensure the removal of any intestinal nematode eggs to well below the WHO guideline value. However, the operational regime used with these systems involved the WSTR being ®lled and drawn down simultaneously during the irrigation season, and this had the disadvantage of introducing an increasingly large proportion of anaerobic pond euent into the well treated WSTR contents, such that towards the end of the irrigation season Ð i.e. closest to the time of crop harvest Ð the irrigation water was of increasingly poorer quality (Liran et al., 1994). Mara and Pearson (1992) proposed a system of three or four sequential batch-fed WSTR in parallel to produce a treated wastewater suitable for unrestricted irrigation (Fig. 1(b)): each reservoir is operated on a cycle of ®ll-rest-use, with faecal coliform die-o to below 1000 per 100 ml occurring in the ®ll and rest phases (Pearson et al., 1996). However, this system is perhaps operationally rather demanding. In this paper we present a hybrid WSP-WSTR system which is both simple to operate and ¯exible in the euent
592
Technical Note
Fig. 1. Wastewater storage and treatment reservoir options: (a) single WSTR for restricted irrigation; (b) sequential batch-fed WSTR in parallel for unrestricted irrigation; and (c) hybrid WSP-WSTR system for both restricted and unrestricted irrigation. A: anaerobic pond; F: facultative pond (a maturation pond may also be necessary in (c) Ð see text).
quality produced. Importantly it minimizes land area requirements whilst ensuring that the entire annual supply of wastewater is available for irrigation, so maximizing crop production.
HYBRID WSP-WSTR SYSTEM
The hybrid WSP-WSTR system shown in Fig. 1(c) is a novel method of wastewater treatment and storage. The wastewater is ®rst treated in a series of WSP. During the non-irrigation season the WSP euent is discharged into a single WSTR, which has a volume equal to that of the wastewater produced during this period. During the irrigation season the WSP euent is used for either restricted or unrestricted irrigation (depending on its faecal coliform level: see design example below) and the WSTR contents for unrestricted irrigation. This strategy ensures the safety of the treated wastewater for both restricted and unrestricted irrigation as it separates the WSP euent and WSTR contents into two streams prior to reuse. It also prevents the contamination of the contents of a single WSTR with partially treated wastewater during the irrigation season, as found in Israel by Liran et al. (1994).
Design example A hybrid WSP-WSTR system is to be designed for an irrigation season of six months. Assume that a unit daily ¯ow of (Q) of 10,000 m3 of wastewater with a BOD5 (Li) of 300 mg lÿ1 is to be treated, and that the mean temperature of the coolest month of the year is 128C and that the mean temperature of the coolest month in the irrigation season is 258C when the net evaporation rate (e) is 10 mm dÿ1. The number of faecal coliforms is taken as 5 107 per 100 ml and the number of human intestinal nematode eggs as 1000 per litre. The design procedure follows that given in Mara et al. (1992) and Mara (1997). Anaerobic pond The design volumetric BOD5 loading rate for 128C is 140 g mÿ3 dÿ1. Thus, for a BOD5 of 300 mg lÿ1, the retention time is 2.1 d and, for a ¯ow of 10,000 m3 dÿ1, the volume is 21,500 m3; assuming a depth of 4 m, the mid-depth area is 5,375 m2. Using the design equation of Ayres et al. (1992) for egg removal, the percentage removal achieved by a 2.1-day pond is 85. Thus the number of eggs leaving the anaerobic pond is 150 lÿ1. At 128C BOD5 removal is 44 percent, so the BOD5 of the anaerobic pond euent is 170 mg lÿ1.
Technical Note
593
Facultative pond
Financial analysis
The design surface BOD5 loading rate (ls) for 128C is 124 kg haÿ1 dÿ1, and the area is given by:
Using discounted cash ¯ow analysis, it is straightforward to determine the ®nancial viability of the WSTR. Assume that its construction cost is US$ 2 millions and that this cost is incurred in year 1; that the irrigation water demand is 1.5 m over the ®ve month irrigation season Ð thus the WSTR contents can be used to irrigate some 116 ha; and that the net crop value (=sale value less cost of production and O&M costs of the WSTR) is US$ 10,000 per ha (based on recent ®gures from northeast Brazil for salad crops and new potatoes). Thus, for a discount rate of 10 percent and discounting over a 20year period, for example, the sum of the present values of future costs is US$2 millions and that of future bene®ts is US$9.71 millions. The bene®t/cost ratio is therefore 4.9, so the WSTR is certainly ®nancially viable. The internal rate of return (the discount rate at which the discounted costs and discounted bene®ts are equal) is, for this example, 58 percent.
