Paper 8 Wastewater treatment and use for irrigation

M. B. Pescod Obe
Professor, Department of Civil Engineering, University of Newcastle upon Tyne

Summary: The paper describes the potential health risks associated with wastewater use for irrigation and identifies helminths as the pathogens of greatest concern. Environmental and agricultural impacts resulting from wastewater irrigation are reviewed and the need for consideration of wastewater quality at the project planning stage is stressed. The new wastewater quality guidelines, introduced by WHO in 1989, are given and attention drawn to the FAO irrigation water quality guidelines applied to wastewater irrigation in the 1992 'Irrigation and Drainage Paper No. 47'. Wastewater treatment, crop selection, application control and human exposure control are discussed as alternative and complementary techniques and strategies for managing treated wastewater use in agriculture. Finally, case-study examples from Jordan, Tunisia, Kuwait and Mexico are used to illustrate some of the strategies.

Introduction

Wherever a community's wastewater is collected in sewers and irrigation water is scarce, raw wastewater is likely to be used by farmers. In the past, this has often had adverse health impacts, causing international agencies, particularly the World Health (WHO) and Food and Agriculture (FAO) Organizations of the United Nations, to become increasingly concerned. It is recognized that the nutrients contained in domestic wastewater will benefit agriculture, so effluent re-use is to be encouraged. However, unacceptable health and environmental risks cannot be tolerated. Considerable progress has recently been made to assess health and environmental risks associated with wastewater use in irrigation and to develop suitable guidelines.

Land application is often the most economical way to dispose of wastewater and sludge - but municipal sewage carries harmful pathogens and may also contain dangerous levels of heavy metals and industrial organic compounds. Direct use of municipal sewage without pretreatment and without applying any other controls will lead to serious health risks and possibly to impairment of the soil's long-term productivity. The problem is especially acute in developing countries where restricting raw wastewater use in irrigation has been difficult and resources to invest in costly wastewater treatment are scarce. Low-cost alternatives are necessary if poorer countries are to take advantage of this additional water resource for crop irrigation in an organized and controlled manner.

Wealthier countries have tended to rely on advanced wastewater treatment technology for health and environmental protection. This is not only costly, but conventional secondary and tertiary sewage treatment processes are notoriously difficult to operate and maintain. Experience with such processes has not been good in developing countries and this approach is unlikely to be successful in the near future. Fortunately, alternative control measures to minimize health and environmental risks are available.

Health risks associated with wastewater use for irrigation

The potential risk of infection being transmitted to plants, animals and humans through land application of wastewater is attributable to the presence of pathogenic organisms in the raw wastewater. Under favourable conditions, enteric pathogens can survive for long periods on crops and in the soil, as indicated in Table 1 from Feachem et al. (1983). However, despite the extensive world-wide practice of nightsoil and sewage sludge application and raw wastewater irrigation, few epidemiological studies have definitely established adverse health impacts from consuming food grown in this way.

Table 1 Survival of excreted pathogens at 20-30°C

Those epidemiological studies that have been conducted, as reported by Shuval et al. (1984) and Gunnerson et al. (1984), have shown that transmission of helminthic infections (Ascaris and Trichuris spp.) has been found to occur where these diseases were endemic in the population and where raw untreated wastewater was used to irrigate salad crops and/or other vegetables that are generally eaten raw. Some evidence suggests that cholera has been transmitted through the same channel. Reports from Melbourne, Australia and Denmark, reviewed by Gunnerson et al. (1984), confirmed that beef tapeworm (Taenia saginata) has been transmitted to people consuming the meat of cattle grazing on wastewater-irrigated fields, or fed crops from such fields. Although the reported incidence of disease transmission to workers on sewage farms has been inconclusive, there is always a potential risk associated with direct contact of wastewater with hands, especially where personal hygiene is not strict. Finally, the inhalation of aerosolized sewage containing pathogens from spray irrigation is a possible mode of disease transmission but no evidence has been presented to confirm this.

The health risks associated with wastewater re-use can show up to different extents in different subgroups of the population. The most important sub-groups to consider are those that consume crops irrigated with the wastewater (consumer risk) and agricultural workers subjected to occupational exposure (occupational risk). It is also important to consider persons of different ages separately, since the risk to children may be different from the risk to adults. The control measures taken depend on whether consumer risk, occupational risk, or both, are to be minimized.

