Last September, the International Water Association conference in Beijing saw the launch of the revised World Health Organization Guidelines for the Safe Use of Wastewater, Excreta and Greywater in Agriculture and Aquaculture (WHO 2006a,b, 2006c,d). Volume 2 of these guidelines deals with the use of sewage or municipal wastewater for agricultural production. The use of wastewater in agriculture started in the 19th century, when cities in Europe and North America introduced the water-carriage system for domestic wastewater. Large sewage farms, as they were called, were established in the UK, USA, France and Germany, followed by India, Australia and Mexico (Mara & Cairncross 1989). Their purpose was to prevent contamination of rivers and to improve soil fertility. Most of these sewage farms were abandoned at the beginning of the 20th century for a number of reasons, notably the need for more land for expanding cities, increased awareness of the potential adverse human health impacts, the introduction of chemical fertilizers, and the development of wastewater treatment technologies. However, with a growing world population and unprecedented urbanization, especially in developing countries, the new driving force behind the use of wastewater in agriculture is that fresh water has become an increasingly scarce and contested resource.
The current revision is the second since the WHO published its first guidelines in 1973. The WHO guidelines remain internationally authoritative although some countries maintain guidelines with a different approach. For example, many states in the US have guidelines for the irrigation of food crops using wastewater, based on the principle that there should be no risk of infection (USEPA 2004), while the WHO guidelines are based on the principle that there should be no additional cases of disease in the population at risk. The WHO guidelines are based on a scientific consensus of the best available evidence and encourage national governments to adapt the guidelines to their own socioeconomic and environmental realities. In the 1989 revision of the WHO guidelines, the microbiological standard was relaxed from a maximum of 100 total coliform bacteria to a geometric mean of 1000 faecal coliform bacteria per 100 ml irrigation water, for crops consumed uncooked. At that time, there was sufficient epidemiological evidence that infection with intestinal helminths poses the major human health risk associated with the agricultural use of untreated urban wastewater and therefore an intestinal nematode guideline of ‘≤1 nematode egg per litre’ was added. The relaxation of the faecal coliform guideline evoked strong reactions, with some advocating a much stricter standard of no detectable faecal coliform bacteria. Others criticized the WHO guidelines as still being too strict and too focused on expensive wastewater treatment technology, thereby not taking into consideration the ground reality in developing countries.
The reality is that in many developing countries untreated wastewater is either used directly in agriculture or disposed of untreated into surface water from where it is used for crop irrigation. There are no reliable figures on the extent and importance of wastewater use in agriculture at the global level but it is estimated that as little as 20% of all wastewater produced globally, undergoes treatment, while estimates for the area of agricultural land irrigated by untreated, partially treated or wastewater-polluted river water range from 3.5 to 20 million hectares (Scott et al. 2004; IWMI 2006). In municipalities with limited financial resources, wastewater treatment almost always has a lower priority than the provision of safe drinking water and improved sanitation. Enforcement of water quality standards is often further complicated by the fact that it is unclear who should enforce these standards; health, agricultural or water supply and sanitation agencies? Restrictions in the crops that can be grown with wastewater are hampered by the fact that farmers grow crops which have a short growing season and a high demand at the local market as these are the most profitable to cultivate. Municipal officers in Pakistan, where untreated wastewater was used in 80% of all cities, indicated that enforcement was next to impossible as wastewater inlets to farms were opened within hours after their closure (Ensink et al. 2004). There have been court rulings in Pakistan in favour of wastewater farmers, considering that access to irrigation water is a fundamental right of farmers and that avoiding the loss of livelihood is over-riding over potential health risks.
Lack of access to regular irrigation water is not the only reason for the use of wastewater. With high concentrations of essential plant nutrients (nitrogen, phosphorus and potassium), wastewater farmers need to apply little to no chemical fertilizer. In most developing countries, irrigation water is subject to rotational schedules and thus rarely available on demand, while a continuous supply of wastewater is ensured. This allows farmers to grow more crops a year and more water-demanding crops, such as vegetables. Vegetables especially fetch high prices at the nearby urban markets and combined with reduced expenditure on fertilizer, income of wastewater farmers is often higher than that of farmers who depend on regular irrigation water. This is illustrated in Faisalabad, Pakistan, where farmers were willing to pay high water fees and land rents, which in some cases were six times higher for untreated wastewater than for regular irrigation water (Ensink et al. 2004). A nationwide survey in Pakistan showed that an estimated 25% of all vegetables grown in the country were irrigated with untreated urban wastewater and that these vegetables, cultivated close to the urban markets, were considerably cheaper than the vegetables imported from different regions of Pakistan (Ensink et al. 2004). Likewise, 60% of the vegetables consumed in Dakar, Senegal were grown with a mixture of groundwater and untreated wastewater within the city limits (Faruqui et al. 2004). In this context, the use of wastewater for peri-urban agriculture provided an opportunity and a resource for livelihood generation.
