Paper 7 Pollution alleviation issues: a case study on the River Ganges

I. M. Griffiths
Thames Water International, National Rivers Authority - Thames Region, Reading, Berkshire

Summary: Thames Water International were employed as technical advisors to the Ganga Project Directorate to assist with the implementation of the Ganga Action Plan, an ambitious plan to clean up the River Ganges. A team of water quality and sewerage experts from Thames Water were charged with evaluating the sources and extent of water pollution in the River Ganges and to assist in drawing up a plan for its control or prevention. In addition to the environmental work the consultancy covered a wide range of disciplines including sewage treatment, sewer maintenance and sludge management. This paper will concentrate on the environmental aspects of the work but relates the findings to the management of sewage disposal. Technology transfer was a major element of this project.

Introduction

The work was sponsored by the Overseas Development Administration and consisted of an advisory programme between the governments of India and the UK. Thames Water environmental and sewage treatment specialists worked with counterpart Indian scientists and engineers from the Ganga Project Directorate and the State Pollution Control Boards to effect technology transfer and to facilitate the requirements of the Ganga Action Plan. The work contained a number of key elements including the development of a computer model to assist with strategic water quality planning. This was linked to a project to design and procure an automatic river quality monitoring (ARQM) system and to undertake river quality surveys with which to establish suitable sites for the siting of the monitors and to provide baseline data for input into the model. In addition, advice was given on the selection of appropriate sewage treatment options and associated maintenance requirements. Priorities were set for the rehabilitation of sewers and sewage treatment works. The siting and design parameters for new treatment plants were also considered. This work required a significant amount of time to be spent in India, gathering information and gaining an understanding of the current situation and practices.

When undertaking a programme of water pollution abatement a thorough understanding of the river environment is an essential precursor. The physico-chemical and flow characteristics of the river, seasonal patterns, existing pollution loadings and the use requirements of the river need to be known before the siting and design of sewage treatment works can be undertaken. This information can then be fed into a computer programme which is capable of simulating the impact of varying the concentrations of pollutants upon the river.

This paper will concentrate on the environmental elements of this work, specifically the design, procurement and commissioning of the network of automatic river quality monitoring stations and the water quality surveys undertaken on the River Ganges. The work began with the author making a preliminary visit to India in August 1986 and subsequently writing an ARQM system specification. River surveys were carried out in April and May 1987 and February, March and April 1989. Further visits were made in February 1990 and 1991 to test and commission the equipment.

The Ganga Action Plan

The Ganga Action Plan is best summarised by the following extract from An Action Plan for the Prevention of Pollution of the Ganga, Department of the Environment, Government of India, revised, 1985:

"Based on a comprehensive survey of the Ganga Basin carried out by the Central Board for the Prevention and Control of Water Pollution (CPCB), an Action Plan for the prevention of pollution of the Ganga was prepared by the Department of the Environment (India) in December, 1984. The Central Ganga Authority (CGA) with the Prime Minister (Rajiv Ghandi) as chairman was set up by Government Resolution in February, 1985. This was a high level body for determining policies and programmes, to allocate resources and mobilise public support for accomplishing the Action Plan. In June 1985, the Ganga Project Directorate (GPD) was established as a wing of the Department of the Environment, to appraise and clear the projects prepared by the field level agencies, release funds and co-ordinate the various activities under the Action Plan on a continuing basis.”

The principle aims of the Action Plan are, "the immediate reduction of pollution load on the river and the establishment of self-sustaining treatment plant systems."

General characteristics of the River Ganges

The River Ganges (Ganga) is the largest and most important river in India. It is 2552 kilometres long and carries the drainage of a vast basin of more than 1 060 000 square kilometres, which is bounded by snow-covered peaks of the Himalayas in the north and the peninsular uplands of the Vindya range to the south (figure 1). It extends over four countries, India, Nepal, Bangladesh and China. It drains 814 400 square kilometres within India, covering more than a quarter of the land area. It is a mayor surface and groundwater resource with an annual flow of 468.7 billion cubic metres, equivalent to approximately one-quarter of India's total water resource. The Ganges basin is the home of one third (approximately 200 million) of the Indian population and is one of the most important pilgrim centres of India (CPCB, 1984).

