9. Wastewater use case studies

9.1 Advanced wastewater treatment: California, USA
9.2 Wastewater treatment in stabilization ponds: Al Samra, Jordan
9.3 Soil-aquifer treatment: Arizona, USA
9.4 Wastewater treatment and crop restriction: Tunisia
9.5 Wastewater treatment and human exposure control: Kuwait
9.6 Crop restriction for wastewater irrigation: Mexico
9.7 Wastewater use in aquaculture: Calcutta, India

9.1 Advanced wastewater treatment: California, USA

9.1.1 Reclaimed wastewater uses
9.1.2 Wastewater reclamation criteria
9.1.3 Wastewater treatment
9.1.4 Monterey wastewater reclamation study for agriculture

9.1.1 Reclaimed wastewater uses

Beneficial use of wastewater has been practised in California since the 1890s, when raw sewage was applied on 'sewer farms'. By 1987, more than 0.899 Mm³/d of municipal wastewater (7-8% of the production) were being used for the applications indicated in Figure 20 (California State Water Resources Control Board 1990). Historically, agricultural use has dominated, and continues to do so, but over the past decade reclaimed wastewater has been increasingly used for landscape irrigation in urban areas and for groundwater recharge. Most of the reclaimed water (78%) is used in the Central Valley and South Coastal regions of California. Two hundred reclamation plants throughout California produce the volume of treated effluent indicated above and save 0.759 Mm³/d of fresh water. The major wastewater reclamation systems are shown in Table 31. In agricultural use of treated effluent, at least twenty different food crops are being irrigated as well as at least eleven other crops and nursery products, as indicated in Table 32.

Figure 20: Types of reuse in California in 1987 (California State Water Resources Control Board 1990)

9.1.2 Wastewater reclamation criteria

Wastewater reclamation criteria have been in force in California since 1978, as issued by the California Department of Health Services (1978). For surface irrigation of food crops the requirement is for the effluent to be adequately disinfected and oxidized so that the median number of coliform organisms does not exceed 2.2 per 100 ml over 7 days, except that orchards and vineyards may be surface irrigated with effluent having a quality equivalent to that of primary effluent. Reclaimed wastewater use for spray irrigation of food crops must be at all times adequately disinfected, oxidized, coagulated, clarified, filtered wastewater with a bacteriological quality such that the 7-day median number of coliform organisms does not exceed 2.2 per 100 ml and the number of coliform organisms does not exceed 23 per 100 ml in more than one sample within any 30-day period. Exceptions to these quality requirements may be allowed by the State Department of Health on an individual case basis where the food crop is to undergo extensive commercial physical or chemical processing sufficient to destroy pathogenic agents before human consumption. For irrigation of fodder, fibre and seed crops the wastewater need only have received primary treatment. However, reclaimed wastewater used to irrigate pasture to which milking cows or goats have access must be at all times adequately disinfected and oxidized to achieve a median number of coliform organisms not exceeding 23 per 100 ml over 7-days.

Table 31: MAJOR WATER RECLAMATION SYSTEMS IN CALIFORNIA

Wastewater Treatment Plant Name	Reclaimed water deliveries m3/d
San Jose Creek WRP	67 101
City of Bakersfield WTP #2	56875
Whittier Narrows WRP	53 648
City of Modesto	48 630
Fresno-Clovis Metropolitan Area Regional Wastewater Facilities	46284
Pomona WRP	32435
Laguna TP	31 560
Michelson WRP	29536
City of Bakersfield WTP #3	26447
City of Tulare WPCF	21 114
Lancaster WRP	18 539
South Tahoe PUD STP	17 184
Total	449 355
Percent of Statewide Total	50

The quality requirements for irrigation of golf courses, cemeteries, freeway landscapes and landscapes in other areas where the public has similar access or exposure are that the effluent should at all times be disinfected and oxidized to a median number of coliforms not exceeding 23 per 100 ml over 7 days and a number of coliforms not exceeding 240 per 100 ml in any two consecutive samples. More stringent quality requirements are applied to reclaimed wastewater used to irrigate parks, playgrounds, schoolyards and other areas where the public has similar access or exposure; here an adequately disinfected, oxidized, coagulated, clarified, filtered wastewater is required, or a wastewater treated by a sequence of unit processes assuming an equivalent degree of treatment and reliability. The effluent quality requirements are a median number of coliforms not exceeding 2.2 per 100 ml over 7 days and a limit of 23 coliforms per 100 ml in any sample. A similar quality is required for reclaimed wastewater used as a source of supply in nonrestricted recreational impoundments but in restricted recreational impoundments the wastewater should be disinfected and oxidized to a median number of coliforms not exceeding 2.2 per 100 ml over 7 days. Reclaimed water used as a source of supply in a landscape impoundment should be disinfected and oxidized to a median number of coliforms not exceeding 23 per 100 ml over 7 days.

For groundwater recharge of domestic water supply aquifers by surface spreading, the reclaimed wastewater must be at all times of a quality that protects public health. The State Department of Health Services advises Regional Water Quality Control Boards on an individual case basis for proposed groundwater recharge projects and for expansion of existing projects. Recommendations are based on all relevant aspects of each project, including treatment provided, effluent quality and quantity, spreading area operations, soil characteristics, hydrogeology, residence time and distance to withdrawal. A public hearing is held prior to making the final determination regarding the public health aspects of each groundwater recharge project.

Table 32: TYPES OF CROPS IRRIGATED WITH RECLAIMED WATER IN CALIFORNIA

Food crops		Non-food crops
Apples	Grapes	Alfalfa
Asparagus	Lettuce	Christmas trees
Avocados	Maize	Clover
Barley	Peaches	Cotton
Beans	Peppers	Eucalyptus trees
Broccoli	Pistachios	Flower seeds
Cabbage	Plums	Hay
Cauliflower	Squash	Maize
Celery	Sugarbeets	Sod
Citrus	Wheat	Trees
		Vegetable seeds

9.1.3 Wastewater treatment

The Office of Water Recycling of the California State Water Resources Control Board recognizes four levels of treatment beyond primary treatment, based on the unit processes and on the types of effluent use taking place:

i. Secondary treatment in stabilization ponds, including disinfection if provided

ii. Other secondary treatment, for example by the activated sludge process, including disinfection if provided

iii. Title 22 tertiary treatment, using filtration and other processes intended to comply with the requirement in the reclamation criteria, published in Title 22 of the California Code of Regulations (California Department of Health Services 1978) for adequately disinfected, oxidized, coagulated, clarified, filtered wastewater, or approved equivalent. Usually secondary effluent is treated by the approved equivalent of 'direct filtration', that is, coagulent addition and mixing directly followed by filtration.

iv. Other tertiary treatment, consisting of any process following secondary treatment, except tertiary treatment intended to comply with wastewater reclamation criteria in Title 22 of the California Code of Regulations.

In the survey reported in the review of California municipal wastewater reclamation in 1987 (California State Water Resources Control Board 1990) all the wastewater treatment plants producing effluents for beneficial uses were found to provide at least secondary treatment. With one exception, chlorination was believed to have been the sole method of disinfection applied. Tertiary treatment processes falling under category (iv) above were found to include filtration, carbon adsorption, denitrification, air stripping and reverse osmosis. A summary of the treatment levels provided for specific types of effluent use is given in Table 33.

9.1.4 Monterey wastewater reclamation study for agriculture

The Monterey Wastewater Reclamation Study for Agriculture (MWRSA) was a 10-year, US $7.2 million field-scale project designed to evaluate the safety and feasibility of irrigating food crops (many eaten raw) with reclaimed municipal wastewater (Sheikh et al. 1990). Demonstration fields at Castroville in the lower Salinas Valley, California were used to study full-scale farm practices using reclaimed municipal wastewater. Two 5 hectare experimental plots provided large amounts of data on crop response which were subjected to statistical analysis. On one plot artichokes were grown, while on the other a succession of broccoli, cauliflower, lettuce and celery was raised over a 5-year period starting in late 1980.

Secondary effluent from the 1500 m³/d Castroville wastewater treatment plant of the Monterey Regional Water Pollution Control Agency was upgraded in a pilot tertiary reclamation plant before being used to irrigate the plots. Two parallel tertiary treatment processes were used, complete treatment in a 'Title 22' (T-22) process and a direct filtration process, termed the 'filtered effluent' (FE) process, both systems being shown in Figure 21. The T-22 train included coagulation, clarification, filtration and disinfection, the full treatment process required for spray irrigation of food crops in the Wastewater Reclamation Criteria (California Department of Health Services 1978). Alum dosages of 50 to 200 mg/l and polymer dosage of 0.2 mg/l were applied in this process. In the FE process, low alum dosages between 0 and 15 mg/l and polymer dosages from 0 to 0.18 mg/l were applied with a combination of either static or mechanical rapid mixing and dual-media gravity filtration at 3.4 l/m²s. The chlorine contact tank had a 90 minute theoretical retention time. Flocculation chambers were added to the FE process in October 1983 to enhance floe formation prior to filtration, producing a filtered effluent flow stream designated FE-F. Dechlorination of final effluent with sulphur dioxide was practised over the first three years but was discontinued in June 1983 to determine the effects, if any, of a chlorine residual on crops and to prevent microbial regrowth. No adverse effects of chlorine residual on crops was observed and further microbial regrowth in storage tanks and pipelines was prevented.

