Documento(s) 6 de 19

¹World Health Organization (WHO), Geneva, Switzerland
²London School of Hygiene and Tropical Medicine (LSHTM), London, UK
³School of Civil Engineering, University of Leeds, Leeds, UK

Abstract

The use of wastewater in agriculture – often untreated or inadequately treated – is occurring more frequently because of water scarcity and population growth. Often the poorest households rely on this resource for their livelihood and food security needs. However, there are negative health implications of this practice that need to be addressed. In 1989 the World Health Organization (WHO) developed guidelines for the safe use of wastewater in agriculture, which are currently being revised based on new data from epidemiological studies, quantitative microbial risk assessments and other relevant information. The revisions being developed are in accordance with the Stockholm Framework that provides a tool for managing health risks from all water-related microbial exposures. The Stockholm Framework encourages a flexible approach to setting guidelines, allowing countries to adapt the guidelines to their own social, cultural, economic, and environmental circumstances. It is important to recognise that in many situations where wastewater is used in agriculture, the effective treatment of such wastewater may not be available for many years. WHO guidelines must therefore be practical and offer feasible risk-management solutions that will minimise health threats and allow for the beneficial use of scarce resources. To achieve the greatest impact on health, guidelines should be implemented with such other health promoting measures as: health education, hygiene promotion, provision of adequate drinking water and sanitation, etc.

Introduction

The use of wastewater in agriculture is growing due to water scarcity, population growth, and urbanisation, which all lead to the generation of yet more wastewater in urban areas. Wastewater can be used to substitute for other better-quality water sources, especially in agriculture – the single largest user of freshwater and wastewater worldwide. However, the uncontrolled use of wastewater in agriculture has important health implications for produce consumers, farmers and their families, produce vendors, and communities in wastewater-irrigated areas. Negative health impacts from the use of untreated or inadequately treated wastewater have been documented in many studies. Less attention has been paid to the positive health impacts of the use of wastewater in agriculture that may result from improved household food security, better nutrition, and increased household income.

Guidelines for the safe use of wastewater in agriculture need to maximise public health benefits while allowing for the beneficial use of scarce resources. Achieving this balance in the variety of situations that occur worldwide (especially in settings where there may be no wastewater treatment) can be difficult. Guidelines need to be adaptable to the local social, economic, and environmental conditions and should be co-implemented with such other health interventions as hygiene promotion, provision of adequate drinking water and sanitation, and other healthcare measures. The Hyderabad Declaration on Wastewater Use in Agriculture (Appendix 1, this volume) recognises these principles and recommends a holistic approach to the management of wastewater use in agriculture.

Following a major expert meeting in Stockholm Sweden in 1999, the International Water Association (IWA) on behalf of the World Health Organization (WHO) published Water Quality: Guidelines, Standards and Health: Assessment of Risk and Risk Management for Water-related Infectious Disease. This publication outlines a harmonised framework for the development of guidelines and standards for water-related microbiological hazards (Bartram et al., 2001; Prüss and Havelaar, 2001). The suggested framework involves the assessment of health risks prior to setting health targets; defining basic control approaches, and evaluating the impact of these combined approaches on public health status (Fig. 4.1). The framework is flexible and allows countries to adjust the guidelines to local circumstances and compare the associated health risks with risks that may result from microbial exposures through drinking water or recreational/occupational water contact (Bartram et al., 2001). It is important that health risks from the use of wastewater in agriculture be put into the context of the overall level of gastrointestinal disease within a given population. Future WHO water-related guidelines will be developed in accordance with this framework.

The regulation of water quality for irrigation is of international importance because trade in agricultural products across regions is growing and products grown with contaminated water may cause health effects at both the local and transboundary levels. Exports of contaminated fresh produce from different geographical regions can facilitate the spread of both known pathogens and strains with new virulence characteristics into areas where such pathogens are not normally found or have been absent for many years (Beuchat, 1998).

Fig. 4.1. Stockholm Framework for assessment of risk for water-related microbiological hazards (adapted from Bartram et al., 2001).

Effective guidelines for health protection should be: feasible to implement; adaptable to local social, economic, and environmental factors; and include the following elements:

• Evidence-based health risk assessment
• Guidance for managing risk (including options other than wastewater treatment)
• Strategies for guideline implementation (including progressive implementation where necessary).

This chapter provides an overview of the current status of wastewater use in agriculture, reviews the evidence on health impacts, and outlines management steps that can be implemented to reduce potential health impacts especially in low-resource settings.

