AND IRRIGATION DRAINAGE
Irrig. and Drain. 54: S103S111 (2005) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ird.190
WHO GUIDELINES FOR SAFE WASTEWATER USEMORE THAN JUST NUMBERSy
RICHARD CARR*,z
World Health Organization, Geneva, Switzerland
ABSTRACT The use of wastewater in agriculture 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. WHO developed Health Guidelines for the Use of Wastewater in Agriculture and Aquaculture in 1989. The Guidelines are currently being revised based on new data from epidemiological studies, quantitative microbial risk assessments and other relevant information. WHO Guidelines contain both microbial guideline values and good practices to reduce health risks. They must be practical and offer feasible risk management solutions that will minimize health threats and allow for the benecial use of scarce resources. It is important that the Guidelines are based on actual health risks and an evaluation of what is a tolerable risk. This will vary from country to country. WHO Guidelines, therefore, need to be adapted to the unique social, economic and environmental factors in each situation. To achieve the greatest impact on health, guidelines should be implemented with other health measures such as: health education, hygiene promotion, provision of adequate drinking water and sanitation, and other measures such as vaccination. Copyright # 2005 John Wiley & Sons, Ltd.
key words: agriculture; guidelines; health protection; sanitation; wastewater use/reuse; WHO
RESUME ` Lutilisation deaux usees dans lagriculture se produit plus frequemment a lheure actuelle en raison du manque deau et de la croissance demographique. Ce sont souvent les foyers les plus demunis qui dependent de cette ressource pour leurs moyens dexistence et leurs besoins en securite alimentaire. Toutefois, cette pratique presente ` pour la sante des implications negatives quil faut aborder. LOMS a redige des Directives de sante relatives a ` lutilisation des eaux usees dans lagriculture et laquaculture en 1989. Ces directives sont en ce moment mises a ` jour a partir de nouvelles donnees originaires detudes epidemiologiques, devaluations quantitatives des risques ` microbiens et dautres informations pertinentes. Les directives de lOMS comportent a la fois des valeurs microbiennes indicatives et des methodes adequates pour reduire les risques de sante. Ces directives doivent etre pratiques et offrir des solutions faisables de gestion des risques susceptibles de minimiser les menaces pour la ` sante, tout en tenant compte de la necessite dutiliser les ressources rares a bon escient. Il est important que ces directives se basent sur les risques de sante reels et sur une evaluation de ce quest un risque tolerable, evaluation ` qui variera dun pays a lautre. Cest pourquoi les directives de lOMS doivent etre adaptees aux facteurs sociaux, ` economiques et environnementaux uniques a chaque situation.
* Correspondence to: Richard Carr, World Health Organization, WSH/PHE/SDE, 20 Avenue Appia, CH-1211 Geneva 27, Switzerland. E-mail: [e-mail] y Directives de lOMS pour une utilisation sans risque des eaux useesplus que de simples chiffres. z The opinions expressed in this paper are those of the author and do not necessarily reect the views and policies of WHO.
Copyright # 2005 John Wiley & Sons, Ltd.
Received 29 March 2005 Accepted 8 April 2005
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Pour obtenir un impact maximum sur la sante, les directives doivent etre mises en uvre avec dautres mesures ` ` de sante telles que: leducation a la sante, la promotion de lhygiene, la fourniture deau potable et dinstallations sanitaires adequates, ainsi que dautres mesures telles que la vaccination. Copyright # 2005 John Wiley & Sons, Ltd.
mots cles: agriculture; directives; protection de la sante; installations sanitaires; utilisation/reutilisation des eaux usees; OMS
INTRODUCTION
The importance of increasing access to sanitation for unserved populations has been given renewed emphasis with the adoption by the United Nations General Assembly of the Millennium Development Goals in 2000 and similar International Development Targets developed at the World Summit on Sustainable Development in Johannesburg in 2002. Because much of the future population growth is expected to occur in or around urban centres, increased sanitation coverage will often take the form of sewerage with a subsequent increase in the generation of wastewater. The use of wastewater in agriculture may actually help to prevent some of the downstream health and environmental impacts especially when the alternative is to discharge wastewater (frequently without adequate treatment) directly into surface waters. The use of wastewater in agriculture is growing due to water scarcity, population growth and the recognition of its resource value. Wastewater can be used to substitute for other better quality water sources, especially in agriculture. However, the uncontrolled use of wastewater in agriculture has important health implications for product 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 arise because of improved household food security, better nutrition and increased household income. WHO Guidelines contain both microbial guideline values and good practices to reduce health risks. They offer feasible risk management solutions that will minimize health threats and allow for the benecial use of scarce resources. The Guidelines are based on actual health risks and an evaluation of what is a tolerable risk. This will vary from country to country. WHO Guidelines are meant to be adapted to the unique social, economic and environmental factors in each situation. This paper will look at some of the issues surrounding the use of wastewater in agriculture, the history and philosophy of the WHO Guidelines, health issues, risk management strategies, and guideline implementation.
BACKGROUND
At least one-tenth of the worlds population is thought to consume foods produced by irrigation with wastewater (Smit and Nasr, 1992). In many countries, wastewater and excreta used in crop production are not adequately treated. It has been estimated that at least 20 million ha in 50 countries are irrigated with raw or partially treated wastewater (van der Hoek, 2004; Hussain et al., 2001). Wastewater and excreta are also used in urban agriculture which often supplies a large proportion of the fresh vegetables sold in many cities, particularly in developing countries. 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 (Faruqui et al., 2004). WHO/UNICEF (2000) estimates 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 gures 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 maximizing health protection when untreated wastewater is used in agriculture. The safe use of wastewater and excreta also has social equity issues. A signicant percentage of wastewater and excreta use in agriculture occurs at the subsistence level. Wastewater and excreta are often seen as resources which help to improve food security and positively impact household nutrition and, thus, health.