Figure 1 General map of the River Ganges

Seasonal and climatic considerations

The subtropical river and climatic conditions are associated with four seasons, characterised as follows (CPCB, 1984):

1. Monsoon season (June to September)

Frequent rainfall, dramatic increase in river level and flow

Air temperature, 25-40°C

High humidity

2. Post - monsoon season (October to November)

River flows decline sharply

Air temperature, 15-35°C

3. Winter season (December to February)

River flows continue to decline

Occasional bursts of winter rainfall

Air temperature, 5-25°C

4. Summer season (March to May)

Flows as for winter

Air temperature, 15-45°C.

This seasonality is reflected in extremes of river depth and flow rate, with the effects being particularly pronounced at Varanasi where the river is constricted by high ground on either side. For example, at Varanasi, during the summer season water depth is approximately 12 metres with a mean flow of 285 cubic metres per second. During the monsoon, depths rise to 20 metres and mean flows increase to 13 454 cubic meters per second.

The subtropical nature of the Indian climate is an important consideration in the design and operation of equipment which must work at high ambient temperatures and variations in air temperature from 5°C during winter nights to 45°C during summer days must be expected.

ARQM system design and specification

The experience gained from the operation and development of the freshwater and tidal ARQM stations in the River Thames catchment provided the basis for the specification of the system for the Ganges. The format of the tidal system was particularly appropriate because of the simplicity of the in-situ (sensors immersed directly in the river) sensor arrangement, designed to operate in harsh conditions which included large fluctuations in water level. Modifications to the format were required to accommodate the subtropical environment and the monitoring needs of the Ganges.

A specification was written for a network of nine monitoring stations to be sited on the Ganges to monitor the effects of water pollution in the vicinity of specified major cities. The stations would be un-manned and would be remote from operational facilities. Mains power would not be available. The project would involve the production of an operational prototype to be tested near Delhi under the supervision of the author. Phased introduction of the nine monitors would follow.

Water quality information provided by the CGA and a review of water quality in the Ganga (CPCB, 1982 and 1984) provided a baseline with which to specify the system. The specification was written in 1986 in the form of an International Tender Document.

Seasonal variations in water level of up to 10 metres had to be taken into account. This factor, combined with unmade banks and the lack of suitable buildings in remote areas excluded the use of fixed stations and meant that a floating platform arrangement anchored to the river-bed or suitable structure offered many advantages. In addition, floating platforms allowed for the possibility of moving outstations at a later date and made them suitable for use on any river in India.

Other factors that had to be taken into account in the design of the floating platforms were as follows:

· provision for anti-fouling measures
· availability and size of craft for positioning and anchoring a platform
· need for specialist advice on anchorage of platforms
· need for the system to be operational during the monsoon, particularly during its 'first flush'. In case this proved impossible, provision had to be made for the removal of the equipment for the duration of the monsoon.

The floating platforms and anchorages were designed in collaboration with marine architects from the Oceanographic Research Centre in Madras. Figure 2 is a schematic drawing of the platform which formed the basis of the detailed design and construction work undertaken by the architects. Figure 3 is a photograph of the equipment in operation on the River Ganges.

Since it was proposed to operate the stations in truly remote locations where mains power would be unobtainable, power consumption had to be kept to a minimum and solar power options were the most suitable choice.

The following factors had to be taken into account in the design of the mounting for the sensors:

· sensors were to be placed directly in the river
· sensors should be mounted on a robust 'lance' assembly to allow for sampling at a depth of 0.5-1.0 m below the surface
· sensors needed to be protected from damaging impacts from floating debris
· sensors needed to be easily removed for servicing and cleaning
· anti-fouling measures
· incorporation of a facility for swinging the probe up and back down in the event of collision with a submerged object.

The ARQM equipment was required to provide the following range of sampling frequencies:

· 24 per day
· 12 per day
· 6 per day
· 4 per day
· 1 per day

The actual frequency of sampling that would be employed in long-term monitoring programmes would depend upon trial results. Dataloggers would be provided in the first instance and options to convert to a telemetry system were specified.