Table 33: LEVELS OF WASTEWATER TREATMENT PROVIDED IN CALIFORNIA FOR TYPES OF EFFLUENT USE

Type of effluent use	Number of water reclamation plants providing indicated treatment
	Oxidation ponds	Other secondary	Title 22 tertiary	Other tertiary	Total
Agricultural Irrigation:
Harvested feed, fibre and seed crops	12	20	1	1	34
Pasture	23	25	4	3	55
Orchards and vineyards	3	4	2	1	10
Tree crops (Christmas trees, firewood, pulp, etc.)	2	1	0	0	3
Nursery and sod crops	0	3	4	1	8
Food crops	0	2	1	0	3
Mixed, other or unknown types of agricultural products	11	19	3	3	36
Landscape Irrigation:
Schools, playgrounds, parks where Title 22 tertiary effluent required	0	0	7	2	9
Freeway and highway landscape	0	0	8	4	12
Golf courses (including golf course impoundments)	4	13	24	8	49
Mixed, other or unknown types of landscape (including street landscape, slope cover,parks where tertiary effluent not required)	2	6	13	3	24
Landscape Impoundments (excluding golf courses)	0	0	1	0	1
Recreational Impoundment	0	1	3	0	4
Wildlife Habitat Enhancement, Wetlands	1	2	2	0	5
Industrial Use:
Cooling water	0	1	2	2	5
Process water	0	0	1	0	1
Construction, dust control, washdown	1	1	1	1	4
Other or unknown types of industrial use	0	1	0	0	1
Groundwater Recharge	0	0	5	0	5
Miscellaneous or unknown types of use or mixed types of above uses	1	4	5	1	11
TOTAL	60	103	87	30	2801

¹ Total exceeds actual number of treatment plants because some plants serve several types of reuse.

Source: California State Water Resources Control Board 1990

Figure 21: Schematic diagrams of tertiary treatment systems used in the MWRSA (Sheikh et al. 1990)

A split plot experimental design allowed the study of two treatment variables: irrigation water type (T-22 effluent, FE effluent and well water) and fertilization rate (no fertilizer and 33%, 66% and 100% of full local fertilizer rate for the crop irrigated). The artichokes were fertilized four times a year and the fertilization regimes for the other row crops varied with each crop's requirements but all row crop areas received an application of fertilizer before planting. Analysis of variance was used to determine if significant differences could be detected between the characteristics of the soils and plants receiving different water types and fertilization rates.

The well water used to irrigate the plots was chemically satisfactory, consistently exhibiting adjusted SAR (SAR_adj) values of less than 4; for the soil at the MWRSA site (a 50% montmorillonite and 50% illite-vermiculite clay mixture) an SAR_adj, of 7 or less is considered to pose no problem, 7-12 would cause increasing problems and greater than 12 would potentially pose a severe problem. The T-22 effluent was generally within the SAR_adj, range 7-12, indicating increasing potential problems, while the FE effluent although usually in this range occasionally exceeded 12, with the potential for severe problems. Salinity in the reclaimed effluents was correspondingly high (611-1621 mg/l) but not so high as to cause soil permeability problems. The reclaimed effluents contained very low levels of heavy metals, an order of magnitude lower than the metal input from impurities in commercial fertilizers. All three types of irrigation water, including the well water, periodically exhibited high total coliform levels. Both the T22 and FE processes were capable of producing reclaimed water meeting the most stringent of the California Wastewater Reclamation Criteria (2.2 MPN coliforms/100 ml) most of the time. Ascaris lumbricoides, Entamoeba histolytica or other parasites were never detected in any of the irrigation waters. During the five years of the field study, the quality of both reclaimed effluents improved as a result of improving treatment plant operations and reclaimed water storage procedures.

In nearly all cases the relative values of chemical constituents of the soil followed the same relative value relationships in the irrigation waters. Chemical parameter concentrations were generally highest in the FE-irrigated soil samples and lowest in samples from the well water-irrigated soil. Higher fertilizer application rates were found to have effects on the concentrations of various soil chemical parameters similar to that of effluent irrigation. None of the data indicated that the soils irrigated with the three waters were being adversely affected and the reclaimed wastewater effluents did not have harmful effects on the soil. Heavy metals concentrations in the irrigated soils were not found to be a problem. The levels of total and faecal coliforms in soils irrigated with the two reclaimed effluents were similar to levels in the well water-irrigated soils and no parasites were ever detected in soil samples.

Analysis of plant edible tissues for heavy metals proved that there was no consistently significant difference between concentrations in plants irrigated with reclaimed wastewater effluents and in those irrigated with well water. In addition, the metal content of artichoke tissues from neighbouring fields showed no relationship with distance from the site of the plots. Analysis of residual tissues produced results similar to those for edible tissues except for accumulation of zinc (higher in edible tissues for all vegetables studied) and cadmium (higher in residual tissues). The levels of total and faecal conforms in plant tissues irrigated with all three waters were generally comparable. No consistently significant difference attributable to water type was observed and the same applied to the presence of parasites, which were detected in plant tissue only during the first year of the study.

Statistically significant differences in crop yield due to irrigation water type were observed in the cases of celery and broccoli, both crops giving higher yields with reclaimed wastewater irrigation. Yields of lettuce and celery showed interaction of water type and fertilization, with reclaimed wastewater irrigation improving yields in unfertilized plots but having little or no effect on plots receiving fertilizer. Artichoke yields were similar with all three irrigation water types. Yields of all five crops levelled off at or below 66% of the standard local fertilizer application rate and application of the full (100%) local fertilization rate did not improve yields further. It would appear that reductions of up to 33% of fertilizer application could be possible when reclaimed wastewater is used for irrigating these crops. Field inspection of crops showed no leaf damage due to residual chlorine in the effluents and no differences in appearance or vigour of plants irrigated with different water types. In cold storage tests for periods up to 4 weeks following harvest, no unexpected deterioration of produce was observed. The quality and shelf life of all the produce irrigated with the reclaimed wastewaters was as good as, and in some instances superior to, the produce irrigated with well water.

The results of this 5-year study have indicated that use of tertiary treated wastewater for food crop irrigation is safe and acceptable. No adverse impacts in terms of soil or groundwater quality degradation were observed. Conventional farming practices were shown to be adequate and the marketability of the produce did not appear to pose any problems. No project-related health problems were detected through medical examinations and the serum banking programme routinely conducted for the project personnel.

9.2 Wastewater treatment in stabilization ponds: Al Samra, Jordan

9.2.1 Septage pretreatment and wastewater transmission
9.2.2 Al Samra stabilization ponds
9.2.3 Performance of Al Samra stabilization ponds

The Al Samra Wastewater Stabilization Pond (WSP) System was commissioned in May 1985 and by 1986 was receiving approximately 57 000 m³/d of domestic wastewater and septage from the Metropolitan Area of Greater Amman, Jordan. In addition to the WSP facility, which is about 40 kilometres northeast of Amman, the system comprises a septage receiving and pretreatment installation, an inverted siphon 38.6 kilometres long and a raw wastewater pumping station.

9.2.1 Septage pretreatment and wastewater transmission

Before wastewater from the Greater Amman area enters the siphon, a septage receiving and pretreatment installation allows the sludge removed from septic tanks and cesspits to be mixed with the wastewater. At Ain Ghazal, a site in Amman primarily occupied by an abandoned activated sludge treatment plant, an average of 5000 m³/d of septage is discharged into an aerated grit chamber. A typical composition of the septage is 1600 mg/l BOD₅, 5700 mg/l COD and 2600 mg/l Suspended Solids. Also located at Ain Ghazal are large screening and grit removal devices for the wastewater. It is important during stormflow conditions to protect the siphon from damage from floating material, grit and large stones in the wastewater, which increases in volume to 148 000 m³/d.

The transmission pipeline from Ain Ghazal to Al Samra is an inverted siphon of 1200 mm diameter with an inlet elevation of 688m, an outlet elevation of 580 m and an elevation of 460 m at its lowest point. It is made of welded steel 8.3 mm thick with a 25 mm concrete lining and has an ultimate capacity of 220,000 m³/d. There are facilities at the low point for draining the siphon into a 50,400 m³ emergency storage pond, which has a capacity larger than the 45,000 m³ of wastewater in the pipe when it is flowing full. The siphon is equipped with blowoff and double acting air valves, line-size access points (at 4 km intervals), a line-size isolation valve at the low point and two flushing outlets, one on either side of the flushing valve. A foam swab and ball can be passed through the siphon from Ain Ghazal to the inlet works at Al Samra for cleaning purposes.

About 1 km upstream of the low point in the siphon a new wastewater pumping station allows wastewater from the Zarqa-Ruseifa area to be introduced into the pipeline. This pumping station had a peak pumping capacity of 14 000 m³/d in 1987 but the ultimate capacity is 72 000 m³/d.

9.2.2 Al Samra stabilization ponds

The general layout of the Al Samra wastewater stabilization ponds is shown in Figure 22 indicating three trains of ponds, each containing two anaerobic ponds, four facultative ponds and four maturation ponds. However, due to the high organic loading on the ponds, in practice the first eight ponds in each train are anaerobic and only the final two behave as facultative ponds. All the ponds are contained by embankments constructed of the indigenous soil, containing 10-20% clay, and no separate lining is provided. Details on the ponds are provided in Table 34.