Background

Worldwide, it is estimated that 18% of cropland is irrigated, producing 40% of all food (Gleick, 2000). A significant portion of irrigation water is wastewater. Hussain et al. (2001) report on estimates that at least 20 million ha in 50 countries are irrigated with raw or partially treated wastewater. Smit and Nasr (1992) estimated that one tenth or more of the world’s population consumes foods produced on land irrigated with wastewater. Wastewater and excreta are also used in urban agriculture. A high proportion of the fresh vegetables sold in many cities, particularly in less-developed countries are grown in urban and peri-urban areas. For example, in Dakar, Senegal, more than 60% of the vegetables consumed in the city are grown in urban areas using a mixture of groundwater and untreated wastewater (see Faruqui et al., Chapter 10, this volume).

In many developing countries, wastewater used for irrigation, is often inadequately treated. For example, WHO/UNICEF (2000) estimate the median percentage of wastewater treated by effective treatment plants to be 35% in Asia, 14% in Latin America, and the Caribbean, 90% in North America and 66% in Europe. Other figures are even lower: for example, Homsi (2000) estimates that only around 10% of all wastewater in developing countries receives treatment. Given these circumstances, WHO guidelines must include feasible strategies for maximising health protection when untreated wastewater is used in agriculture.

Evidence Base

Health effects

Previous WHO guidelines (see Table 4.1; WHO, 1989) were based on a number of available epidemiological studies, many of which were reviewed by Shuval et al. (1986). The evidence at that time suggested that the use of untreated wastewater in agriculture presented a high actual risk of transmitting intestinal nematodes and bacterial infections especially to produce consumers and farm workers; but that there was limited evidence that the health of people living near wastewater-irrigated fields was affected. There was less evidence for the transmission of viruses and no evidence for the transmission of parasitic protozoa to farm workers, consumers or nearby communities. The review of epidemiological evidence by Shuval et al. (1986) also indicated that irrigation with treated wastewater did not lead to excess intestinal nematode infections among field workers or consumers (WHO, 1989).

In 2002, Blumenthal and Peasey completed a critical review of epidemiological evidence on the health effects of wastewater and excreta use in agriculture for WHO. A sub-set of analytical epidemiological studies were selected that included the following features: well-defined exposure and disease, risk estimates calculated after allowance for confounding factors, statistical testing of associations between exposure and disease, and evidence of causality (where available). These were used as a basis for estimating threshold levels below which no excess infection in the exposed population could be expected. Further information on the risks of infection attributable to exposure, and in particular on the proportion of disease in the study population attributable to exposure (and therefore potentially preventable through improvement in wastewater quality), was used to inform proposals on appropriate microbiological guidelines for wastewater reuse in agriculture. A summary of the results of this epidemiological review are presented in Table 4.2.

Table 4.1. Recommended revised microbiological guidelines for treated wastewater use in agriculture^a,b.

Sources: Adapted from Blumenthal et al., 2000a; WHO, 1989.

^a Values in brackets are the 1989 guideline values.
^b In specific cases, local epidemiological, socio-cultural and environmental factors should be taken into account and the guidelines modified accordingly.
^c Ascaris and Trichurisspecies and hookworms; the guideline is also intended to protect against risks from parasitic protozoa.
^d During the irrigation season (if the wastewater is treated in WSP or WSTR which have been designed to achieve these egg numbers, then routine effluent quality monitoring is not required).
^e During the irrigation season (faecal coliform counts should preferably be done weekly, but at least monthly).
^f A more stringent guideline (≤ 200 faecal coliforms/100 ml) is appropriate for public lawns, such as hotel lawns, with which the public may come into direct contact.
^g This guideline can be increased to ≤1 egg/l if (i) conditions are hot and dry and surface irrigation is not used, or (ii) if wastewater treatment is supplemented with antihelminthic chemotherapy campaigns in areas of wastewater re-use.
^h In the case of fruit trees, irrigation should cease 2 weeks before fruit is picked and no fruit should be picked off the ground. Spray/sprinkler irrigation should not be used.

Wastewater is often a resource for the poor and in many cases the water and nutrients it contains can have important – yet largely uncharacterised – impacts on food security (Buechler and Devi, 2003). Improving nutrition, especially for children, is very important in maintaining the overall health of individuals and communities. Malnutrition is estimated to have a significant role in the deaths of 50% of all children in developing countries – 10.4 million children under the age of 5 die each year (Rice et al., 2000; WHO, 2000). Malnutrition affects approximately 800 million people, or 20% of all people in the developing world (WHO, 2000). Malnutrition may also have long-term effects on the health and social development of a community, and leads to both stunted physical growth and impaired cognitive development (Berkman et al., 2002).

Improving the living standards of the poor through developing irrigation (with wastewater or freshwater) can lead to better health, in some cases, even when irrigation leads to an increase in disease vectors (van der Hoek et al., 2001a). For example, a study in Tanzania showed that a village where a rice irrigation scheme had been developed had more malaria vectors than a nearby savannah village but a lower level of malaria transmission (Ijumba, 1997).

Table 4.2. Summary of health risks associated with the use of wastewater in irrigation.