Water quality surveys of the River Ganges

These investigations represent part of a large programme of surveys carried out in 1987 and 1989 in the vicinity of major cities known to contribute significant pollution loads to the Ganges. All survey sites were potential sites for the introduction of ARQM. These were in the vicinity of Allahabad and Kanpur (surveyed in 1987) and Kannauj, Kanpur, Patna and Barauni. Three sites on the Hoogli Estuary near Calcutta were also surveyed in 1989 and as noted above, surveys were carried out at Varanasi in both 1987 and 1989 (Themes Water International, 1987 and 1989).

Figure 2 Schematic drawing of river quality monitoring station

Topographical, physico-chemical and bacteriological surveys were undertaken across transects at strategic points in the vicinity of the cities studied with effort being concentrated upstream and downstream of the major effluents. A series of depth profiles was recorded at selected transect locations, the number of measurements being dependent upon the variability of the river. The depth profiles showed river bottom features identified by echo sounder. At full transect sites profiles of the flood plain were fixed using the electronic distance measurer.

The results of the river surveys at Varanasi are summarised in Figures 4, 5 and 6 which are examples of a cross-sectional dissolved oxygen profile, bacteriological results and a management summary diagram indicating the distribution of pollution in the river.

Figure 4 Examples of cross-sectional dissolved oxygen profile of the River Ganges at Varanasi, 1987

Figure 5 Bacterial counts: thermo-tolerant coliforms/100 ml, Varanasi, April 1987

Figure 6 Schematic diagram of water depth profiles and pollution streaming: summary of survey findings, Varanasi, 1989

Discussion

Reasons for the use of ARQM on the River Ganges

In order to fulfil the aim of the Ganga Action Plan to improve the water quality status of the River Ganges, it is essential to have comprehensive information on the river's quality on a 2 hour basis. ARQM systems will assist in gathering this information.

ARQM serves to complement the limited laboratory facilities in India and the system specified has an advantage of being based on and adapted from a proven system in operation on the River Thames. The modular design should assist in maintenance and the in-situ configuration will allow the ARQM stations to be easily moved to new sites if required.

The survey work involved in locating suitable monitoring sites improved the basic knowledge of the water quality of the river and the Indian team trained during the surveys has continued to undertake detailed surveys at other sites on the Ganges. The survey equipment used was given to the GPD by the Overseas Development Agency.

Implementation of the ARQM system

The contract to manufacture equipment was awarded to Envirotech (India) Ltd following an international tendering exercise according to World Bank rules. The Indian company tendered a competitive price and some advantage was seen by the Indian government in awarding the contract to an indigenous company. Most of the specialist components were from USA or UK. Considerable delays in manufacture ensued and although the company was competent in electronics and process control it had no experience of constructing equipment to operate in the aquatic environment and considerable redesign of the 'wet end' of the ARQM was required before any acceptable reliability was achieved.

A simple modular approach to equipment design and maintenance was taken with a view to achieving as 'appropriate a technology' as possible for the Indian environment. The use of in-situ probes and the exclusion of ammonia monitoring assisted in this. The commissioning trials of the equipment seem promising but the reliability of the equipment and the ease of servicing cannot be assessed fully until the system is operational. The use of solar power should enable the equipment to operate at truly remote sites and should be well suited to the Indian climate.

The experience of operating ARQM on floating piers on the tidal Thames was particularly relevant and formed the basis for the design which utilises in-situ deployment of the sensors. The harsh environment, including fluctuations in water level, fast current speeds, probability of physical and biological fouling and remote locations meant that the technology developed for the tidal Thames system was applicable. Dissolved oxygen, temperature and conductivity sensors of a similar type to that used on the tideway were supplemented with turbidity and pH, additional factors important to the Indian water quality objective scheme. Measurement ranges were also adjusted to the Indian requirement. Most of the ARQM stations on the tidal Thames are mains powered with the exception of the self-contained floating monitoring station at Crossness. This is solar powered and although considerable scaling down of solar panels and batteries was possible for the subtropical climate, the technology was directly transferable.

Financial and data communications restrictions prevented the immediate installation of telemetered data collection from the Indian stations, although provision was made in the station design to add this at a later date. Dataloggers were installed and earlier experiences in the Thames catchment with their use assisted in the development of working practices, data collection routines and in data storage and presentation.

Finally, assistance in staff training, equipment commissioning and in setting up secure working practices has assisted in the development of ARQM in India.