Af 10LiQ=ls 10 170 10,000=124 137,000 m2 :
1
The retention time is given by: yf Af D=Q
2
where D = facultative pond depth, m (taken here as 1.75 m).Thus: yf 137,000 1:75=10,000 24 d: The corresponding egg removal is 99.96 percent, so the number of eggs in the facultative pond euent is (0.0004 150), i.e. <<1 lÿ1 and so that WSP euent is suitable for restricted irrigation. During the irrigation season the euent ¯ow, allowing for evaporation, would be 8,630 m3 dÿ1 (i.e., an evaporative loss of 14 percent). Wastewater storage and treatment reservoir The WSTR must store the facultative pond euent for six months, so its volume is (8630 365/2), i.e. 1,575,000 m3. Assuming a depth of 10 m, its area is 157,500 m2. Allowing a minimum rest period at 258C of one month (Mara et al., 1996), the WSTR contents are available for unrestricted irrigation for ®ve months. Design summary The design comprises an anaerobic and a facultative pond and a single WSTR, each with the following mid-depth areas, depths and retention times: Anaerobic pond: 0.54 ha, 4 m, 2.1 d Facultative pond: 13.70 ha, 1.75 m, 24 d WSTR: 15.75 ha, 10 m, 183 d. Inclusion of a maturation pond The faecal coliform count of the facultative pond in the above design example can be readily determined, using Marais' (Marais, 1974) equations, as 23,000 per 100 ml at 258C, which is the design temperature for the irrigation season. To produce an euent suitable for unrestricted irrigation (61000 faecal coliforms per 100 ml) would require a 3.6day maturation pond with a mid-depth area of 20,700 m2 and a depth of 1.5 m; the size of the maturation pond is minimised since its euent is required to have 61000 faecal coliforms only during the irrigation season (in winter, when the euent has >1000 per 100 ml, it is used to ®ll the WSTR). Thus, at relatively little additional cost, both the WSP euent and the WSTR contents could be safely used for unrestricted irrigation. Local agricultural requirements will normally determine whether the maturation pond should in fact be included.
CONCLUSIONS
(1) The hybrid WSP-WSTR system can be successfully used for the production of microbiologically safe euents for both restricted and unrestricted irrigation. The WSP euent is used to ®ll the WSTR in the non-irrigation season and for restricted irrigation in the irrigation season, when the WSTR contents are used for unrestricted irrigation. If the WSP system includes a maturation pond (or ponds), then the whole year's wastewater can be treated for use for unrestricted irrigation. (2) The area of land irrigated, and hence the quantity of crops produced, are greatly increased. Wastewater is conserved for microbiologically safe reuse, rather than being discharged to the environment. (3) The ®nancial viability of the WSTR is amply demonstrated in the typical design example used. Its internal rate of return was calculated as 58 percent. (4) The system can be readily adapted to the storage and treatment of conventional secondary euents: during the non-irrigation season the conventional euent can be used to ®ll the WSTR and during the irrigation season it can be treated in a maturation pond designed to achieve the required degree of helminth egg removal to ensure its safety for restricted irrigation. AcknowledgementÐThe ®nancial support of the former Overseas Development Administration (Research Scheme R5676) is gratefully acknowledged. REFERENCES
Ayres R. M., Alabaster G. P., Mara D. D. and Lee D. L. (1992) A design equation for human intestinal nema-
594
Technical Note
tode egg removal in waste stabilization ponds. Water Research 26(6), 863±865. Bartone C. R. and Arlosoro S. (1987) Reuse of pond euents in developing countries. Water Science and Technology 19(12), 289±297. Idelovitch E. (1978) Wastewater reuse by biological±chemical treatment and groundwater recharge. Journal of the Water Pollution Control Federation 50, 2723±2740. Juanico M. and Shelef G. (1991) The performance of stabilization reservoirs as a function of design and operation parameters. Water Science and Technology 23(7± 9), 1509±1516. Juanico M. and Shelef G. (1994) Design operation and performance of stabilization reservoirs for wastewater irrigation in Israel. Water Research 28, 175±186. Liran A., Juanico M. and Shelef G. (1994) Coliform removal in a stabilization reservoir for wastewater irrigation in Israel. Water Research 28, 1305±1314. Mara D. D. (1997) Design Manual for Waste Stabilization Ponds in India. Lagoon Technology International Ltd., Leeds. Mara D. D. and Pearson H. W. (1992) Sequential batchfed euent storage reservoirs: A new concept of wastewater treatment prior to unrestricted crop irrigation. Water Science and Technology 26(7/8), 1459±1464. Mara D. D., Alabaster G. P., Pearson H. W. and Mills S. W. (1992) Waste Stabilization Ponds: A Design
Manual for Eastern Africa. Lagoon Technology International Ltd., Leeds. Mara D. D., Pearson H. W., Oragui J. I., Cawley L. R., de Oliveira R. and Silva S. A. (1996) Wastewater Storage and Treatment Reservoirs in Northeast Brazil. TPHE Research Monograph No. 12. University of Leeds (Department of Civil Engineering), Leeds. Marais G.v.R. (1974) Faecal bacterial kinetics in waste stabilization ponds. Journal of the Environmental Engineering Division, American Society of Civil Engineers 100(EE1), 119±139. Pearson H. W., Mara D. D., Arridge H. and Cawley L. R. (1996) Pathogen removal in experimental deep euent storage reservoirs. Water Science and Technology 33(7), 251±260. Shuval H. I., Adin A., Fattal B., Rawitz E. and Yekutiel P. (1986) Wastewater Irrigation in Developing Countries: Health Eects and Technical Solutions. Technical Paper No. 51. The World Bank, Washington, DC. Shuval H., Lampert Y. and Fattal B. (1997) Development of a risk assessment approach for evaluating wastewater reuse standards. Water Science and Technology 35(11± 12), 15±20. WHO (1989) Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture. Technical Report Series No. 778. World Health Organization, Geneva.