Sources: Blumenthal and Peasey, 2002; Blumenthal et al., 2000a; Armon et al., 2002

The village with the irrigation scheme had more resources to buy food, children had a better nutritional status, and the villagers were more likely to buy and use mosquito nets (Ijumba, 1997). Similar results may also be applicable to the development of wastewateruse schemes in some countries.

Microbial guideline derivation

Worldwide many different microbial standards for wastewater use in agriculture have been developed. Most guidelines lay heavy emphasis on microbial standards, but it should be recognised that other strategies for managing health risks may also be effective. Based on an approach that used empirical epidemiological studies, microbiological studies of the transmission of pathogens, and quantitative microbial risk assessment (see Table 4.3), Blumenthal et al. (2000a) proposed revisions to the WHO microbiological guidelines for treated wastewater use in agriculture (Table 4.1). The main differences from the 1989 WHO guidelines are new recommendations for a faecal coliform (FC) value for restricted irrigation (≤10⁵ FC/100 ml) and new FC and nematode egg limits in certain conditions when children are exposed.

Risk assessment

The health risk from pathogens in wastewater can be estimated by using a quantitative microbial risk assessment (see Table 4.3) based on data derived from the following evaluations:

• Hazard identification (HI)
• Exposure assessment (EA)
• Dose-response analysis (DRA)
• Risk characterisation (RC)

Quantitative microbial risk assessment (QMRA) provides a technique for estimating the risks from a specific pathogen associated with a specific exposure pathway. QMRA is a sensitive tool that can estimate risks that would be difficult to measure and therefore provides a useful supplement to epidemiological investigations that are less sensitive and more difficult to perform. However, QMRA is only as good as the data available and the assumptions made.

A number of QMRA have been performed for the use of wastewater in agriculture. Table 4.4 presents some information on the estimated health risks associated with different levels of indicator organisms (Escherichia coli) present in the wastewater. Escherichia coli is almost always found in human and animal faeces and thus indicates the presence of faecal contamination in water. The presence of E. coli in a water sample will often (but not always) mean that other excreta-related pathogens are also present. It is easier to measure E. coli concentrations and assume that this represents a group of similar pathogens than to measure concentrations of individual pathogens.

Tolerable Risk and Decision-making

A level of risk can be estimated for almost any exposure, in other words, there is no such thing as zero risk, only very low risks. Because a level of risk can always be estimated, it is important that a risk tolerable to society be defined. To facilitate the comparison of different health outcomes (e.g. diarrhoea compared to cancer) risks can be framed in terms of disability adjusted life years (DALYs) which are a measure of years lost due to premature death and/ or disability caused by a disease (Prüss and Havelar, 2001).

For water-related exposures, WHO has determined that a disease burden of 1 × 10^-6 DALYs per person per year (one ‘microDALY’) from a disease caused by either a chemical or infectious agent transmitted through drinking water is a tolerable risk (WHO, 2004). This level of health burden is equivalent to a mild illness (e.g. watery diarrhoea) with a low case-fatality rate (e.g. 1 in 100,000) at an approximate 1 in 1000 (10^-3) annual risk or 1 in 10 lifetime risk of disease in an individual (Havelaar and Melse, 2003; WHO, 1996, 2004). For exposure to a carcinogen, this level of disease burden is broadly equivalent to a 10^-5 lifetime excess risk of renal cancer (1 excess case of cancer/100,000 individuals exposed to the chemical over a lifetime) (Havelaar and Melse, 2003). The third edition of the WHO Guidelines for Drinking Water Quality will use the approach described above to define tolerable risks (WHO, 2004).

Tolerable risks also need to be put into the context of all exposures leading to disease. For example, Mead et al. (1999) estimated that the average person (including all age groups) in the USA suffers from 0.8 episodes of acute gastroenteritis (GI) (characterised by diarrhoea, vomiting or both) per year (i.e. an 8 × 10^-1 annual GI risk). The incidence rates of GI among adults worldwide are generally within the same order of magnitude (Murray and Lopez, 1996), but children living in high-risk situations where poor hygiene, sanitation and water quality prevail have more frequent gastrointestinal illnesses. Kosek et al., (2003) found that children under the age of 5 in developing countries experienced a median of 3.2 annual episodes of diarrhoea per child (an annual risk of 3.2 × 10⁰).

Table 4.3. Types of evidence used to develop microbial guidelines.

Sources: Blumenthal and Peasey, 2002; Petterson and Ashbolt, 2003; Teunis etal., 1996.

Risks of viral infection and diarrhoeal disease associated with contact with wastewater of different qualities have been estimated by QMRA techniques (Table 4.4). Guidelines should take these levels of risk into account. For example, if the background GI incidence rate in adults in a given population is 0.8 episodes per year, then treating wastewater to ≤2.2 total coliforms/100 ml (see Table 4.4) will potentially only add an extra 10^-7 annual episodes of viral diarrhoea to the background level, i.e. the background level will increase from 0.8 to 0.8000001. Such a small increase is impossible to detect and, in any case, contributes virtually nothing to the background level. This implies that it is not necessary to treat wastewater to such a high quality.

