The use of domestic wastewater for crop production has been practised for several centuries in one form or another. Prior to the 1940s, most wastewater use occurred on "sewage farms" or areas specifically designated for such use. One of the oldest in the world is the Werribee Farm which serves the City of Melbourne, Australia. This large well-managed farm was established in 1897 and is still in operation today, irrigating some 10 000 ha with wastewater. The impetus for these "sewage farms" was to minimize or prevent pollution in rivers and conserve water and nutrients to improve agriculture (Shuval, 1991). Few of these "sewage farms" still exist today; most were ill-conceived, inadequately funded and poorly regulated, and were eventually abandoned because of public health concerns.
In the mid 1940s, domestic wastewater use again gained increased attention, especially in arid and semi-arid areas that suffer from insufficient overall water supplies. Although the same early motivations for wastewater use remained, the newer areas using wastewater were focused on ensuring they minimized or prevented potential public health problems. The principal concern was use of wastewater on crops normally eaten raw. The change in focus was driven by a better understanding of public health problems and the desire to improve public health standards.
The need to improve public health protection prompted a number of state health departments in the United States to establish guidelines and regulations to control the public health aspects of wastewater use in agriculture. These initial guidelines provided a rational basis for continuing wastewater use by agriculture while meeting strict public health criteria. One important criterion was to restrict the use of partially treated sewage to crops that are generally cooked before being consumed and allow only water that has gone through advanced wastewater treatment and microbial disinfection to be applied to crops normally eaten raw.
Many nations adopted the very strict microbial standards for wastewater use that were developed in California (USA) and elsewhere. In reality these microbial standards were almost unattainable in most wastewater treatment systems, therefore many poorer or developing countries abandoned plans for wastewater use (Shuval et al., 1986a). The primary reason was the realization that producing effluent with a microbial quality sufficient for unrestricted irrigation required costly sophisticated treatment technology. Some of these countries shifted their focus in wastewater use to unrestricted areas of use coupled with crop restrictions. Most, however, did not have a strong institutional structure to control cropping. The result has been little improvement in public health conditions associated with wastewater use. Untreated or partially treated wastewater continues to be used directly for unrestricted irrigation or is discharged to surface water channels where unintended use by agriculture occurs when water is appropriated for irrigation use.
Over the past 20 years there has been a strong revival of interest in the controlled use of wastewater for crop irrigation. In addition to consumer health protection, the main reasons are:
scarcity of alternative water supplies;
need to increase local food production;
need to improve rural health standards; and
concern for the increasing degree of surface water contamination that is limiting further development and making further progress in eliminating public health problems difficult (Witt and Reiff, 1991).
The last of the above reasons is driven by an increased public awareness of the need for clean water supplies and rivers. This perception, coupled with the population explosion in the urban areas, has resulted in strongly competing demands for water supplies, especially the best quality supplies. Agricultural and rural communities are often left to use the least desirable water supplies including those that have been contaminated with an increasing level of urban wastewater discharges. For example, Mexico is studying the cost-benefit of doubling irrigation with wastewater in the next decade. This would be a means of releasing clean water supplies to cover the domestic needs of nearly 30 million people (Cifuentes et al., 1991/92). This situation is likely to continue in many developing countries until reliable treatment and disposal works are in place. The level of contamination in rivers and irrigation water supplies may be a serious constraint for developing countries as they strive to produce an adequate and safe food supply in the future.
During the next 20 years while reliable wastewater treatment facilities are planned and constructed, agricultural and water resource planners must face two dilemmas:
manage food production and cropping with the water resource system at the present level of microbial contamination; and
plan for future wastewater use schemes which minimize or prevent the present level of microbial contamination and allow the wastewater to be a crop production resource.
There is sufficient information and technology available to plan and execute a wastewater use scheme properly. The key to a successful programme is to control potential public health problems. The differences in approach are centred on where the control or application of public health standards takes place: the point of treatment and discharge; the area where wastewater is used; or the actual point of use for crop production. In reality, the lack of treatment and well defined areas using wastewater in most developing countries will focus control for the near term on the point of use.
The simplest approach is to control the quality of wastewater at its point of treatment and discharge. This places regulation and control at the institutional level as treatment is normally conducted by a public agency. The quality of the discharge can then be regulated to fit the type of use. This alternative assumes that the treatment system is well managed and maintained and produces a reliable quality of effluent. This approach is utilized in the United States, Canada, and Europe and in many cases requires an advanced level of treatment technology. In most developing countries for the present, the lack of treatment works makes this a long-term goal. New approaches to treatment technology in developing countries (Shuval et al., 1986a) will assist in implementing this technology sooner than originally planned, but financial constraints are still likely to make this a long-term effort.
The alternative to controlling the quality of wastewater at its point of treatment and discharge is to control the place where wastewater can be used. This alternative moves around the need for immediate treatment of the wastewater and places an emphasis on controlling where the discharge is used. Under this alternative, wastewater use would be within a defined area and the emphasis would shift to controlling the type of crop production in that area. This approach requires a broader based institutional structure and a strong ability to control cropping in the wastewater use area. The key element is a defined area where cropping restrictions can be practised. This approach is utilized in several developing countries including Tunisia, Mexico, Peru and Kuwait.
The two previous alternatives assume there is either a strict control of wastewater treatment or a well defined area of use. In most developing countries lack of treatment, poorly maintained treatment works, lack of well defined use areas and unrestricted discharges to rivers and canals make using these two alternatives ineffective in the near term. Until treatment works are installed and wastewater use areas defined, most developing countries will be faced with trying to control cropping on a broad scale. Where unrestricted discharges occur to rivers and canals and widespread water appropriation occurs from these water bodies, the dilemma will be whether or not a strong institutional structure is available to implement and enforce cropping restrictions on a broad scale. An alternative approach to crop restrictions is to identify safe production areas and then utilize market pressures to implement a programme that promotes a crop produced in this safe environment.
This document describes an approach that promotes safe production areas. The programme involves evaluating the quality of irrigation water presently used, identifying safe production areas for high-risk crops, such as vegetable crops, and, through a water quality certification programme, promoting the safety of that produce. The goal is to use market pressures to promote safe vegetable products. The programme described here is based upon on an effort in Chile (1992) to control the quality of water used in vegetable production as the irrigation water in Chile was identified as a major mechanism in the spread of cholera and other gastrointestinal diseases (FAO, 1993; Shuval, 1993).