Figure 22: Al Samra pond layout (Al-Salem 1987)

In order to reduce water losses by seepage and evaporation during the early phase of operation of the ponds, only two trains were used. After start-up in May 1985, effluent overflowed from the two trains by August 1985. Initially, in September 1985, the rate of seepage was estimated to be 8.54 mm/d but this declined to approximately 0.36 mm/d by December 1986. During 1986, loss of water by evaporation was 12.6%, with maximum rate of evaporation 14.4 mm/d in July and minimum 0.3 mm/d in November.

Table 34: EFFECTIVE POND SIZES AND RETENTIONS AT A FLOW RATE OF 68 000 m³/d

Pond	Total depth (m)	Effective depth (m)	3 trains			2 trains
			Area (ha)	Volume (m3x105)	Retention (d)	Area (ha)	Volume (m3x105)	Retention (d)
Al	5.0	3.0	9.5	2.85	4.2	6.3	1.90	2.8
A2	5.0	3.0	9.5	2.85	4.2	6.3	1.90	2.8
Fl	2.25	1.5	21.75	3.26	4.8	14.5	2.17	3.2
F2	2.0	1.5	21.75	3.26	4.8	14.5	2.17	3.2
F3	1.5	1.5	21.75	3.26	4.8	14.5	2.17	3.2
F4	1.5	1.5	21.75	3.26	4.8	14.5	2.17	3.2
Ml	1.25	1.25	18.75	2.34	3.4	12.5	1.56	2.3
M2	1.25	1.25	18.75	2.34	3.4	12.5	1.56	2.3
M3	1.25	1.25	18.75	2.34	3.4	12.5	1.56	2.3
M4	1.25	1.25	18.75	2.34	3.4	12.5	1.56	2.3
TOTAL			181.00	28.10	41.2	120.6	18.72	27.6

Source: Al-Salem (1987)

9.2.3 Performance of Al Samra stabilization ponds

The composition of the Amman wastewater as it enters the siphon at Ain Ghazal and as it discharges to the inlet works at Al Samra is shown in Table 35. Clearly, the transmission pipeline is acting as an anaerobic digester during the 18 hours travel time, with reductions in BOD₅, COD and SS of 14, 25 and 16%, respectively.

Table 35: COMPOSITION OF AMMAN WASTEWATER

Parameter	At entry to siphon (Ain Ghazal) mg/l	At Al Samra Inlet Works mg/l
BOD5	766	623
TSS	899	754
COD	1829	1376
CaCO3	848	645
TOC	224	193
TDS	1172	1127
Total N as N	150	103
NH4-N	101	91
Total P as P	25	22
SO4	93	60
H2S	18.2	21.9

Source: Al-Salem (1987)

The performance of the Al Samra stabilization ponds ' is influenced by temperature, with an average water temperature of 15°C in the cold season (December-March) and 24°C in the hot season (August-November). Figure 23 shows the BOD₅ removal during 1986 for both the cold and the hot seasons. Pond loadings during this year were as shown in Table 36.

In terms of overall performance in 1986, the Al Samra ponds were highly efficient, removing 80% and 91% of the incoming BOD₅ on the basis of unfiltered and filtered final effluent samples, respectively. This was the situation with only two trains of ponds in operation when the design organic loading was being exceeded by 57% and the hydraulic loading was 25% greater than design. At the same time, a 4.6 log reduction in faecal coliforms was achieved in passage through the ponds (Al-Salem 1987).

Figure 23: Summary of BOD removal through the ponds (Al-Salem 1987)

Table 36: AL SAMRA POND ORGANIC LOADINGS - 1986

Pond No.	Hot season		Cold season
	Vol. Loading g BOD5/m3d	Areal loading kg BOD5/ha.d	Vol. loading g BOD5/m3d	Areal loading kg BOD5/ha.d
A2-1	120	5925	120	5923
A2-2	50	2444	70	2376
F2-1		915
F2-2		775 (408)
F2-3		560 (307)
F2-4		477 (176)
M2-1		465 (164)
M2-2		552 (121)
M2-3		439 (118)
M2-4		544 (95)

() Figures in brackets denote loadings based on filtered BOD₅ samples.

Source: Al-Salem (1987)

The microbiological performance of the Al Samra ponds has been described in more detail for the period December 1986 to March 1987 by Saqqar and Pescod (1990). Table 37 shows total coliform and faecal coliform reductions through the pond series for the period concerned. It is clear that the final effluent (after Pond M4) did not meet the WHO (1989) guidelines figure of £ 1000 faecal coliforms/100 ml for most of the study period, in spite of having passed through the series of ponds with a minimum theoretical retention time of 34 days. The 4-month geometric means of the total and faecal coliform die-off coefficient (K_b) ranged from 0.22-0.76 and 0.11-0.68, respectively, with the level of K_b increasing through the pond sequence. Linear regression analysis of the data indicated that retention time, pond BOD₅ concentration, pH and depth had a significant effect on K_b. Data on nematode egg removal during January and February 1987 are given in Table 38, showing that nematode eggs were absent from the final effluent (Pond M4 outlet) over the period and indicating that the WHO (1989) guidelines value of £ 1/litre could be achieved with the theoretical retention time of 34 days, but not after 24.7 days (Pond F4 outlet).

Table 37: MONTHLY GEOMETRIC MEANS FOR TOTAL AND FAECAL COLIFORMS

Month	DEC 1986		JAN 1987		FEB 1987		MARCH 1987
Average monthly water temp. °C	12.1		11.8		14.9		15.1
Monthly geometric mean	Total coliforms No/100 ml	Faecal coliforms No/100 ml	Total coliforms No/100 ml	Faecal coliforms No/100 ml	Total coliforms No/100 ml	Faecal coliforms No/100 ml	Total coliforms No/100 ml	Faecal coliforms No/100 ml
Effluent of Pond Al	6.5x107	2.22x107	9.59x107	1.50x107	9.42x107	1.90x107	7.52x107	1.78x107
Effluent of Pond A2	2.59x106	9.20x105	4.28x107	6.18x106	5.57x107	1.0x107	3.23x107	7.94x106
Effluent of Pond F2	4.02x106	4.73x105	7.15x106	1.02x106	7.05x106	9.98x105	6.94x106	7.84x105
Effluent of Pond F4	6.38x105	6.53x104	1.24x106	1.76x105	8.78x105	1.12x105	6.30x105	9.65x104
Effluent of Pond M2	8.21x104	8180	2.36x105	31 020	1.28x105	17252	4.87x104	13924
Effluent of Pond M4	12289	1022	27838	4423	13 176	2631	3908	814

Source: Saqqar and Pescod (1991)

9.3 Soil-aquifer treatment: Arizona, USA

9.3.1 Project details
9.3.2 Quality improvements

9.3.1 Project details

The city of Phoenix, in south-central Arizona, has been carrying out extensive testing of experimental soil-aquifer treatment (SAT) systems since 1967. Part of the effluent from the two major sewage treatment plants in the Phoenix area, both activated sludge plants with chlorination, was intended to be renovated by the SAT process and exchanged for high quality groundwater in a nearby irrigation district, which the city would then use to augment its municipal water supply. The Phoenix SAT system was to consist of a series of infiltration basins arranged in two parallel strips with wells on a line midway between the strips, as illustrated in Figure 24. To test the feasibility of the SAT system, a small test project was installed in 1967 followed by a larger demonstration project installed in 1975. The latter project was intended to form part of a future operational project with a basin area of 48 ha and a projected capacity of about 50 million m³/year (Figure 25).

Table 38: NEMATODE* EGG COUNTS IN AL SAMRA PONDS SYSTEM

Date	Sample location
	Raw wastewater eggs/I	A2 outlet eggs/I	F4 outlet eggs/I	M4 outlet eggs/I
24/1/1987	71	7	7	0
31/1/1987	53	10	7	0
7/2/1987	50	10	10	0
14/2/1987	141	36	24	0
20/2/1987	350	10	0	0
Geometric mean	99	12	6	0**
Removal efficiency	-	88%	94%	100%

* Ascaris lumbricoides, Trichuris trichiura, Ancylostoma duedenale and Necator americanus.
** Arithmetic mean
Source: Saqqar and Pescod (1991)

Figure 24: Infiltration basin SAT system (Bouwer 1987)

The first, test, project was installed in the Salt River bed at Flushing Meadows. It consisted of six parallel, long, narrow infiltration basins of about 0.13 ha each. The soil was about 1 m of loamy sand underlain by sand and gravel layers, with the groundwater table at a depth of around 3 m. Monitoring wells 6-9 m deep were installed at various points between the basins and away from the basins. A flooding and drying schedule of 9 days flooding - 12 days drying was adopted. Renovated water was sampled from the aquifer below the basins and after it had moved laterally for some distance through the aquifer. The scheme and results are reviewed in Bouwer et al. (1974) and in Bouwer et al. (1980).

The 23rd Avenue demonstration project (Bouwer and Rice 1984) was installed in 1975 on the north side of the Salt River bed. It consisted of the 16 ha area of lagoons, shown at the west side of Figure 25, split lengthwise into four infiltration basins of 4 ha each. Here, the soil lacks the loamy sand top layer of the Flushing Meadows site and, thus, the soil profile consists mostly of sand and gravel layers. The water table depth over the period of study ranged from 5 m to 25 m but was mostly about 15 m below ground level. Monitoring wells for sampling renovated water were installed at the centre of the project to depths of 18, 24 and 30 m, and on the north side of the basin complex to depths of 22 m. In addition, a large production well (capacity about 10 000 m³/day) was drilled at the centre of the project with the casing perforated over the 30 to 54 m depth range.