However, with the same background rate of GI in adults, use of untreated wastewater would add an additional 0.2–0.6 annual GI episodes that would have a substantial impact on the level of GI, increasing it from 0.8 to 0.99 or 1.39– i.e. increases of 25% and 76%, respectively. Treating the wastewater to the WHO guideline level of 1000 FC/100ml would add an extra 10^-4–10^-5 infections, increasing the level from 0.8 to 0.8001, or 0.80001 annual episodes, that again does not perceptibly change the background level. This emphasises that the background levels of disease should be taken into consideration when microbial guideline values are established. The costs incurred in reaching different levels of risk must also be considered. Achieving such very low levels of risk through more advanced wastewater treatment technologies substantially increases costs (Fattal Shuval, 1999).

The Stockholm Framework requires that the risk of gastrointestinal illness in a given population be considered in the context of total risk from all exposures (i.e. drinking water, recreational water contact, and contaminated food). This facilitates making risk-management decisions that address the greatest risks first. For example, it will have very little impact on the disease burden if the number of cases of salmonellosis attributed to the use of wastewater in irrigation is halved if 99% of the cases are transmitted in other ways, most notably through contaminated food (Bartram et al., 2001).

It is important to note that water quality requirements for the use of wastewater in unrestricted irrigation are often stricter than surface water quality requirements for unrestricted irrigation. In many places surface water would not meet WHO FC guideline targets for unrestricted irrigation (UNEP, 1991; Mara and

Table 4.4. Estimated risks from the use of untreated or treated wastewater in irrigation of viral infection per person per year for various concentrations of E. coli^a.

^a E. coli concentrations in wastewater do not necessarily correspond to viral concentrations in wastewater.
^b Risks are based on either the consumption of irrigated raw vegetables (CV) or contact with the wastewater during/after irrigation (WC).
^c Total coliforms in chlorinated secondary effluent used for unrestricted crop irrigation.
^d Total coliforms in chlorinated tertiary effluent used for golf course irrigation.

Cairncross, 1989). Thus in some cases, strict wastewater quality standards for irrigation will paradoxically encourage the use of more contaminated water for irrigation resulting in greater health risks. For example, in irrigated areas near Santiago, Chile, 60% of the river water used for irrigation contained in excess of 10,000 FC/100 ml (ten times the recommended WHO standard) (FAO, 1993). Additionally, the United States Environmental Protection Agency (USEPA) recommends a standard for irrigation with treated wastewater of ≤2.2 total coliforms/100 ml, but when surface waters are used for irrigation a standard of ≤1000 FC/100 ml is required (USEPA, 1973). However, a percentage of FC in surface waters may not originate from sewage effluents or waste discharges, especially in tropical/sub-tropical regions, and this may have significant implications in terms of human health risk assessment (WHO, 1996).

In some places where freshwater is scarce people often drink water that is of a quality that does not meet drinking water standards, and would not meet strict standards (e.g. California Title 22 standards) for unrestricted irrigation. For example, in some areas in the southern Punjab, Pakistan, groundwater supplies are too brackish to drink, so people rely on irrigation water for their drinking water supplies. In one study, 58% of the water from the village reservoirs contained >100 E. coli/ 100 ml (van der Hoek et al., 2001b). In these circumstances it would be highly inappropriate to expect that wastewater be treated to a higher quality than drinking water. Clearly, as the Stockholm Framework suggests, interventions that would yield higher health benefits should be given more priority.

Water quality guidelines need to be adapted to the social, economic, and environmental conditions of each country. When countries with high levels of excreta-related disease background levels and inadequate resources for wastewater treatment adopt overly strict water quality standards for use in agriculture, it may lead to a lower level of health protection because, in these circumstances, the standards may not significantly change the background level of disease and/or may be viewed as unachievable and thus ignored entirely.

Chemical Guidelines

In many countries, industrial wastewater is often mixed with the municipal wastewater used for irrigation. Industrial wastes may contain toxic organic and inorganic chemicals that can be taken up by the crops. The health risks associated with chemicals found in wastewater and sludge may need to be given more attention, particularly as industrialisation increases in developing countries. To minimise adverse health and environmental effects from toxic substances, industrial wastes should be adequately pre-treated to remove these chemicals, or should be treated separately from municipal wastewater and excreta.