The method of application, the interval between successive applications, and the interval between the last application and harvesting, all affect the likely degree of crop contamination and the environmental dispersion of excreted pathogens. Agricultural crops intended for human consumption pose potential risks to farm workers, those who handle the products and those who consume them. If they are fodder crops, farm workers and those who consume the resulting meat or milk are at potential risk; in the case of industrial products (for example, sugar beet), only farm workers and product handlers are at risk. Where sprinkler irrigation is used, people living near the irrigated fields are potentially at risk from pathogens present in wind-dispersed aerosol droplets.

The greatest risk occurs when crops - such as salad crops - are eaten raw, especially if they are root crops (radishes) or grow close to the soil (lettuces). Pathogen survival times can be greater than the crop growing time, so contamination is highly likely unless the wastewater is treated to a very high standard.

Significant host immunity only occurs with the viral diseases and some bacterial diseases (for example, typhoid). The role of immunity is most not)ceable in the case of viral infections, where infection at an early age is very common (even in communities with high standards of personal hygiene). As a result, the adult population is largely immune to the disease, and frequently also to infection.

The relative importance of such potential health risks from wastewater re-use depends on alternative access routes to excreted pathogens, such as lack of safe water supply. If there are no such routes, wastewater re-use will be entirely responsible for the risk induced. However, Shaval et al. (1986) have pointed out that negative health effects have only been detected in association with the use of raw or poorly-settled wastewater, while inconclusive evidence has suggested that appropriate wastewater treatment could provide a high level of health protection. In respect of the health impact of wastewater use for irrigation, these workers rank pathogenic agents in the following order of priority of concern:

High risk	Helminths
	(high incidence of excess infection)	(Ancylostoma, Ascaris, Trichuris and Taenia)
Medium risk	Enteric bacteria (low incidence of excess infection	(Cholera vibrio, Salmonella typhosa, Shigella and possibly others)
Low risk (low incidence of excess infection)	Enteric viruses

Environmental and agricultural impacts of wastewater use for irrigation

Wastewater has an important role to play in water resources management as a substitute for freshwater in irrigation. By releasing freshwater sources for potable water supply and other priority uses, wastewater re-use contributes to water conservation and takes on an economic dimension.

Those pollutants which, if discharged directly to the environment in raw wastewater, would create serious pollution problems (especially organic matter, nitrogen, phosphorus and potassium) serve as nutrients when applied in irrigation water. Studies in many countries have shown that, with proper management, crop yields may increase by irrigating with raw wastewater, as well as with primary and secondary treated effluents. For an irrigation rate of 2 m/year, commonly required in semi-arid areas, typical concentrations of 15 mg/l of total N and 3 mg/l of total P in well-treated sewage (say, after treatment in a properly designed series of stabilization ponds) correspond to annual N and P application rates of 300 and 60 kg/ha, respectively. Such nutrient inputs will reduce or eliminate the need for commercial fertilizers. The organic matter, biological oxygen demand (BOD), added through wastewater irrigation will serve as a soil conditioner over time, increasing the capacity of the soil to store water.

Discharging untreated or partially treated wastewater to the environment can give rise to pollution in surface and ground waters, and on land. Planned re-use of wastewater for irrigation prevents such problems and reduces the resulting damages which, if quantified, can partly offset the costs of the re-use scheme. Also, by substituting wastewater irrigation for groundwater irrigation in those areas where over-use of groundwater resources is causing problems (such as salt water intrusion in coastal areas), additional environmental benefits might result.

Groundwater contamination might arise from using wastewater in irrigation, and from applying sewage sludge to land. Nitrates are a particular problem in many countries; the risk of contaminating groundwater through wastewater irrigation depends on local conditions as well as on the rate of application. Where a deep, homogeneous, unsaturated zone overlies the saturated layer of the aquifer, most pollutants will be removed in the unsaturated layer and there will be a very low risk of contaminating the groundwater. A high-risk situation will arise only where a shallow and/or a highly porous unsaturated zone exists above the aquifer, especially if this zone is fissured.

Municipal wastewater is likely to contain chemical pollutants wherever industrial discharges are allowed into the sewerage system. Of particular concern are those that are toxic to man, plants and aquatic biota. Heavy metals and refractory organics fall into this category. Boron, a constituent of synthetic detergents, is an important phytotoxin, especially of citrus crops, and should be monitored when wastewater is used for irrigation. Preventing chemical pollutants from entering sewerage systems is the best solution but this is difficult to achieve unless industrial zones are isolated and provided with their own wastewater treatment plants.