A flooding and drying schedule of 14 days flooding - 14 days drying was mostly used and water depth in the basins was 15-20 cm. During flooding, infiltration rates were typically between 0.3 and 0.6 m/day, yielding a total infiltration or hydraulic loading rate of about 100 m/year. Initially, the sewage treatment plant effluent was allowed to flow through the 32 ha lagoon shown on the east of Figure 21 before entering the demonstration infiltration basins. However, this gave problems of soil clogging in the infiltration basins due to heavy growth of algae in the lagoon, especially during summer. Unicellular algae Carteria klebsii were particularly toublesome, not only forming a 'filter cake' on the bottom of the basins but also raising pH (due to CO₂ removal) and causing precipitation of CaCO₃, which further aggravated the soil clogging. Eventually, a bypass canal was constructed around the 32 ha lagoon, as shown on Figure 25, reducing the retention time from a few days to about half an hour. After the bypass channel was put into operation, hydraulic loading rates for the infiltration basins increased from 21 m/year to almost 100 m/year.

Figure 25: 23rd Avenue SAT project, Phoenix (Bouwer 1987)

At a loading rate of 100 m/year, 1 ha of infiltration basin can handle 10⁶ m³ of effluent/year. Hence, the 150 000 m³/day of effluent from the 23rd Avenue wastewater treatment plant would require 55 ha of infiltration basins. Almost all the required area could be provided by converting the 32 ha lagoon into infiltration basins, as indicated in Figure 25. The resulting 48 ha could then handle 48 million m³/year of effluent. Wells for pumping the renovated water from the aquifer could be located on the centreline through the project area (Figure 25). At a capacity of 10,000 m³/day per well, 12 wells would be required in the operational full-scale project to remove renovated water at the same rate as wastewater infiltrates into the aquifer from the basins, thus creating an equilibrium situation.

9.3.2 Quality improvements

In both experimental projects, most improvements in effluent quality occurred in the vadose zone (the unsaturated zone between the soil surface and the groundwater table). The following details on quality improvements are taken from Bouwer (1987).

Suspended solids

The suspended solids content of the renovated water at the Flushing Meadows project was less than 1 mg/l. From the 23rd Avenue project it averaged about 1 mg/l for the large production well. Most of these solids probably were fine aquifer particles that entered the well through the perforations in the casing. The suspended solids content of the secondary effluent at the 23rd Avenue project averaged about 11 mg/l.

Total dissolved solids

The total salt content of the renovated water increased slightly as it moved through the SAT system (from 750 to 790 mg/l at the 23rd Avenue project). Evaporation from the basins (including from the soil during drying) should increase the TDS content by about 2%. The rest of the increase probably was due to mobilization of calcium carbonate due to a pH drop from 8 to 7 as the effluent moved through the vadose zone.

Nitrogen

At the Flushing Meadows project, nitrogen removal from the effluent as it seeped through the vadose zone to become renovated water was about 30% at maximum hydraulic loading (100-200 m/year), but 65% when the loading rate was reduced to about 70 m/year, by using 9-day flooding-12-day drying cyles, and by reducing the water depths in the basins from 0.3 to 0.15 m. The form and concentration of nitrogen in the renovated water sampled from the aquifer below the basins were slow to respond to the reduction in hydraulic loading (Bouwer et al. 1980). In the 10th year of operation (1977), the renovated water contained 2.8 mg/l of ammonium nitrogen, 6.25 mg/l nitrate nitrogen and 0.58 mg/l organic nitrogen, for a total nitrogen content of 9.6 mg/l. This was 65% less than the total nitrogen of the secondary sewage effluent, which averaged 27.4 mg/l (most as ammonium) in that year. At the 23rd Avenue project, the total N content in the secondary sewage effluent averaged about 18 mg/l, of which 16 mg/l was as ammonium. The 2-week flooding-drying cycles must have been conducive to denitrification in the vadose zone, because the total N content of the renovated water from the large centre well averaged about 5.6 mg/l, of which 5.3 mg/l was as nitrate, 0.1 mg/l as ammonium, 0.1 mg/l as organic nitrogen and 0.02 mg/l as nitrite. The nitrogen removal was thus about 70%. This removal was the same before and after the secondary effluent was chlorinated, indicating that the low residual chlorine of the effluent by the time it infiltrated into the ground apparently had no effect on nitrogen transformations in the soil.

The flooding and drying sequence that maximizes denitrification in the vadose zone depends on various factors and must be evaluated for each particular system. Pertinent factors include the ammonium and carbon contents of the effluent entering the soil, infiltration rates, cation-exchange capacity of soil, exchangeable ammonium percentage, depth of oxygen penetration in the soil during drying and temperature. The combined laboratory and field data from the Flushing Meadows experiments showed that, to achieve high nitrogen removal percentages, the amount of ammonium nitrogen applied during flooding must be balanced against the amount of oxygen entering the soil during drying. Flooding periods must be long enough to develop anaerobic conditions in the soil. Infiltration rates must be controlled to the appropriate level for the particular effluent, soil and climate at a given site. Most of the nitrogen transformations in the Flushing Meadows studies occurred in the upper 50 cm of the vadose zone.

Phosphate

Phosphate removal increased with increasing distance of underground movement of the sewage effluent. After 3 m of downward movement through the vadose zone and 6 m through the aquifer, phosphate removal at the Flushing Meadows project was about 40% at high hydraulic loading and 80% at reduced hydraulic loading. Additional lateral movement of 60 m through the aquifer increased the removal to 95% (that is, to a concentration of 0.51 mg/l phosphate phosphorus versus 7.9 mg/l in the effluent). After 10 years of operation and a total infiltration of 754 m of secondary effluent, there were no signs of a decrease in phosphate removal. At the 23rd Avenue project, phosphate phosphorus concentrations in the last few years of the research averaged 5.5 mg/l for the secondary effluent and 0.37 mg/l for the renovated water pumped from the centre well. The shallower wells showed a higher phosphate content, indicating that precipitation of phosphate continued in the aquifer. For example, renovated water sampled from the 22 m deep north well showed phosphate phosphorus concentrations that averaged 1.5 mg/l. Most of the phosphate removal was probably due to precipitation of calcium phosphate.

Fluoride

Fluoride removal paralleled phosphate removal, indicating precipitation as calcium fluoride. At the Flushing Meadows project, fluoride concentrations in 1977 were 2.08 mg/l for the effluent, 1.66 mg/l for the renovated water after it had moved 3 m through the vadose zone and 3-6 m through the aquifer and 0.95 mg/l after it had moved an additional 30 m through the aquifer. At the 23rd Avenue project, fluoride concentrations averaged 1.22 mg/l in the secondary effluent and 0.7 mg/l in the renovated water from the centre well.

Boron

Boron was not removed in the vadose zone or the aquifer and was present at concentrations of 0.5 to 0.7 mg/l in both effluent and renovated water. The lack of boron removal was due to the absence of significant amounts of clay in the vadose zone and aquifer.

Metals

At the Flushing Meadows project, movement of the secondary effluent through 3 m of vadose zone and 6 m of aquifer reduced zinc from 193 to 35 m g/1, copper from 123 to 16 m g/1, cadmium from 7.7 to 7.2 m g/1 and lead from 82 to 66 m g/1 (Bouwer, Lance and Riggs, 1974). Cadmium thus appeared to be the most mobile metal.

Faecal coliforms

The secondary effluent at the Flushing Meadows project was not chlorinated and contained 105-10⁶ faecal coliforms/100 ml. Most of these were removed in the top metre of the vadose zone but some penetrated to the aquifer, especially when a new flooding period was started. The deeper penetration of faecal coliforms at the beginning of a flooding period was attributed to less straining of bacteria at the soil surface because the clogged layer had not yet developed. Also, the activity of native soil bacteria at the end of a drying period was lower, producing a less antagonistic environment for the faecal coliforms in the soil when flooding was resumed. Faecal coliform concentrations in the water after 3 m of travel through the vadose zone and 6 m through the aquifer were 10-500/100 ml when the renovated water consisted of water that had infiltrated at the beginning of a flooding period and 0-1/100 ml after continued flooding. Additional lateral movement of about 100 m through the aquifer was necessary to produce renovated water that was completely free of faecal coliforms at all times.

At the 23rd Avenue project, faecal coliform concentration in the secondary sewage effluent entering the infiltration basins was 10,000/100 ml prior to November 1980, when the effluent was not yet chlorinated but was first passed through the 32 ha lagoon. This concentration increased to 1.8 x 10⁶/100 ml when the unchlorinated effluent was bypassed around the lagoon and flowed directly into the infiltration basins. It then decreased to 3500/100 ml after the effluent was chlorinated and still bypassed around the lagoon. The corresponding faecal coliform concentrations in the water pumped from the large centre well from a depth of 30-54 m averaged 2.3, 22 and 0.27/100 ml, respectively. The corresponding ranges were 0-40, 0-160 and 0-3/100 ml, respectively. Considerable faecal coliform concentrations were observed in the renovated water from the shallower wells, especially when the faecal coliform concentration of the infiltrating effluent was 1.8 x 10⁶/100 ml. At that time, water from the 18 m deep well showed coliform peaks following the start of a new flooding period that regularly exceeded 1000/100 ml and at one time even reached 17,000/100 ml. Thus, a considerable number of faecal coliforms passed through the vadose zone. However, chlorination of the effluent with resulting reduction of the faecal coliform concentration to 3500/100 ml prior to infiltration, and additional movement of the water through the aquifer to the centre well, produced renovated water that was essentially free of faecal coliforms.