So what is new in the current guidelines and have the revisions been able to meet past criticism? A major change is that the controversial faecal coliform guidelines has given way to a tolerable burden of disease and a quantitative microbiological risk assessment approach. In this approach, the risk of disease from exposure to a specific pathogen is estimated and based on that the reduction in faecal coliform concentrations that need to be achieved to guarantee safe use of wastewater in agriculture is calculated. There is also now a much stronger focus on the realities of wastewater use in developing countries with a clear acknowledgement of the livelihood factor associated with wastewater irrigation. The revised guidelines take natural die-off of pathogens on produce into consideration and provide a multiple barrier risk reduction approach and a wider range of risk reduction measures such as localized irrigation techniques, and food preparation measures like washing or peeling of produce. However, the guidelines specifically state that these measures, as well as chemotherapy and vaccinations, are complementary and should not be seen as alternatives to wastewater treatment. This seems to be confirmed by the fact that all different risk reduction scenarios that are presented as a matter of example, include wastewater treatment technology. This seems to imply that municipalities, where untreated wastewater is currently being used, have only two alternatives to protect public health: the removal of farmers from their land, or to turn a blind eye to the practice. This seems short-sighted because based on the evidence presented in the guidelines, a last (wastewater) irrigation 5 days before harvest and the concomitant die-off of pathogens on produce, can under certain climatic conditions, be more effective than wastewater treatment.
The use of wastewater in agriculture poses obvious health risks and there is a strong need for regulation through water quality standards and health protection measures. The WHO guidelines are the most appropriate for developing countries, as they are based on actual risk and will therefore not result in needlessly strict and expensive treatment technologies to achieve standards. Shuval et al. (1997) estimated that the extra expenditure required to treat wastewater from WHO to USEPA water quality standards was US$ 3–30 million per case of prevented (enteric) disease. This investment seems hard to justify in a developing country where the burden of disease as a result of wastewater irrigation can be expected to be just a fraction of that from poor access to improved water supply and sanitation, and as a result investments in drinking water supply, sanitation and hygiene promotion will be much more cost-effective and should receive priority over wastewater treatment.
There is, however, a strong need for more a sound epidemiological evidence, as Blumenthal and Peasey (2002) pointed out in their review of past research on wastewater use in agriculture. Past studies have been found wanting in epidemiological and statistical rigour and evidence is heavily biased towards industrialized countries with the bulk of the available evidence coming from studies conducted in Israel and the US and little to no sound epidemiological evidence from developing countries. There is therefore a strong need for more field based studies under different environmental, climatic and socioeconomic conditions, that combine a water quality or produce quality survey with an epidemiological study on health risks in farmers and consumers, as this will improve the credibility of the WHO guidelines.
The WHO rightly points out that realistic national water quality standards should be established because standards that are too strict tend to be ignored. However, the same might be the case for a guideline which still relies heavily on wastewater treatment. Although the WHO guidelines since their inception in 1973 have offered other measures for risk reduction, only wastewater treatment has been implemented under field conditions, while the effectiveness of other measures, like for example the use of localized (drip) irrigation technology, to reduce risk has often only been tested under experimental conditions and therefore the acceptability of other measures to farmers, policy makers and consumers remains unknown. As a result, municipalities are reluctant to officially condone the use of wastewater if wastewater treatment cannot be provided.
Wastewater is an important resource for the livelihood and food security of poor urban and peri-urban households. Technically it is possible to treat wastewater to drinking water standards, but this is not a realistic option in many developing countries (or indeed in many industrialized countries). The challenge is to manage wastewater in a way that makes it safe even when full wastewater treatment is not available. Policy makers and municipalities should therefore focus on low-cost decentralized treatment alternatives and wastewater application methods, like for example (subsurface) drip irrigation which can reduce human exposure. This should be combined with well-tested strategies such as treatment of people at high risk of helminth infection, promotion of washing and cooking of agricultural produce before consumption, and general improvements in hygiene practices, sanitation and water supply.