A possible long-term problem of wastewater irrigation is build-up of toxic materials or salinity in the soil. As the unsaturated zone removes chemical pollutants - particularly heavy metals their concentration in the soil will increase with time and, after many years of irrigation, it is possible that toxic levels could develop and be absorbed by a crop. Soil salinization is common in arid regions where irrigation water is saline; wastewater irrigation could cause this over the long term, thereby rendering the land unusable for agriculture.

The chronic effects of long-term exposure to low levels of toxic chemicals, through consuming groundwater into which these materials have leached, is also of concern. Although studies have indicated that only negligible amounts of such toxic chemicals normally move 30 cm beyond the point of application within the soil, it is possible that long-term effluent re-use and eventual accumulation of toxic materials in the soil might ultimately lead to their mobilisation and increase groundwater concentrations. Numerous studies have indicated that the content of certain toxic metals in plant tissues is directly proportional to the concentration of such metals within the soil root zone. Thus, long-term application of wastewater in irrigation poses the risk of plants having high levels of toxic materials in their tissues. The FAO 'Irrigation and Drainage Paper No. 29' (Ayers and Westcot, 1985) recommends some maximum concentrations for phytotoxic elements in irrigation water.

Wastewater quality guidelines for irrigation use

Health protection measures which can be applied in agricultural use of wastewater include the following, either singly or in combination:

· Wastewater treatment
· Crop restriction
· Control of wastewater application
· Human exposure control and promotion of hygiene.

In the past, wastewater treatment has been widely adopted as the major control measure in controlled effluent use schemes, with crop restriction being used in a few notable cases. A more integrated approach to the planning of wastewater use in agriculture will take advantage of the optimal combination of the health protection measures available and allow for any soil/plant constraints in arriving at an economic system suited to the local socio-cultural and institutional conditions.

A WHO (1989) Technical Report on 'Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture' discusses the integration of the various measures available to achieve effective health protection. Limitations of the administrative or legal systems in some countries will make some of these approaches difficult to apply, whereas shortage of skilled technical staff in other countries will place doubt upon reliance on wastewater treatment as the only control mechanism. To achieve greater flexibility in the use of wastewater application as a health protection measure, irrigation systems must be developed to be capable of delivering low quality wastewater and restrictions on irrigation technique and crops irrigated must become more common.

Many schemes have been proposed for the classification of irrigation water. In the FAO 'Irrigation and Drainage Paper No. 29' (Ayers and Westcot, 1985) irrigation water is classified into three groups, based on its salinity, sodicity, toxicity and miscellaneous hazards, which help to identify potential crop production problems associated with the use of conventional water sources. These guidelines are equally applicable to evaluate wastewaters for irrigation purposes and the recent FAO 'Irrigation and Drainage Paper No. 47' (Pescod, 1992) on 'Wastewater Treatment and Use in Agriculture' adopts the same criteria for chemical constituents, such as dissolved solids, relative sodium content and toxic ions. Such guidelines stress the management needed to use wastewater of a certain quality successfully and must take account of the local conditions at the planning stage of wastewater irrigation schemes.

Effluent quality guidelines for health protection

The WHO (1989) Technical Report recommended microbiological quality guidelines for wastewater use as irrigation water, as shown in Table 2. These guidelines were based on the consensus view of a WHO Scientific Group of environmental specialists and epidemiologists that the actual risk associated with the use of treated wastewater for irrigation is much lower than previously perceived and that earlier standards and guidelines concerned with health control were unjustifiably restrictive, particularly in respect of faecal coliforms.

The new guidelines are stricter than previous standards in respect of the requirement to reduce the numbers of helminth eggs (Ascaris and Trichuns species and hookworms) in effluents for Category A and B conditions to a level of not more than one per litre. Also implied by the guidelines is the expectation that protozoan cysts will be reduced to the same level as helminth eggs. Although no bacterial pathogen limit is imposed for Category C conditions where farm workers are the only exposed population, on the premise that there is little or no evidence indicating a risk to such workers from bacteria, some degree of reduction in bacterial concentration is recommended for any effluent use situation.

The WHO Scientific Group considered the new approach to effluent quality would increase public health protection for the large numbers of people who were now being infected in areas where crops eaten uncooked are being irrigated in an unregulated, and often illegal, manner with raw wastewater It was felt that the recommended guidelines, if adopted, would achieve this improvement and set targets which are both technologically and economically feasible. However, the need to interpret the guidelines carefully and modify them in the light of local epidemiological, socio-cultural and environmental factors was also pointed out.