Viruses

At the Flushing Meadows project, the virus concentrations of unchlorinated secondary effluent averaged 2118 plaque-forming units (PFU)/100 l (average of six bimonthly samples taken for 1 year). They included poliovirus, echo virus, coxsackie and reoviruses. No viruses could be detected in renovated water sampled after 3 m of movement through the vadose zone and 3-6 m movement through the aquifer. At the 23rd Avenue project, virus concentrations in the renovated water from the centre well averaged 1.3 PFU/100 l before chlorination of the secondary effluent and 0PFU/100 l after chlorination of the secondary effluent. The combined effects of chlorination and SAT thus apparently resulted in complete removal of viruses.

Organic carbon

At the Flushing Meadows project, the biochemical oxygen demand (BOD₅) of the effluent water after moving 3 m through the vadose zone and 6 m through the aquifer was essentially zero, indicating that almost all biodegradable carbon was mineralized. However, the renovated water still contained about 5 mg/l total organic carbon (TOC), as compared to 10-20 mg/l of TOC in the secondary effluent. At the 23rd Avenue project, the TOC concentration of the secondary effluent averaged 12 mg/l where it entered the infiltration basins and 14 mg/l at the opposite ends of the basins. This increase was probably due to biological activity in the water as it moved through the basins. The renovated water from the 18 m well (intake about 5 m below the bottom of the vadose zone) had a TOC content of 3.2 mg/l and that from the centre well (which pumped from 30 to 54 m depth) had a TOC content of 1.9 mg/l, indicating further removal of organic carbon as the water moved through the aquifer. The TOC removal in the SAT system was the same before and after chlorination of the secondary effluent, indicating that chlorination had no effect on the microbiological processes in the soil.

The concentration of organic carbon in the renovated water of 1.9 mg/l was higher than the 0.2-0.7 mg/l typically found in unpolluted groundwaters, which contain mostly humic substances, such as fulvic and humic acids. The renovated water from the SAT process could thus contain a number of synthetic organic compounds, some of which could be carcinogenic or otherwise toxic.

Removal of trace organic compounds in the vadose zone

The nature and concentration of trace organics in the secondary sewage effluent and in the renovated water from the various wells of the 23rd Avenue project were determined by Stanford University's Environmental Engineering and Science Section, using gas chomatography and mass spectrometry. The studies were carried out for 2 months with unchlorinated effluent and then for 3 months with chlorinated effluent, taking weekly or biweekly samples. As could be expected, the results showed a wide variety of organic compounds, including priority pollutants, many in concentrations of the order of m g/1 (Bouwer et al. 1984, and Bouwer and Rice 1984).

Chlorination had only a minor effect on the type and concentration of organic compounds in the sewage effluent. Of the volatile organic compounds, 30-70% were lost by volatilization from the infiltration basins. Soil percolation removed 50-99% of the non-halogenated organic compunds, probably mostly by microbial decomposition. Concentrations of halogenated organic compounds decreased to a lesser extent with passage through the soil and aquifer. Thus, halogenated organic compounds (including the aliphatic compounds chloroform, carbon tetrachloride, trichloroethylene and 1,1,1-trichloroethane and the aromatic dichlorobenzenes, trichlorobenzenes and chlorophenols) were more mobile and refractory in the underground environment than the non-halogenated compounds, which included the aliphatic nonanes, hexanes and octanes, and the aromatic xylenes, C3-benzenes, styrene, phenanthrene and diethylphthalate.

Other organic micropollutants

In addition to the aliphatic and aromatic compounds mentioned, other compounds tentatively identified in organic extracts of the samples of secondary sewage effluent and renovated water using gas chromatography-mass spectrometry were: fatty acids, resin acids, clofibric acid, alkylphenol polyethoxylate carboxylic acids (APECs), trimethylbenzene sulphonic acid, steroids, n-alkanes, caffeine, Diazinon, alkylphenol polyethoxylates (APEs) and trialkylphosphates. Several of the compounds were detected only in the secondary effluent and not in the renovated water. A few others - Diazinon, clofibric acid and tributylphosphate -decreased in concentration with soil passage but were still detected in the renovated water. The APEs appeared to undergo rather complex transformations during ground infiltration. They appeared to be completly removed by soil percolation during the prechlorination period but, after chlorination, two isomers were found following soil passage, while others were removed.

The results of these studies showed that SAT is effective in reducing concentrations of a number of synthetic organic compounds in the sewage effluent but that the renovated water still contains a wide spectrum of organic compounds, albeit at very low concentrations. Thus, while the renovated water is suitable as such for unrestricted irrigation and recreation, recycling it for drinking would require additional treatment, such as activated carbon filtration, to remove the remaining organic compounds. The water would also have to be disinfected and reverse osmosis may be desirable.

The Phoenix studies have proved that SAT can produce a renovated water meeting US public health, agronomic and aesthetic requirements for unrestricted irrigation, including irrigation of vegetable crops that are consumed raw. In these studies, chlorinated secondary effluent was applied to the infiltration basins because that was the effluent available. However, Bouwer (1987) asserts that the secondary, biological, treatment stage is not necessary because the SAT system can handle relatively large amounts of organic carbon. Instead, primary treated effluent, possibly with additional clarification through lime precipitation, might be satisfactory. Thus SAT might well provide a simple, low-cost method of producing an effluent suitable for agricultural use, where land is available and hydrogeological conditions are favourable.

9.4 Wastewater treatment and crop restriction: Tunisia

9.4.1 Current and future use of wastewaters in Tunisia
9.4.2 Studies of treated wastewater irrigation and sewage sludge application
9.4.3 Legislation

9.4.1 Current and future use of wastewaters in Tunisia

Wastewater use in agriculture has been practised for several decades in Tunisia and is now an integral part of the national water resources strategy. The volume of treated wastewater available in 1988 was 78 million m³ and in the year 2000 it will probably exceed 125 million m³ (Bahri 1988). In 1988, wastewater was being treated in 26 treatment plants, mainly located on the coast so as to prevent sea pollution, and by 1996 there should be 54 treatment plants. Of the existing sewage treatment plants, 16 are activated sludge, 2 trickling filters, 5 stabilization ponds and 3 oxidation ditches.

Use of treated effluents is seasonal in Tunisia (spring and summer) and the effluent is often mixed with groundwater before being applied to irrigate citrus and olive trees, forage crops, cotton, golf courses and hotel lawns. Irrigation with wastewater of vegetables that might be consumed raw is prohibited by the National Water Law (Code des Eaux). A regional Department for Agricultural Development (CRDA) supervises all irrigation water distribution systems and enforces the Water Code. At the present time, an area of about 1750 ha is being irrigated with treated wastewater, at the locations indicated in Table 39. The major irrigation areas around Tunis are shown on Figure 26. La Cherguia activated sludge plant receives sewage from part of the Tunis metropolitan area and discharges its effluent to the La Soukra irrigation area 8 kilometres away.

Figure 26: Current and future irrigation with treated wastewater in the Tunis metropolitan area (Strauss and Blumenthal 1989)

Table 39: ACTUAL AND FUTURE USE OF TREATED WASTEWATER IN TUNISIA

Location		Area ha	Wastewater treatment plant			Crops
			Name	System	Capacity m3/d
Existing
Tunis
	Soukra	600	Cherguia	AS	60000	Citrus trees
Nabeul
	Oued Souhil	250		As	14400	Citrus trees
			SE4
Sousse
	Sousse North	43	Sousse N	AS	13 000	Golf course
	Sousse South	205	Sousse S	TF	18700	Forage crops
Monastir
	Monastir		Monastir	TF	2600	Golf course
Under Implementation
Tunis						Citrus trees
	Soukra	200	Cherguia	AS	60000	Cereals
	Cebala	2670	Choutrana	OD	43000	Forage crops
			Cotiere N	SP	15 800	Industrial crops
Mornag		940	Meliane S	OD	37 500	-
Nabeul		330	SE2	AS	3500
			SE4	AS	14400	Citrus trees
			SE3	OD	3 500
Hammamet		140	SE1	AS	6600	Golf course
Sousse
	Sousse North	120	Sousse N	AS	13 000	Forage, trees
	Sousse South	200	Sousse S	TF	18700	Forage, trees
Sfax		270	Sfax	SP	24000	Forage, trees
Kairouan		240	Kairouan	AS	12000	Forage crops Industrial crops
Cafsa		157	Cafsa	SP	4500	Forage, trees
Planned
Moknine		100	Moknine	SP	2400
Sfax		130	Sfax	SP	24000
Tunis		15000	Sedjoumi

AS: Activated sludge;
TF: Trickling filters;
OD: Oxidation ditches;
SP: Stabilization ponds
Source: Bahri (1988)

Many new projects are now being implemented or planned and the wastewater irrigated area will be increased to 6700 ha, allowing 95% of the treated wastewater to be used in agriculture. The most important developments will take place around Tunis, where 60% of the country's wastewater is produced and 68% of the effluent-irrigated area will occur.