It is difficult to assess the health impacts from toxic chemicals in wastewater used for irrigation because of the difficulty in associating chronic exposure to chemicals and chemical mixtures to diseases with long latency periods. However, in some parts of China, the use of heavily contaminated industrial wastewater for irrigation is thought to be associated with health problems. For example, in these areas a 36% increase in hepatomegaly (enlarged liver), and a 100% increase in both cancer and congenital malformation rates were observed compared to those problems in control areas where industrial wastewater was not used for irrigation (Yuan, 1993). Heavy metals in the wastewater can also pose a health risk, e.g. in Japan, China and Taiwan rice accumulated high concentrations of cadmium (and other heavy metals) when it was grown in soils contaminated with irrigation water containing substantial industrial discharges (Chen, 1992). In Japan, Itai-itai disease – a bone and kidney disorder – associated with chronic cadmium poisoning, occurred in areas where rice paddies were irrigated with water from the contaminated Jinzu river (WHO, 1992).

WHO is currently developing standards for a selection of harmful chemicals that might be found in wastewater. In many situations the safety of the wastewater for use in irrigation will need to be determined on a case-by-case basis, depending on the type of chemicals suspected to be present. Chemical analysis of such wastewater may be necessary. Chemical guideline values will be presented in the revised guidelines.

Strategies for Managing Health Risk

The protection of public health can best be achieved by using a ‘multiple barrier’ approach that interrupts the flow of pathogens from the environment (wastewater, crops, soil etc.) to people. Human pathogens in the fields do not necessarily represent a health risk if other suitable health protection measures can be taken. These measures may prevent pathogens from reaching the worker or the crop or, by selection of appropriate crops (e.g. cotton), may prevent pathogens on the crop from affecting the consumer (Mara and Cairncross, 1989). The measures available for health protection can thus be grouped into five main categories:

• Waste treatment
• Crop restriction
• Irrigation technique
• Human exposure control
• Chemotherapy and vaccination.

It will often be desirable to use a combination of several methods. For example, crop restriction may be sufficient to protect consumers, but will need to be supplemented by additional measures to protect agricultural workers. Sometimes partial treatment to a less-demanding standard may be sufficient if combined with other measures. The feasibility and efficacy of any combination will depend on many factors that must be carefully considered before any option is put into practice (Mara and Cairncross, 1989). These factors will include the following:

• Availability of resources (labour, funds, land)
• Existing social and agricultural practices
• Market demand for wastewater-irrigated products
• Existing patterns of excreta-related disease. For example, if sufficient funds and/or sufficient land are not available for wastewater treatment, some of the other three types of health protection measure will be needed.

Treatment

When municipal or domestic wastewater is used in agriculture, the removal or inactivation of excreted pathogens is the principal objective of wastewater treatment. Conventional wastewater treatment options (primary and secondary treatments), as applied in developed countries, have traditionally focused on the removal of environmental pollutants [e.g. suspended solids, or biological oxygen demanding (BOD) substances] and not on pathogens. Many of these processes may be difficult and costly to operate properly in developing-country situations due to their high energy, skilled labour, infrastructure and maintenance requirements (Carr and Strauss, 2001). In some cases, tertiary treatment (e.g. filtration and/or disinfection) will be required to reduce the concentrations of pathogens in the effluents to WHO-recommended microbial guideline values. In some situations, the quality of primary or secondary treated effluents could be improved by retaining them for 5 days in a single polishing (maturation) pond to reduce the risk of disease transmission (Mara and Cairncross, 1989).

There is a need for research and development work to improve the helminth egg removal efficacy of conventional systems to meet microbial standards. Such processes as lime treatment, chemically enhanced primary treatment (CEPT), upward-flow anaerobic sludge blanket, sand filtration, and storage in compartmentalised reservoirs deserve further study (Mara and Cairncross, 1989). Parr et al. (2000) present a brief overview of some wastewater treatment options that might be suitable for developing countries.

CEPT is a treatment technique that uses specific chemicals (e.g. ferric chloride plus an anionic polymer) to facilitate particle coagulation and flocculation. Improving these processes increases the removal of suspended solids, BOD and intestinal nematode eggs (Morrissey and Harleman, 1992; Harleman and Murcott, 2001). Studies in Mexico City showed that CEPT was capable of producing effluents with 2–5 nematode eggs/l. When CEPT effluents were filtered through polishing, sand filters effluents with <1 nematode egg/l were produced at significantly lower cost than in a conventional secondary treatment system (primary plus activated sludge) (Harleman and Murcott, 2001).

Waste stabilisation ponds (WSP) have been used successfully in many situations for treating wastewater. When designed and operated properly, WSP are highly effective in removing pathogens and can be operated at low cost where inexpensive land is available. Ponds for FC and helminth removal can designed using specific equations (Mara, 1997; Ayres et al., 1992), examples of their use are given by Mara in Blumenthal et al. (2000b). However, WSP should be designed, operated and maintained in such a way as to prevent disease vectors from breeding in them.