The effluent quality guidelines in Table 2 are intended as design goals for wastewater treatment systems, rather than standards requiring routine testing of effluents. Wastewater treatment processes achieving the recommended microbiological quality consistently as a result of their intrinsic design characteristics, rather than by high standards of operational control, are to be preferred. In addition to the microbiological quality requirements of treated effluents used in agriculture, attention must also be given to those quality parameters of importance in respect of groundwater contamination and of soil structure and crop productivity. Although heavy metals may not be a problem with purely domestic sewage effluents, all these elements are potentially present in municipal wastewater.

Table 2 Recommended microbiological quality guidelines for wastewater use in agriculture

Wastewater treatment

The most appropriate wastewater treatment to be applied before effluent use in agriculture is that which will produce an effluent meeting the recommended microbiological and chemical quality guidelines both at low cost and with minimal operational and maintenance requirements. Adopting as low a level of treatment as possible is especially desirable in developing countries, not only from the point of view of cost but also in acknowledgement of the difficulty of operating complex systems reliably. In many locations it will be better to design the re-use system to accept a low grade of effluent rather than to rely on advanced treatment processes producing a reclaimed effluent which continuously meets a stringent quality standard.

Nevertheless, there are locations where a higher-grade effluent will be necessary and it is essential that information on the performance of a wide range of wastewater treatment technology should be available. The design of wastewater treatment plants is usually based on the need to reduce organic and suspended solids loads to limit pollution of the environment. Pathogen removal has very rarely been considered an objective but, for re-use of effluents in agriculture, this must now be of primary concern and processes should be selected and designed accordingly. Treatment to remove wastewater constituents that may be toxic or harmful to crops is technically possible but is not normally economically feasible. Unfortunately, few performance data on wastewater treatment plants in developing countries are available and even then they do not normally include effluent quality parameters of importance in agricultural use.

The FAO 'Irrigation and Drainage Paper No. 47' (Pescod, 1992) deals with wastewater treatment alternatives and stresses the need for reliability of treatment. In the case of developing countries, without experience in operating wastewater treatment plants and short of trained manpower, conventional wastewater treatment processes will be less likely to produce satisfactory effluents consistently than natural low-rate biological treatment systems. Such systems, particularly wastewater stabilization ponds, tend to be lower in cost and less sophisticated in operation and maintenance. Although they tend to be land intensive, they are generally more effective in removing pathogens and do so reliably and continuously if properly designed and not overloaded. Ponds are recommended in the WHO (1989) health guidelines as the preferred method of wastewater treatment for effluent use in irrigation.

Strategies for managing treated wastewater use in agriculture

Success in using treated wastewater for crop production will largely depend on adopting appropriate strategies aimed at optimizing crop yields and quality, maintaining soil productivity and safeguarding the environment. Several alternatives are available and a combination of these alternatives will offer an optimum solution for a given set of conditions. The user should have prior information on effluent supply and its quality to ensure the formulation and adoption of an appropriate on-farm management strategy.

Basically, the components of an on-farm strategy in using treated wastewater will consist of a combination of:

· crop selection
· selection of irrigation method
· adoption of appropriate management practices.

Furthermore, when the farmer has additional sources of water supply, such as a limited amount of normal irrigation water, he will then have an option to use both the effluent and the conventional source of water in two ways, namely:

· by blending conventional water with treated effluent
· using the two sources in rotation.

Crop selection provides the opportunity to overcome salinity hazards, toxicity hazards and health hazards and details of each aspect are provided in the FAO 'Irrigation and Drainage Paper No. 47' (Pescod, 1992). However, in terms of health control, although crop restriction protects the consuming public it does not protect farm workers and their families. Therefore, it is not adequate on its own and should be complemented by other measures, such as partial wastewater treatment, controlled wastewater application and/or human exposure control. Crop restriction is feasible under the following conditions:

· where a law-abiding society or strong law enforcement exists

· where a public body controls waste allocation

· where an irrigation project has strong central management

· where there is adequate demand for the crops allowed under crop restriction - and they fetch a reasonable price

· where there is little market pressure in favour of excluded crops, such as those in Category C.