9.4.2 Studies of treated wastewater irrigation and sewage sludge application

In the period 1981 to 1987, the Ministries of Agriculture and Public Health, with assistance from the United Nations Development Programme (UNDP), carried out studies designed to assess the effects of using treated wastewater and dried, digested sewage sludge on crop productivity and on the hygienic quality of crops and soil. Treated wastewaters and dried, digested sludge from the La Cherguia (Tunis) and Nabeul (SE4) activated sludge plants were used in the studies and irrigation with groundwater was used as a control. At La Soukra, tests were conducted on sorghum (Sorghum vulgare) and pepper (Capsicum annuum) using flood irrigation and furrow irrigation, respectively. Clementine and orange trees were irrigated at Oued Souhil (Nabeul). In order to assess the long-term effects of irrigation with treated waste-water, investigations were carried out on the perimeter area of La Soukra, where irrigation with treated effluent had been practised for more than 20 years. The programme of studies not only produced useful results but was also valuable from the point of view of the training of specialists and technicians (Bahri 1988).

The average quality characteristics of the treated wastewater from La Cherguia and sewage sludge from Soukra and Nabeul are given in Tables 40 and 41. The effluent contains moderate to high salinity but presents no alkalization risk and trace element concentrations are below toxicity thresholds. The sewage sludge had a fertilizing potential, due to the presence of minerals and organic matter, but was of variable consistency. Evaluation of the fertilizing value of the effluent in relation to crop uptake suggests that the mean summer irrigation volume of 6000 m³/ha would provide an excess of nitrogen (N) and potassium (K₂O) but a deficit of phosphorus (P₂O₅). The fertilizing value of 30 tonnes dry weight of sewage sludge per ha would be an excess of N and P₂O₅ and a deficit of K₂O. Application of treated effluent and sludge would balance the fertilizing elements but would provide an excess over crop requirements. Excess nitrogen would be of concern from the point of view of crop growth and in relation to groundwater pollution.

Table 40: AVERAGE CHARACTERISTICS OF TREATED WASTEWATERS (TW) AND WELL WATERS (WW) USED FOR IRRIGATION (in mg/l) IN LA SOUKRA COMPARED TO FAO RECOMMENDED MAXIMUM CONCENTRATIONS

Parameter	TW	WW	FAO
pH	7.6	7.6	6.5-8.5
EC	2.97	2.61	3.0
TDS (g/l)	1.82	1.71	2.0
SM	13.4	4.3
COD	51	-
HCO3	370.0	228.5	600
SO4	363.0	87.5	1000
Cl	554.0	648.0	1100
Ca	154.5	249.0	400
Mg	56.5	48.5	60
K	36.5	3.0
Na	366.0	214.0	900
SAR	6.4	3.2	15
N (total)	2.5-43	-	30
NH4	0.26-50.5	0.09
NO3	1.33-83.5	92.8
NO2	0.07-5.0	0.08
P (total)	4.10	-
PO4	11.6	0.06
Cd	-	-	0.01
Co	0.05	0.04	0.05
Cr	0.02	-	0.1
Cu	0.03	0.02	0.1
Fe	0.33	0.11	5
Mn	0.05	0.01	0.2
Ni	0.06	0.05	5
Pb	0.19	0.16	2
Zn	0.12	0.04
TC/100 ml	10e4-10e6
FC/100 ml	10e4-10e6
FS/100 ml	10e4-10e6
Salmonella	No
Cholerae	No

EC: electrical conductivity (in dS/m at 25°)
TDS: total dissolved solids
SM: suspended matter
COD: chemical oxygen demand
SAR: sodium adsorption ratio (in molalities)
TC: total coliforms
FC: faecal coliforms
FS: faecal streptococci
Source: Bahri (1988)

Application of treated wastewaters and sewage sludge at the La Soukra and Oued Souhil experimental stations, where the soils are alluvial and sandy-clayey to sandy, has not adversely affected the physical or bacterial quality of the soils. However, the chemical quality of the soils varied considerably, with an increase in electrical conductivity and a transformation of the geochemical characteristics of the soil solution from bicarbonate-calcium to chloride-sulphate-sodium (Bahri 1988). Trace elements concentrated in the surface layer of soil, particularly zinc (Zn), lead (Pb) and copper (Cu), but did not increase to phytotoxic levels in the short term of the study period. Rational use of sewage sludge would require standards to be developed for the specific soils, based on limiting concentrations of trace elements.

The use of treated wastewater resulted in annual and perennial crop yields higher than yields produced by groundwater irrigation. Sewage sludge application increased the production of sorghum and pepper and resulted in the crops containing higher concentrations of N, P and K and some minor elements (Fe, Zn and Cu). Bacterial contamination of citrus fruit picked from the ground irrigated with treated wastewater or fertilized with sewage sludge was significantly higher than the level of contamination of fruit picked from the trees. Natural bacterial die-off on sorghum plants was more rapid in summer than in autumn. Tests on pepper did not indicate particular contamination of the fruit.

Irrigation with treated wastewaters was not found to have an adverse effect on the chemical and bacteriological quality of shallow groundwater, although the initial contamination of wells was relatively high and subject to seasonal variation. Investigations on the peripheral area of La Soukra did not indicate significant impacts on soils, crops or groundwaters.

Table 41: AVERAGE CHEMICAL COMPOSITION OF THE SEWAGE SLUDGE (% DRY MATTER) COMPARED TO EUROPEAN STANDARDS

Parameter	Soukra-Nabeul sludge	AFNOR Standards 044-041
H2O	25-50
pH	7.2-7.9
EC (1/5)	3.8-7.1
VM %	17-42
C (organic) %	10-20
N (total) %	1-2.5
C/N	6-8
P (total) %	0.5-1.0
Ca %	5-9
Mg %	0.1-0.8
K %	0.2-0.3
Na %	0.1-0.4
Cd ppm	4.0-7.0	20
Co ppm	16-30
Cr ppm	51-78	1000
Cu ppm	150-320	1000
Fe %	7.6-18	-
Hg ppm	0.6-1.8	10
Mn ppm	103-320	-
Ni ppm	21-52	200
Pb ppm	192-526	800
Zn ppm	400-982	3000
Cr+Cu+Ni+Zn	560-1200	4000
FC/g fresh sludge	10e3-10e4
FS/g fresh sludge	10e3-10e5

EC: electrical conductivity (in dS/m at 25°)
VM: volatile matter
FC: faecal coliforms
FS: faecal streptococci
Source: Bahri (1988)

9.4.3 Legislation

The Tunisian Water Law, enacted in 1975, provides the legal framework for treated wastewater use. The Code prohibits the use of raw sewage in agriculture and the irrigation of vegetables that are eaten raw with treated wastewater. Another relevant legal document is an enactment, issued in 1985, regulating substances released to the environment, which refers to wastewater use. In 1989, an Act more specifically regulating the use of treated wastewater in agriculture was introduced. The implementation and enforcement of the Decree is the responsibility of the Ministries of Public Works, Agriculture, Economy and Public Health.

The 1989 Act requires that treated wastewater use in agriculture be authorized by the Ministry of Agriculture, after preliminary inquiry from the Ministry of Public Health and notification from the National Environmental Protection Agency (Bahri 1988). Specified in the document is the frequency of physico-chemical and biological analyses. Irrigation of vegetables and of any crop that might be consumed raw is forbidden. It also stipulates that crops irrigated with treated wastewater must be tested by the Ministry of Public Health. In areas where sprinkler irrigation is to be adopted, buffer zones surrounding the irrigated area must be created. Direct grazing on land irrigated with treated wastewater is prohibited. Quality standards have been issued in a separate document, in which the crops that might be irrigated with treated wastewater are specified (forage and industrial crops, cereals, trees) and the precautions that must be taken to prevent contamination of workers, residential areas and consumers are detailed.

9.5 Wastewater treatment and human exposure control: Kuwait

9.5.1 Background to treated effluent use in Kuwait
9.5.2 Master plan for effluent utilization
9.5.3 Project outputs and controls

9.5.1 Background to treated effluent use in Kuwait

Untreated sewage has been used for many years to irrigate forestry projects far from the inhabited areas of Kuwait. Effluent from the Giwan secondary sewage treatment plant was used to irrigate plantations on an experimental farm from 1956 (Agriculture Affairs and Fish Resources Authority, Kuwait 1988). Following extensive studies by health and scientific committees within the country and by international consultants and organizations (WHO and FAO), the government of Kuwait decided to proceed with a programme of sewage treatment and effluent use. In all, by 1987 four sewage treatment plants were in operation: the 150 000 m³/day Ardiyah sewage treatment plant (secondary stage) was commissioned in 1971, the 96 000 m³/day coastal villages and the 65 000 m³/day Jahra sewage treatment plants were commissioned in 1984 and a small (10 000 m³/day) stabilization ponds treatment plant has also been installed on Failaka Island. The effluent from the Ardiyah, coastal villages and Jahra, activated sludge treatment plants was upgraded in the middle 1980s by the provision of tertiary treatment, consisting of chlorination, rapid gravity sand filtration and final chlorination.

Initially, the treated secondary effluent from the Ardiyah plant was distributed to the experimental farm of the Department of Agriculture at Omariyah. Trials were undertaken in the early 1970's to compare crop yields from irrigation with potable water, brackish water and treated effluent. An 850 ha farm was established in 1975 by the United Agricultural Production Company (UAPC), under Government licence, especially for the purpose of utilizing the treated wastewater. The directors of this close shareholding company represented the main private organizations involved in Kuwait agriculture, in particular the local dairy, poultry and livestock farming organization. In 1975, only part of the area was under cultivation, with forage (alfalfa) for the dairy industry the main crop, using side-roll sprinkler irrigation. However, aubergines, peppers, onions and other crops were grown on an experimental basis, using semiportable sprinklers and flood and furrow irrigation.