Water quality of the River Ganges

The programme of surveys provided a considerable amount of information about the water quality status of the Ganges and its tributary the Yamuna (Themes Water International, 1987 and 1989. In general, the water quality of the Ganga and its major tributary, the Yamuna, is able to support a wide diversity of plant and animal species. Its fish and invertebrate communities are exceptional (Jhingran, 1978) and freshwater dolphins, extensive bird populations and reptiles were evident during the survey. High flows and the resultant dilution assist to give great powers of self purification.

Localised areas of gross pollution are associated with major cities where a variety of demands upon the river are made. In these cities the riverside is intensively used for religious bathing, drinking water, disposal of domestic and industrial waste and animal husbandry. It is in the cities and major towns where gross pollution coincides with intensive water use that environmental problems and major public health risks occur.

The major seasonal changes experienced in the subtropical environment must be taken into account when assessing water quality. The surveys were undertaken during the dry season when river flows are at their lowest and temperatures at their highest. The gross pollution from urban areas was expected to have maximum effect at this time. However, the monsoon regime of flow will have a considerable effect upon water quality (Payne, 1986). At the height of the monsoon flows, considerable dilution and river cleansing takes place. This is used by some factory complexes, for example at Barauni where effluents stored in temporary lagoons are flooded away in the monsoon (Mohan, personal communication).

At the onset of the monsoon considerable quantities of silt and other polluting matter are displaced down the river in a short period of time, the 'first flush' effect (Ittekkot et al., 1985). This has been noted on the Yamuna, downstream of Delhi where the river becomes anaerobic during the dry season and sewage sludges settle on the river bed. At the first rains this septic water, sludge and run off from the city sweeps down the river causing gross pollution resulting in major fish kills for tens of kilometres downstream (Trevedi, personal communication). The ARQM may be very important in assessing the effects of this first flush effect. Because the Yamuna has relatively little flow in the dry season the polluting effects on the river downstream are not extensive. Fish populations can recolonise from the unpolluted tributaries once the first flush has passed.

During the post monsoon period, the river flows recede and nutrients are rapidly assimilated by plant and animal activities. During the dry season, the river has the chemical and physical appearance of an oligotrophic environment. However, closer examination of the benthic invertebrate communities shows that high productivity occurs during the post monsoon period (Andrews, personal communication).

The 'oligotrophic' nature of the river in the dry season and the nature of the flora and fauna present make the river very vulnerable to damage from eutrophication. The indigenous fauna and flora are unlikely to withstand a greatly increased pollution load. The river is currently protected by the lack of mains sanitation which, combined with insufficient water resources in the majority of large conurbations, prevents the pollution load from reaching the river. The current pollution loads are a fraction of what might be expected from cities of comparable population in the West. In addition, during the dry season non point source pollution loads to the Ganges are negligible (Payne, 1986).

There are some indications that pollution loads are already increasing. For example at the major industrial city of Kanpur, the Aver is reaching saturation point and water quality is poor, although it never becomes anaerobic (Themes Water International, 1987). At other sites, the river rapidly recovered from the pollution loads generated at each city which, although causing local pollution problems, never extensively threatened the ecosystem to the same extent as at Kanpur.

The immediate problems at Kanpur may be alleviated in the short term by improving the sewage treatment works (the civil engineering is already underway as part of the Ganga Action plan). However, the overall polluting load on the river must be maintained at a low level and the requirements for effluent standards may have to be extremely strict to maintain the vulnerable riverine community.

There is some evidence to suggest that the fish and reptile community has already been damaged by man's influence and migratory fish populations are impoverished. It was likely that large migratory fish runs occurred. Now only meagre catches of small cyprinid fish are taken from the river (Jhingran, 1978). Unlike the River Thames there is no evidence to suggest that water quality forms a complete barrier, either in the estuary or the freshwater Ganges.

This complex climatic and flow regime requires a totally different pollution control strategy from that used in temperate climates, such as the UK, which is so often applied to the Indian environment. It is hoped that ARQM and associated river quality investigations will provide more information on natural, seasonal or man-made water quality changes, which will act as a basis for the management and formulation of practical solutions to public health, pollution control and environmental protection on the River Ganges.