Adopting crop restriction to protect health in re-use schemes will require a strong institutional framework and capacity to monitor and enforce regulations. Farmers must be advised why such crop restriction is necessary and be assisted in developing a balanced mix of crops to use fully the constant production of partially treated wastewater National agricultural planning should take the crop production potential of restricted re-use schemes into account so that production excesses are avoided.

Wastewater application control could, theoretically, allow a raw wastewater to be used for irrigation but, in practice, this would require the development of irrigation systems to deliver low-grade effluent through subsurface systems. Flooding irrigation involves the least investment, but probably exposes field workers to the greatest risk. If the effluent is not of the quality required for C´ategory B, sprinkler irrigation should not be used, except for pasture or fodder crops, and border irrigation should not be used for vegetables. Subsurface or localized irrigation can give the greatest degree of health protection, as well as using water more efficiently and often producing higher yields. However, it is expensive, and a high degree of reliable treatment is required to prevent the small holes (emitters) through which water is slowly released into the soil from dogging. Bubbler irrigation, a technique developed for localized irrigation of tree crops, avoids the need for small emitter apertures to regulate the flow to each tree.

Human exposure control

Four groups of people can be identified as being at potential risk from the agricultural use of wastewater:

· agricultural field workers and their families
· crop handlers
· consumers (of crops, meat and milk)
· those living near the affected fields.

Agricultural field workers' exposure to hookworm infection can be reduced by in-field use of appropriate footwear. Immunization is not feasible against helminthic infections, nor against most diarrhoeal diseases, but immunization of highly exposed groups against typhoid and hepatitis A may be worth considering. Additional protection may be afforded by providing adequate medical facilities to treat diarrhoeal diseases, and by regular chemotherapeutic control of intense nematode infections in children and control of anaemia. Chemotherapy and immunization cannot be considered totally adequate, but could be beneficial as temporary palliative measures. Tapeworm transmission can be controlled by meat inspection.

Risks to consumers can be reduced by thorough cooking and by high standards of hygiene. Food hygiene is a theme to be included in health education campaigns. Local residents should be kept fully informed about the location of all fields where wastewaters are used, so that they can avoid entering them and also prevent their children from doing so. There is no evidence that those living near wastewater-irrigated fields are at significant risk from sprinkler irrigation schemes. However, sprinklers should not be used within 50-100 m of houses or roads.

Case-study examples

Wastewater treatment in stabilization ponds: Al Samra, Jordan

The Al Samra Wastewater Stabilization Pond (WSP) System was commissioned in May 1985 and by 1986 was receiving approximately 57 000 m³/day of domestic wastewater and septage from the Metropolitan Area of Greater Amman, Jordan. This system comprises three trains of ponds, each designed to contain two anaerobic ponds (A-1 and A-2), four facultative ponds (F-1, F-2, F-3 and F-4) and four maturation ponds (M-1, M-2, M-3 and M-4). 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.

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). In terms of overall performance in 1986, the Al Samra ponds were highly efficient, removing 80% and 91% of the incoming BODs 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).

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 3 shows total coliform and faecal coliform reductions through the pond series for the period concerned. It is clear that the final effluent (after Pond M - 4) 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. Linear regression analysis of the data indicated that retention time, pond BOD5 concentration, pH and depth had a significant effect on faecal coliform die-off. Data on nematode egg removal during January and February 1987 showed that nematode eggs were absent from the final effluent (Pond M-4 outlet) over the period and indicated 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 F-4 outlet).

Table 3 Monthly geometric means for total and faecal coliforms (numer per 100 ml)

Wastewater treatment and crop restriction: Tunisia

Wastewater use in agriculture has been practiced for several decades in Tunisia and is now an integral part of the national water resources strategy. Use of treated effluents is seasonal (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. The 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 km away. 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 effluentirrigated area will occur.

In the period 1981-87, 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 valgare) and pepper (Capsicum annum) 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 wastewater, investigations were carried out on the perimeter area of La Soukra, where irrigation with treated effluent had been practiced 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 effluent contains moderate to high salinity but presents no alkalization risk and trace element concentrations are below toxicity thresholds. The sewage sludge from Soukra and Nabeul 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 (as K2O) but a deficit of phosphorus (as P2O5). The fertilizing value of 30 tonnes dry weight of sewage sludge per hectare 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.

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 changed considerably, with an increase in electrical conductivity and a transformation of the geochemical characteristics of the soil solution from bicarbonate-calcium to chloride-sulphatesodium (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 piked 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.