9.5.2 Master plan for effluent utilization

The Government strategy for implementation of the Effluent Utilization Project was to give the highest priority to development of irrigated agriculture by intensive cultivation in enclosed farm complexes, together with environmental forestry in large areas of low-density, low water-demand tree plantations. By 1976, however, the total cropped area in Kuwait was only 732 ha and the country relied heavily on food imports and imports of both fresh and dried alfalfa were considered to be unnecessarily high. In late 1977, the Ministry of Public Works initiated the preparation of a Master Plan for effective use of all treated effluent in Kuwait, covering the period up to the year 2010 (Cobham and Johnson 1988).

The overall plan recommendations are shown in Table 42. For the western and northern sites (Jahra and Ardiyah effluents, respectively) it was suggested that the first priority should be devoted to developing an integrated system of forage (used in a high concentrate ration dairy enterprise) and extensive vegetable production on the UAPC farm, so that full utilization would be made of existing and potential facilities as soon as possible. This utilization should be based on: modern irrigation techniques, strengthening of shelter belts, provision of adequate effluent storage facilities, trial of different irrigation equipment, investigation of the relative merits of vegetable production on intensive and extensive scales and improvement of both management and technical husbandry skills. Second priority, it was recommended, should be given to developing fresh forage/hay production in rotation with vegetables on the other agricultural sites identified. Once the two major priorities had been achieved, as much prime and subsistence environmental protection forestry as possible should be planted. Provided that trials concerning commercial timber yielded positive results, it was suggested that an area of at least 213 ha of maximum production forestry should be included.

Table 42: THE MASTER PLAN - LAND USE IMPLICATIONS

		1980 (ha)	2010 (ha)
Western and northern sites
Agriculture
	Forage: dairy enterprise	70	670
	Forage: open market sale	149	589
Horticulture
	Extensive vegetables	50	200
Forestry
	'Maximum production' forestry	20	213
	'Environmental protection' at recommended irrigation rate	3808	7826
	'Environmental protection' forestry at subsistence irrigation rate	401	-
Others
	Existing trial site, vegetable areas	46	46
Subtotal		4544	9544
Coastal village sites
Forestry
	'Maximum production' 'Environmental protection' forestry	52	787
		1673	1673
Subtotal		1725	2460
Failaka Island
	'Environmental protection' forestry	176	284
Total		6445	12288

Source: Cobham and Johnson (1988)

The resource implications of the Master Plan were assessed, including the tree nursery production required for the new forestry areas, the infrastructural requirements (including boundary walls and roads), the irrigation equipment and machining needs for forage, vegetables, etc. Construction of works for effluent utilization according to the Master Plan began in mid-1981 but delays in the provision of permanent power supplies to all 12 sites deferred commissioning of the project until 1985. A data monitoring centre receiving treated effluent from Ardiyah and Jahra has been provided and includes two 170 000 m³ storage tanks, pumping station, administration building incorporating laboratories for monitoring effluents and soils and workshops for maintenance and stores.

9.5.3 Project outputs and controls

The ultimate project design provides for the development of 2700 ha of intensive agriculture and 9000 ha of environmental forestry (Agriculture Affairs and Fish Resources Authority, Kuwait 1988). In 1985, the treated effluent supplied to the experimental farm and irrigation project was used to irrigate the following:

Fodder plants - alfalfa, elephant grass, Sudan grass, field corn (maize), vetch, barley, etc.

Field crops - field corn (maize), barley, wheat and oats.

Fruit trees - date palms, olive, zyziphus and early salt-tolerant vines (sprinklers were not used for fruit trees).

Vegetables - potatoes, dry onions, garlic, beet and turnip were irrigated by any method; vegetables which are to be cooked before consumption, such as egg plant, squash, pumpkin, cabbage, cauliflower, sweetcorn, broad beans, Jews mallow, Swiss chard, etc., were irrigated in any way but not by sprinkler; vegetables which are eaten raw, such as tomatoes, water melons and other melons, were irrigated with tertiary-treated sewage effluent by drip irrigation with soil mulching.

The yield of green alfalfa was 100 tonnes/ha per year and the total production from the agricultural irrigation project, using primarily treated sewage effluent, was 34,000 tonnes of vegetables and green fodder plants, including dehydrated alfalfa and barley straw. At this production level, a reasonable supply of some vegetables was made available to the local market, the total demand for green alfalfa for animals was satisfied and some of the needs for dehydrated fodder were met.

For forestry irrigation, the systems include storage tanks and pumping stations incorporating fertilizer injection. Treated effluent is supplied via control points to blocks of forestry. Header mains downstream of control points feed 12.5 mm polyethylene drip lines fitted with pressure-compensating drip emitters (two per tree) discharging 4 l/h operating over a 0.7-3.5 bar inlet pressure range. Up to now, only environmental protection forestry has been developed although there is the potential to produce high yields of commercial forestry using treated effluent irrigation. Annual productivity levels which can be achieved in irrigated sand areas have been estimated to range from 5-25 m³/ha for Prosopis and Tamarix (Cobham and Johnson 1988).

In Kuwait, the decision was taken to exclude all amenity uses for the treated effluent and to restrict agricultural use to safe crops. Furthermore, areas of tree and shrub planting and the agricultural farm were to be fenced to prevent public access. An efficient monitoring system for the treated effluent, the soil and the crops has been implemented since the experimental farm was initiated. The guidelines for tertiary-treated effluent quality used in irrigation are:

Suspended solids	10 mg/l
BOD5	10 mg/l
COD	40 mg/l
Cl2 residual	about 1 mg/l after 12 hours at 20°C
Coliform bacteria	10 000/100 ml for forestry, fodder and crops not eaten raw 100/100 ml for crops eaten raw

Even the tertiary-treated effluent meeting these guidelines is not to be used to irrigate salad greens or strawberries. Cadmium was the only heavy metal of concern and special attention was given to monitoring the effluent and crops for this element and to measuring Cd in the kidneys of animals fed on forage irrigated with treated sewage effluent. Agricultural workers dealing with sewage effluent are medically controlled as a pre-employment measure and given periodic (6 monthly) examinations and vaccinations. No outbreaks of infectious disease have occurred since this procedure began in 1976. The impact of treated effluent irrigated vegetables on the consumer has not been possible to assess because no segregation of vegetables produced in this way is effected in the market.

9.6 Crop restriction for wastewater irrigation: Mexico

9.6.1 Historical use of sewage for irrigation
9.6.2 Mezquital valley irrigation district 03 experience
9.6.3 Health impacts

9.6.1 Historical use of sewage for irrigation

Use of raw sewage for irrigation in the Mezquital Valley of the Tula River Basin began in 1886 (Sanchez Duron 1988). However, it was not until 1945 that the Ministry of Agriculture and Water Resources established the Number 03 Mezquital Irrigation District to manage the distribution of wastewater from Mexico City for irrigation purposes. Irrigation is essential in this Irrigation District because rainfall is limited and poorly distributed over the year, most falling between July and September. Sewage from Mexico City mixed with variable proportions of surface water collected in reservoirs within the basin has enabled farmers in the Mezquital Valley to provide agricultural produce for the capital city.

Six Irrigation Districts currently make use of wastewater and surface runoff from urban areas and the Government has developed plans for wastewater use in 11 more, as indicated in Table 43. Four Irrigation Districts receive wastewater and runoff from Mexico City, which on average amounts to 55 m³/sec. Irrigation District 03, the most significant, comprises 16 municipalities with a population of 300 000 in 1985. A complex network of canals serves the area, allowing intensive cultivation year round taking advantage of the supply of wastewater.

9.6.2 Mezquital valley irrigation district 03 experience

At different times and places in the District, the following types of irrigation water might be used separately or in combination:

River water	containing little or no contamination from urban wastewater.
Impounded river water	diverted from reservoirs, or river reaches downstream receiving spillway overflows, containing wastewater discharged into the reservoirs from the main collector canals.
Wastewater	from the main collector canals, composed of sewage and urban storm runoff.