Where effective treatment is not available, it may be possible to consider other options that improve microbial water quality, such as storage reservoirs that partially treat wastewater through simple sedimentation. For example, in Mexico, irrigation with untreated or partially treated wastewater was estimated to be directly responsible for 80% of all Ascaris infections and 30% of diarrhoeal disease in farm workers and their families, but, when wastewater was retained in a series of reservoirs there was minimal risk of either Ascaris infection or diarrhoeal disease (Cifuentes et al., 2000). The use of reservoirs has the added advantage that wastewater can be stored for use in the dry season – the time of peak irrigation demand.

Crop restriction

Crop restriction can be used to protect the health of consumers when water of sufficient quality is not available for unrestricted irrigation. For example, water of poorer quality can be used to irrigate such non-vegetable crops as cotton, or crops that will be cooked before consumption (e.g. potatoes).

Crop restriction does not, however, provide protection to farm workers and their families where low-quality effluents are used in irrigation or where wastewater is used indirectly, i.e. through contaminated surface water (Blumenthal et al., 2000b). Crop restriction is therefore not an adequate single control measure, but should be considered as part of an integrated system of control. To provide protection for both workers and for the consumers, it should be complemented by such other measures as partial waste treatment, controlled application of wastes, or human exposure control (Mara and Cairncross, 1989).

Crop restriction is feasible and is facilitated in several circumstances including the following (Mara and Cairncross, 1989):

• Where a law-abiding society or strong law enforcement exists
• Where a public body controls allocation of the wastes, and has the legal authority to require that crop restrictions be followed
• Where an irrigation project has strong central management
• Where there is adequate demand for the crops allowed under crop restriction, and where they fetch a reasonable price
• Where there is little market pressure in favour of excluded crops.

Crop restriction has been used effectively in Mexico, Peru and Chile (Blumenthal et al., 2000b). In Chile when implemented with a general hygiene education programme the use of crop restriction reduced the transmission of cholera from the consumption of raw vegetables by 90% (Monreal, 1993).

Waste application methods

The choice of wastewater application method can have impact on the health protection of farm workers, consumers, and nearby communities. Spray/sprinkler irrigation has the highest potential to spread contamination on crop surfaces and affect nearby communities. Bacteria and viruses (but not intestinal nematodes) can be transmitted through aerosols to nearby communities. Where spray/sprinkler irrigation is used with wastewater it may be necessary to set up a buffer zone, e.g. 50–100 m from houses and roads, to prevent health impacts on local communities (Mara and Cairncross, 1989).

Farm workers and their families are at the highest risk when furrow or flood irrigation techniques are used. This is especially true when protective clothing is not worn and earth is moved by hand (Blumenthal et al., 2000b).

Localised irrigation techniques, e.g. bubbler, drip, trickle offer farm workers the most health protection because they apply wastewater directly to the plants. Although these techniques are generally the most expensive to implement, drip irrigation has recently been adopted by some farmers in Cape Verde and India (FAO, 2001; Kay, 2001).

Vaz da Costa Vargas et al. (1996) demonstrated that stopping irrigation 1–2 weeks before harvest can effectively reduce crop contamination. However, this is likely to be difficult to implement in unregulated circumstances because many vegetables (especially lettuce or other leafy vegetables) need watering up to the point of harvest to increase their market value. This technique may be possible for some fodder crops that do not have to be harvested at the peak of their freshness (Blumenthal et al., 2000b).

Human exposure control

The following 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 affected fields.

Agricultural field workers are at high potential – and often actual – risk, especially from parasitic infections. Exposure to hookworm infection can be reduced, even eliminated, by the use of less-contaminating irrigation methods (see above) and by the use of appropriate protective clothing, i.e. shoes for field workers and gloves for crop handlers. Rigorous health education programmes are needed (Blumenthal et al., 2000b; Mara and Cairncross, 1989). Field workers should be provided with adequate water for drinking and hygiene purposes, in order to avoid the consumption of, and any contact with, wastewater. Similarly, safe water should be provided at markets for washing and ‘freshening’ produce. Consumers should cook vegetables and meat, boil milk, and practise good personal and domestic hygiene measures to protect their health.

Health education campaigns that focus on improving personal and domestic hygiene should target produce consumers, farm workers, produce handlers and vendors. Hand washing with soap should be emphasised. It may be possible to link health education and hygiene promotion to agricultural extension activities or other health programmes, e.g. immunisation (Blumenthal et al., 2000b).

Chemotherapy and vaccination

Immunisation against helminthic infections and most diarrhoeal diseases is currently not feasible. However, for highly exposed groups, immunisation against typhoid and hepatitis A may be worth considering.

Additional protection can be provided if adequate medical facilities to treat diarrhoeal diseases, are available and by regular chemotherapy. This might include chemotherapeutic control of intense nematode infections in children and control of anaemia in both children and adults, especially women and post-menarche girls. Chemotherapy must be reapplied at regular intervals to be effective. The frequency required to keep worm burdens at a low level (e.g. as low as those in the rest of the population) depends on the intensity of the transmission, but treatment may be required 2–3 times a year for children living in endemic areas (Montresor et al., 2002; Mara and Cairncross, 1989). Albonico et al. (1995) found that re-infection with helminths could return to pre-treatment levels within 6 months of a mass chemotherapy campaign if the prevailing conditions did not change.