Wastewater treatment and human exposure control: 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 (Agricultural 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 had 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 1970s 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 semi-portable sprinklers and flood and furrow irrigation.

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).

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. 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/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.

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 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	- 49 mg/l
Cl: 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 (six-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.

Crop restriction for wastewater irrigation: Mexico

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. 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.

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. 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).

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% in exposed versus 4% in control group) and Ascaris lumbricoides (50% in exposed versus 16% 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.

References

AGRICULTURAL AFFAIRS AND FISH RESOURCES AUTHORITY, Kuwait (1988) Treated sewage effluent for irrigation in Kuwait. In: Treatment and Use of Sewage Effluent for Irrigation.

PESCOD, M. B. and ARAR, A. (eds), Butterworths, Sevenoaks, Kent.

AL-SALEM, S. S. (1987) Evaluation of the Al Samra Waste Stabilization Pond System and its Suitability for Unrestricted Irrigation. Paper prepared for the Land and Water Development Division, FAO, Rome.

AYERS, R. S. and WESTCOT, D. W. (1985) Water Quality for Agriculture. FAO Irrigation and Drainage Paper No. 29, Rev. 1, Food and Agriculture Organization, Rome.

BAHRI, A. (1988) Present and future state of treated wastewaters and sewage sludge in Tunisia. Paper presented at Regional Seminar on Wastewater Reclamation and Re-use, 11-16 December, Cairo.

COBHAM, R. O. and JOHNSON, P. R. (1988) The use of treated sewage effluent for irrigation: case study from Kuwait. In: Treatment and Use of Sewage Effluent for Irrigation. PESCOD, M. B. and ARAR, A. (eds), Butterworths, London.

FEACHEM, R. G., BRADLEY, D. J., GARELICK, H. and MARA, D. D. (1983) Sanitation and Disease: Health Aspects of Excreta and Wastewater Management. John Wiley & Sons, Chichester, UK.

GUNNERSON, C. G., SHWAL, H. I. and ARLOSOROFF, S. (1984) Health effects of wastewater irrigation and their control in developing countries. In: Proceedings of the Water Re-use Symposium III, San Diego, AWWA Research Foundation, Denver.

PESCOD, M.B. (1992) Wastewater Treatment and Use in Agriculture. FAO Irrigation and Drainage Paper No. 47, Food and Agriculture Organization of the United Nations, Rome.

SANCHEZ DURON, M. (1988) Mexican experience in using sewage effluent for large scale irrigation. In: Treatment and Use of Sewage Effluent for Irrigation. PESCOD, M. B. and ARAR, A. (eds), Butterworths, Sevenoaks, Kent.

SAQQAR, M. M. and PESCOD, M. B. (1990) Microbiological performance of multi-stage stabilization ponds for effluent use in agriculture. Water Science Technology, 23: 1517-1524.

SHUVAL, H. I., ADIN, A., FATTAL, B., RAWITZ, E. and YEKUTIEL, P. (1986) Wastewater Irrigation in Developing Countries: Health Effects and Technical Solutions. Technical Paper No. 51, World Bank, Washington, DC.

SHUVAL, H.I., YEKUTIEL, P. and FATTAL, B. (1984) Epidemiological evidence for helminth and cholera transmission by vegetables irrigated with wastewater: Jerusalem - a case study. In: Proceedings of the Twelfth IAWPRC Conference, Amsterdam.

STRAUSS, M. and BLUMENTHAL, U. J. (1989) Human Waste Use in Agriculture and Aquaculture: Utilization Practices and Health Perspectives. IRCWD Report No. 08/89. International Reference Centre for Waste Disposal, Dubendorf, Switzerland.

WHO (1989) Health Guidelines for the Use of Wastewater in Agriculture and Aguaculture. Technical Report No. 778, WHO, Geneva.

Discussion

There were a number of questions posed over the problems and risks associated with the use of wastewater for irrigation. It was said that there was a risk of contaminating crops from subsurface delivery of wastewater only in the case of root crops. Salinity was not necessarily a problem. Heavy metals do not generally present a problem in urban wastewater in developing countries but where they did exist in significant quantities they would have to be dealt with. The tight WHO standard for Ascaris eggs was difficult to measure and further work was being done on developing techniques. Helminths would survive for one month but the use of a settling pond could be an efficient way in which to deal with them, although this leads to the question of how to use the sediment. On the potential for using wastewater in aquaculture the WHO 'Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture' was referred to.