Table 43: IRRIGATION DISTRICTS WITH CURRENT AND PLANNED USE OF WASTEWATER

Dist.No.	Name of district	State	Area irrigated (ha)	Total area which can be irrigated (ha)	Annual wastewater flow available as % of total irrigation water supplied	Major crops grown
Wastewater reuse existing
031	Tula2	Hidalgo	43000	48000	>100	Alfalfa, maize, wheat, oats, green tomatoes chillies
09	Cd. Juárez	Chihuahua	3000	17500	3.5	Cotton, alfalfa, oats, wheat
281	Tulancingo	Hidalgo	300	1 100	54	Pasture, maize, alfalfa
30	Valesquillo	Puebla	17 600	33 800	58	Maize, alfalfa, beans, chillies
881	Chiconautla-Chalco-Texcoco	Mexico	4 300	4300	> 100	Maize, alfalfa, oats, beet root
1001	Alfajayucan3	Hidalgo	14700	28900	> 100	Maize, beans, wheat, green tomatoes
Wastewater reuse planned
10	Culican y Humaya	Sinaloa		223000	1.3	Wheat, sorghum, sugarcane
11	Alto Río Lerma	Guanajato		102000	5.6	Wheat, sorghum, maize, beans
14	Río Colorado	Baja California Norte		207000	1.5	Cotton, wheat, barley, alfalfa
16	Estado de Morelos			34 600	2.6	Rice, maize, green tomatoes, sugarcane
17	Región Lagunera	Coahuila & Durango		150000	2.1	Cotton, maize, wheat, alfalfa
20	Morelia y Querendaro	Michoacán de Ocampo		33900	7.2	Maize, wheat, sorghum, barley
26	Bajo Río San Juan	Tamaulipas		79500	1.5	Maize and sorghum
41	Río Yaqui	Sonora		93 800	1.3	Wheat, cotton, alfalfa
61	Zamora	Michoacán de Ocampo		17900	2	Wheat, peas, potatoes, strawberries
75	Valle del Fuerte	Sinaloa		223000	0.2	Cotton, knapweed, wheat, sugarcane
82	Ráo Blanco	Veracruz		1 600	2.6	Maize, watermelons, green tomatoes

¹ Using wwastewate and runoff from Mexico City
² 15 800 users in District
³ 21 800 users in District
Source: Strauss and Blumenthal (1989)

Hence, the concentrations of chemical constituents and pathogenic organisms in the irrigation water will vary spatially and temporally. Large impounding reservoirs (such as Endho) providing relatively long retention times for wastewater will serve as treatment devices, settling out solids and reducing pathogen levels. Nevertheless, in general, faecal coliform levels in the irrigation water are 10⁶-10⁸/100 ml.

No treatment of sewage is provided before it is transported the 60 kilometres from Mexico City to Irrigation District 03 and, clearly, little improvement in faecal coliform levels has occurred before it is applied as irrigation water. In trying to achieve public health protection, reliance is placed on the application of crop restrictions rather than wastewater treatment. Every year, each farmer specifies the crops he is going to plant and irrigate with water allocated by the Irrigation District. The Ministry of Health sets the basic rules for crop restriction and the District's directing committee specifies in detail the crops which may not be cultivated under its jurisdiction (Strauss and Blumenthal 1989). In Irrigation District 03, banned crops are: lettuce, cabbage, beet, coriander, radish, carrot, spinach and parsley. Adherence to these restrictions is monitored mainly by the District's canal and gate operators, who are in close contact with farmers. Maize, beans, chili and green tomatoes, which form the staple food for the majority of the population, do not fall under these restrictions and neither does alfalfa, an important fodder crop in the area.

During the agricultural year 1983-84, 52 175 ha in Irrigation District 03 were harvested to produce 2 226 599 tonnes of food crops, with a value of more than US $33 million. The yields of the crops were greater than those obtained 10 years before, except for pasture, and it is believed that fertility conditions, measured on the basis of productivity, are better than before (Table 44). In addition, it is thought that the high content of organic matter and plant nutrients in the wastewater have improved the physical and chemical properties of the shallow soils in the District. The high rate of application of irrigation water has increased soil organic matter and systematically leached the soils, preventing the accumulation of soluble salts (Sanchez Duron 1988).

Table 44: CROPS, AREAS HARVESTED AND YIELDS IN IRRIGATION DISTRICT NUMBER 03

Crops		Area harvested (ha) and yield (kg/ha)
		1970-71	1975-76	1980-81	1983-84
Maize (corn)	Harvested (ha)	17914	21 023	17907	18 371
	Yield (kg/ha)	3 938	3 896	4566	4581
Beans	Harvested (ha)	1 266	1 222	1 646	1 028
	Yield (kg/ha)	1 259	1 768	1 521	1 430
Wheat	Harvested (ha)	7293	2 634	2005	399
	Yield (kg/ha)	1 919	3 119	3 225	3 134
Alfalfa	Harvested (ha)	12708	15 206	20339	19515
	Yield (kg/ha)	95 300	89 154	91 175	96481
Oats	Harvested (ha)	2998	691	1 002	2489
	Yield (kg/ha)	18 150	19898	32470	25 348
Barley	Harvested (ha)	-	832	1 812	1 268
	Yield (kg/ha)	-	19620	19939	16823
Pastures	Harvested (ha)	13	11	65	109
	Yield (kg/ha)	142500	107000	44276	98 832

Source: Sanchez Duron (1988).

9.6.3 Health impacts

Mexican experience with raw wastewater irrigation suggests that successful enforcement of crop restriction has provided health protection for the general public, including crop consumers. Past studies on the health impact of the use of raw wastewater in agriculture in the Mezquital Valley have shown no consistent significant excess prevalence of gastrointestinal complaints or protozoan (apart from amoebiasis) or helminthic infections in children from communities irrigating with wastewater compared with children from a control community using clean water for irrigation. A study on the health effect of the use of wastewater on agricultural workers in Guadalajara concluded that a high prevalence of parasitic diseases in both exposed and control group workers was due to poor environmental sanitation, poor hygienic habits and lack of health education. However, a significant excess prevalence of infection in the exposed group was found for Giardia lamblia (17 per cent in exposed vs 4 per cent in control group) and Ascaris lumbricoides (50 per cent in exposed vs 16 per cent in control group). This led Strauss and Blumenthal (1989) to recommend further epidemiological studies on the increased health risk to farm workers and at least partial treatment of wastewater, to remove helminth eggs and protozoan cysts, in future wastewater use schemes in Mexico.

9.7 Wastewater use in aquaculture: Calcutta, India

9.7.1 The Calcutta system
9.7.2 Health impacts

9.7.1 The Calcutta system

The East Calcutta sewage fisheries are the largest single wastewater use system involving aqua-culture in the world. An historical account of the development of this system has been given by Edwards (1985 and 1990). Ghosh (1984) presented the data on the range of size and numbers of sewage fisheries in Calcutta as shown in Table 45. In 1945, the area of sewage-fed fish ponds was about 4628 ha, in a wetlands area of about 8000 ha, but the fish pond area had been reduced to about 3000 ha by 1987 due to urban reclamation and conversion of fish ponds to rice paddies. Ownership of the ponds is in the hands of about 160 city dwellers, who employ nearly 4000 families as fishermen, and there are several fishermens' cooperatives (Strauss and Blumenthal 1989).

Table 45: SIZE OF CALCUTTA SEWAGE FISHERIES BASED ON 1984 RECORD OF LICENSING, DIRECTORATE OF FISHERIES, WEST BENGAL

Size, ha	Number	Percent of total
<4	35	20
> 4-8	35	20
>8-12	43	24
>12-16	7	4
> 16-20	9	5
> 20-40	18	10
> 40-60	15	9
>60-80	9	5
>80	5	3
Total	176	100

Source: Ghosh (1984)

The fish ponds receive raw sewage from Calcutta on a batch basis and fishermen have developed appropriate operational techniques through experience. Olah et al. (1986) have described the technique adopted in operating 5.7 ha ponds, 0.7 m deep. Initially, screened raw sewage is allowed to flow into the ponds and after 12 days the pond contents is disturbed by repeated netting and manual agitation with split bamboos for oxidation, mixing and 'quick recovery' of water quality. After 25 days from initial filling with sewage, the ponds are ready to be stocked with fish. Thereafter, sewage is applied 7 days/month for 3 hours during the morning, to fertilize the ponds, at an estimated rate of 130 m³ sewage/ha d. The ponds are stocked with a polyculture of fingerlings of catla (Catla catla), mrigal (Cirrhinus mrigala), rohu (Labeo rohita), common carp (Cyprinus carpio) and tilapia (Oreochromis mossambicus) ranging in size from 20-30 g at a total density of 3.5 fish/m² and total initial stocked weight of 869 kg. Intermediate harvesting is started after 120 days of rearing, using a seine net, and continued up to pond draining after 300 days, in March and April.

Estimates of total production and yield of fish from the Calcutta fisheries vary from 4,516 tonnes of fish from 6993 ha of fisheries in 1948 (approximately 0.6 tonnes/ha year) to 4-9 tonnes/ha year in 1984 (Edwards 1990). The fisheries supply the city markets with 10-20 tonnes offish per day, providing 10-20 per cent of the total demand. In addition, some degree of natural treatment is applied to the sewage and, in spite of the threat to the existing fisheries through urban development, workers on the wetlands project feel that much more sewage could be handled in this way and the greater part of Calcutta's demand for pond fish could be produced.

9.7.2 Health impacts

Total coliform counts of 10⁵-10⁶/100 ml in the influent sewage to the Calcutta fish ponds and 10²-10³/100 ml in the pond water have been reported. Vibrio parahaemolyticus, the second most important diarrhoea-causing agent (after V. cholerae) in the Calcutta area, has been found in the intestines offish from the sewage-fed ponds (Strauss and Blumenthal 1989). Nevertheless, no epidemiological studies have been carried out in Calcutta to assess the risk attributable to the use of sewage in aquaculture ponds.

Diarrhoeal diseases, typhoid fever and hepatitis A are the diseases of greatest concern, although protozoan cysts (Giardia and Cryptosporidium) are likely to be present in the upper layers of pond water and constitute a risk. With the relatively low levels of total coliforms in the pond water over the growing season, the fish are likely to be of good enough quality for human consumption providing they are well cooked and high standards of hygiene are maintained during their preparation (Strauss and Blumenthal 1989). Studies on Vibrio parahaemolyticus have indicated that it could be transmitted to fish consumers or fish farmers during the summer months. On the whole, the public health effects of sewage fertilization of aquaculture ponds in Calcutta remain unclear and further microbiological and epidemiological studies are required.