Chemotherapy and immunisation cannot normally be considered adequate strategies to protect farm workers and their families exposed to raw wastewater or excreta. However, where such workers are organised within structured situations, such as on government or company farms, these treatments could be beneficial as palliative measures, pending improvement in the quality of the wastes used, or the adoption of other control measures, e.g. protective clothing (Mara and Cairncross, 1989).

Guideline Implementation

The scarcity of surface and groundwater in many countries has led, or is leading to the development of national plans for the rational allocation, utilisation and protection of available water resources. The objective of such plans is to ensure, as far as is practically possible, the maximum economic yield from the use of an increasingly scarce resource. Human wastes are relevant to these national water plans as they can alter the physicochemical and microbiological quality of water, and thus place restrictions on its use. The incorporation of protocols for waste use planning into national water plans is important, especially when water is scarce, not only to protect water quality but also to minimise treatment costs, to safeguard public health, and to obtain the maximum possible agricultural benefit from the nutrients and organic matter contained in the wastes (Mara and Cairncross, 1989).

Human wastes are already used for crop production in many countries, mostly informally and without official recognition by the health authorities. The Hyderabad Declaration on Wastewater Use in Agriculture (Appendix 1, this volume) recognises this reality. Where the practice is traditional or has arisen spontaneously, untreated or insufficiently treated wastes are commonly used. Experience in many countries has shown that simply to ban the practice is not likely to have much effect, if any, on its prevalence or on the level of public health risk involved. On the contrary, banning the practice is unlikely to stop it, but may make it more difficult to supervise and control, and may also interfere with disease surveillance and health care among those most exposed to the risk of infection. A more promising approach is to provide support to improve existing use practices, not only to maximise health protection, but also to increase productivity, as the major stakeholders are usually relatively poor farmers and consumers (Mara and Cairncross, 1989).

Additional legal controls will often be required, but, it is easier to make regulations than to enforce them. In drafting new regulations (or in choosing which existing ones to enforce) it is important to plan for the institutions, staff and resources necessary to ensure they are followed. Perhaps even more important is to ensure that the regulations are realistic and achievable in the context in which they are to be applied. It will often be advantageous to adopt a gradual approach, or to test a new set of regulations by persuading a local administration to pass them as by-laws before they are extended to the rest of the country (Mara and Cairncross, 1989). Some of the problems countries encounter when setting up and implementing standards have been reviewed by von Sperling and Fattal (2001).

Measures to protect public health are particularly difficult to implement when there are many individual sources or owners of the waste, whether these are individual septic tank overflows or farmers with riparian rights to pump from a river so polluted that it comprises only slightly diluted wastewater. If the wastewater can be brought under unified control by: installing a sewerage system, establishing a treatment plant (or plants), or diverting the wastewater from the river to a treatment works, this will give the controlling authority much greater power to influence the ways in which the wastewater is subsequently used, and thus to maximise health protection (Mara and Cairncross, 1989).

Implementation of the WHO Guidelines for the Safe Use of Wastewater in Agriculture and Aquaculture (WHO, 1989) will be of maximum benefit in protecting public health when they are integrated into a comprehensive public health programme that includes other sanitary measures including education and outreach that aim to change personal and domestic hygiene behaviour. For example, if the guidelines are followed in the field but produce is ‘freshened’ with contaminated water in the market, some of the potential health gains are likely to be erased.

Steps that will facilitate developing a guideline implementation plan are presented below. A sample action plan for incremental adoption of WHO guidelines is presented in Box 4.1. Further discussion of stepwise guideline implementation can be found in von Sperling and Fattal (2001).

Guideline implementation plan

1. Design and conduct a survey of wastewater and excreta use practices throughout the country or in specific districts. The survey could contain questions concerning:

• The availability and types of wastewater treatment available
• The types of crops grown in the area (whether they are eaten cooked or raw)
• Techniques for wastewater and excreta

Box 4.1. Sample action plan for incremental adoption of WHO guidelines^a

Strengthen local capacity
Assemble a team of health and agricultural outreach workers who can work with farmers and villagers to improve health and agricultural practices and develop feasible crop restriction strategies and other interventions as necessary.

Health and hygiene education
Expand existing hygiene and sanitation outreach programmes to include information on potential health effects of wastewater use; educate farmers, produce vendors and consumers about food safety and hygiene.

Crop restriction
Work with farmers to develop feasible and health protective crop restrictions, especially in the areas of highest risk (e.g. where undiluted raw wastewater is used).

Waste application
Determine the safety level of current practices. As resources/technologies permit, shift to safer wastewater/ excreta application practices where there is less human contact (e.g. drip and bubbler irrigation).