Conclusions

River surveys yielded considerable information on the quality of the River Ganges. In general the water quality of the river is good and it is able to support a wide variety of plant and animal species. Localised areas of gross pollution are associated with major cities where a variety of demands on the river are made. Here gross pollution coincides with intensive water use and public health and environmental risks occur.

The river appears vulnerable to extensive damage if pollution loads increase significantly. The river is currently protected by the lack of mains sanitation which, combined with insufficient water resources in the majority of large conurbations, prevents the pollution load from reaching the river. The current pollution loads are a fraction of what might be expected from cities of comparable population in the west. In addition, during the dry season non point source pollution loads to the Ganges are negligible (Payne, 1986). Some evidence of the deleterious effect of increasing pollution loads were noted at Kanpur.

The research and development work undertaken in the River Thames catchment was invaluable for the transfer of the technology to the River Ganges. It enabled a structured approach to be taken to the specification of the system so as to ensure that a reliable and serviceable monitoring system was developed.

The integrated nature of the project enabled a full overview of the pollution control issues to be taken. The water quality data from the survey were fed back into predictive mathematical models enabling the design criteria for the sewage treatment plants to be calculated. Remedial work could be prioritised and short-term solutions have been progressed. These include the reciting of outfalls away from potable abstractions and bathing areas in Varanasi and Allahabad.

Water quality monitoring programmes are now underway on the River Ganges. Three prototype monitors are currently undergoing extended trials. A data archive to collect, interpret and store the data from the ARQM stations and the complementary manual sampling programmes has been developed and is being commissioned. In addition, biological monitoring methods have been developed and are in use.

Acknowledgements

The author is grateful to Thames Water International, the Ganga Project Directorate, the State Pollution Control Boards and those people involved in the project from the UK and India. The views expressed in this paper are those of the author and are not necessarily those of Thames Water International, the ODA or the National Rivers Authority.

References

CENTRAL BOARD FOR THE PREVENTION AND CONTROL OF WATER POLLUTION (CPCB) (1982) Basin Sub-Basin Inventory of Water Pollution; The Yamuna Sub-Basin, Part I. Department of the Environment, India, New Delhi.

CENTRAL BOARD FOR THE PREVENTION AND CONTROL OF WATER POLLUTION (CPCB) (1984) Basin Sub-Basin Inventory of Water Pollution; The Ganga Basin, Part II. Department of the Environment, India, New Delhi.

GOVERNMENT OF INDIA (1985) An Action Plan for the Prevention of Pollution of the Ganga, (Reviled duly 1985). Central Ganga Authority, Department of the Environment, India, New Delhi.

ITTEKKCOT, V., SAFIULLAH, S., MYCKE, B. and SIEFERT, R. (1985) Seasonality and geochemical significance of organic matter in the River Ganges, Bangladesh. Nature, 317, 800-803.

JHINGRAN, V. G. (1978) Fish and Fisheries of India. Hindustan Publishing Corporation (India), Delhi, India.

PAYNE, A. L. (1986) The Ecology of Tropical Lakes and Rivers, John Wiley & Sons, Chichester, UK.

THAMES WATER INTERNATIONAL (1987) River Quality Surveys to Establish the Location of Automatic Monitoring Stations at Varanasi Allahabad and Kanpur, June 1987. Report No. 2, for the Central Ganga Authority. Thames Water International, Reading, UK.

THAMES WATER INTERNATIONAL (1989) River Ganga and Hoogli Estuary surveys, February to April 1989. Report No. 3, for the Central Ganga Authority. Thames Water International, Reading UK.

Discussion

The criteria used for improving river water quality was discussed. Criteria were related to intended uses including drinking, sustaining fish and particular to India, for bathing. It was said that there is a tendency for pesticides and other potential effluents to accumulate on the land during the dry season but it is flushed into the river during the first rains of the monsoon. Some industries used lagoons for storage of their effluent and relied on these being flushed out during the monsoon. This was probably an acceptable procedure at present, provided that the waste did not contain a toxic component. It was asked whether the changes in flora and fauna in the Varanasi indicated that the river was more seriously polluted than the paper suggested. Fish stocks were smaller than expected but it was said that this was due primarily to river barrages which have disrupted migration patterns. Lack of data and difficulties of access to that which existed was recognised as a serious problem.

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.