Human exposure control
Expand hygiene and health education programmes in affected communities. Require protective clothing at larger wastewater/excreta use projects and where feasible. Provide clean water at markets for ‘freshening produce‘. Inspect general hygiene at food markets.

Treatment
Introduce or upgrade treatment at strategic locations, phase in over a period of time (e.g. 10–15 years).

Examples
First stage of treatment: natural purification processes (e.g. abstraction suitable distance downstream from discharge); irrigation storage reservoirs designed for pathogen removal, waste stabilisation ponds, primary treatment plus additional treatment (e.g. storage reservoir, chemically enhanced coagulation, coagulation + rapid sand filtration). Second stage of treatment: waste stabilisation ponds, conventional secondary treatment (e.g. activated sludge, trickling filter, etc.), aeration ponds, etc. Third stage of treatment: waste stabilisation ponds, conventional secondary treatment + storage reservoir or disinfection, advanced processes (e.g. membrane filtration).

Microbial wastewater quality standards
Phase in WHO microbial wastewater quality standards over suitable period of time according to treatment capabilities. For example, the initial standards may be set at ≤10⁵ FC/100 ml and ≤5 viable intestinal nematode eggs/l for unrestricted irrigation (and/or with specific crop restrictions). As resources become available to build treatment facilities the standard could be tightened to ≤10⁴ FC/100 ml and ≤1 viable intestinal nematode egg/l, and eventually to the current recommendations (≤10³ FC/100 ml and ≤1 viable intestinal nematode egg/l).

Other health interventions
Initiate or expand vaccination campaigns in affected areas, e.g. typhoid, hepatitis A. Complement hygiene and sanitation programmes with periodic antihelminthic drug campaigns (this works well where antihelminthic drugs are widely available at low cost and where wastewater and excreta use is limited to distinct areas in a country, e.g. Pakistan (Feenstra et al., 2000). Mass antihelminthic drug campaigns against intestinal nematode infection may need to be considered at least once per year in areas where 50–70% of the school-aged children are infected with soil-transmitted helminthic infections. Where the prevalence of these infections exceeds 70% in school-aged children and more than 10% of the individuals are moderately or heavily infected, then children should be treated 2–3 times a year (Montresor et al., 2002).

Industrial effluents
Initial efforts should be made to identify sources of industrial discharges. Phase in an approach that first requires large polluters to clean up their wastes or divert them from the municipal waste stream and eventually requires all of the industrial discharges to be treated separately.

^a. For more discussion on progressive guideline implementation see von Sperling and Fattal (2001).

application, e.g. bucket, furrow, sprinkler, other

• An assessment of human exposure to wastewater and excreta during agricultural practices, e.g. do fieldworkers wear protective clothing? do they practice good hygiene?
• Evaluation/prioritisation of health risks in the context of the national burden of disease, associated with the use of wastewater and excreta in agriculture.
  Quantitative: scientific studies of disease, review of clinical data, outbreak information, prevalence data, etc.
  Qualitative: interviews with health staff (doctors, nurses, pharmacists), farmers, families, community workers, teachers, etc.

3. National or district-level workshops to formulate appropriate (realistic) strategies for mitigating health impacts that include relevant stakeholders, e.g. farmers.

4. Develop national or other action plan/policy for the safe use of wastewater and excreta in agriculture.

5. Strengthen institutional capacities – designate responsible authority(-ies) to monitor and enforce safe wastewater and excreta use practices.

6. Review and revise national plan/policy as needed.

Conclusions

Developing realistic guidelines for using wastewater in agriculture involves the establishment of appropriate health-based targets prior to defining appropriate risk-management strategies. Establishing appropriate health-based targets primarily involves an assessment of the risks associated with wastewater use in agriculture, using evidence from available studies of epidemiological and microbiological risks, and risk-assessment studies. Considerations of what is an acceptable or tolerable risk are then necessary; these may involve the use of internationally derived estimates of tolerable risk, but these need to be put into the context of actual disease rates in a population related to all the exposures that lead to that disease, including other water- and sanitation-related exposures together with food-related exposure. Positive health impacts resulting from increased food security, improved nutrition, and additional household income should also be considered. Individual countries may therefore set different health targets, based on their own contexts.

Strategies for managing health risks to achieve the health targets include wastewater treatment to achieve appropriate microbiological quality guidelines, crop restriction, waste application methods, control of human exposure, chemotherapy, and vaccination. Phased implementation of the WHO microbial water quality standards may be necessary as treatment is gradually introduced and improved over a period of time, e.g. 1–15 years. For optimal public health effect, the guidelines should be co-implemented with such other health interventions as hygiene promotion, provision of adequate drinking water and sanitation, and other healthcare measures.

Note: The opinions expressed in this chapter are those of the authors and do not necessarily reflect the views or policies of WHO.

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