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The regulation of water quality for irrigation is of international importance because agricultural products grown with contaminated water may cause health effects at both the local and international levels. International trade in agricultural products across regions is growing. 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 the pathogens are not normally found or have been absent for many years (Beuchat, 1998). For food exports, it is important that the wastewater is treated to the WHO recommended levels for unrestricted irrigation. This is to ensure that the risks of consuming such food are low for consumers who may not have immunity to locally endemic diseases. For food products to be consumed locally, national authorities may adapt the guideline values to t their own circumstances.
HISTORY OF WHO GUIDELINES
To protect public health and facilitate the rational use of wastewater and excreta in agriculture and aquaculture WHO developed the document Reuse of Efuents: Methods of Wastewater Treatment and Public Health Safeguards during the period up to 1973. The rst WHO Guidelines (WHO, 1973) were developed in the absence of good epidemiological studies and borrowed essentially a low-risk approach from the USA. The Guidelines specied a low microbial limit (100 coliforms 100 ml1) and gave specic recommendations on treatment (Havelaar et al., 2001). In the two decades after the initial Guidelines were developed, the use of wastewater in agriculture expanded in many arid and semi-arid countries. Increasing use and the health and safety questions concerning this practice became driving forces for conducting a number of epidemiological studies. A thorough review of epidemiological studies was prepared by Shuval et al. (1986). As epidemiological evidence was compiled it became clear that the previous Guidelines needed to be revised. The following issues needed to be considered: Current bacterial standards were overly conservative and not based upon actual health risks as determined by epidemiological studies; Overly strict standards were impossible to achieve in many situations and were often ignored and Guidelines needed to include risk management approaches which would complement available treatment processes or could be used in the absence of wastewater treatment to reduce health risks. Based upon these considerations the WHO Guidelines were updated in 1989 (WHO, 1989; Mara and Cairncross, 1989). These Guidelines have been very inuential and many countries have adopted or adapted them for their wastewater and excreta-use practices. WHO is currently updating the 1989 Guidelines to incorporate new health evidence including risk assessment. The revised Guidelines will include more information about dening tolerable risks to society based upon the actual disease situation in any given country. This will help policy makers to better evaluate the risks in their countries and develop strategies to address the greatest health risks rst. In order to better package the Guidelines for appropriate audiences it was decided to present the Guidelines in three separate volumes: Guidelines for the Safe Use of Wastewater and Excreta in Aquaculture; Guidelines for the Safe Use of Wastewater in Agriculture and Guidelines for the Safe Use of Excreta and Grey Water.
WHO GUIDELINE PHILOSOPHY
WHO Guidelines are based upon best available scientic evidence and broad participation. The Guidelines incorporate a riskbenet approach and are developed around good practices. The Guidelines are meant to be adapted to local social, economic and environmental factors. Where the Guidelines relate to technical issues, for example wastewater treatment, technologies that are readily available and achievable (from both technical and economic standpoints), are explicitly noted. Overly strict standards may not be sustainable and, paradoxically, may lead to less health protection because they may be viewed as unachievable under local circumstances and, thus, ignored. The Guidelines therefore strive to maximize overall public health benets and the benecial use of scarce resources.
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It is important to recognize that in many situations where wastewater is used in agriculture, effective treatment of wastewater may not be available for many years. WHO Guidelines must therefore be practical and offer feasible risk management solutions that will maximize health protection and facilitate the benecial use of scarce resources. To achieve the greatest benets to health, Guidelines should be implemented with other health measures such as: health education, hygiene promotion, provision of adequate drinking water and sanitation, and other health-care measures. In order to properly interpret and apply the Guidelines in a manner appropriate to local conditions, a broad-based policy approach is required that will include legislation as well as positive and negative incentives to alter behaviour and monitor and improve situations. Local, national and international standard-setting bodies may develop standards that differ between regions and within regions according to differences in these factors.
EVIDENCE FOR HEALTH IMPACTS
In 1989, the evidence at that time suggested that the use of untreated wastewater in agriculture presented a high actual risk of transmission of intestinal nematodes and bacterial infections especially to product consumers and farm workers; there was limited evidence that the health of people living near wastewater irrigated elds 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 eld workers or consumers (WHO, 1989). In 2002, Blumenthal and Peasey completed a critical review of epidemiological evidence of the health effects of wastewater and excreta use in agriculture for WHO (Blumenthal and Peasey, 2002). This review was 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 the exposure, and in particular 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. The results of this epidemiological review are presented in Table I. In many countries, industrial wastewater is often mixed with municipal wastewater and is used for irrigation. Industrial wastes may contain toxic organic and inorganic chemicals that can be taken up by crops or enter groundwater resources. It is difcult to assess the impacts on health because of the problem of associating chronic exposure to chemicals with diseases having long latency periods. However, health effects from both organic chemicals and heavy metals have been observed in some countries where industrial wastewater has been used for irrigation. The health risks associated with chemicals found in wastewater and sludge may need to be given more attention, particularly as industrialization increases in developing countries. The best defence against exposure to toxic chemicals during crop irrigation is to prevent their addition to the wastewater in the rst place. This can be achieved by treating industrial wastewater separately, preventing it from being discharged into municipal wastewater streams or requiring pretreatment of the industrial wastewater before it is added to the municipal wastewater stream. In general, wastewater of domestic origin is not likely to contain toxic chemicals in concentrations likely to lead to health problems for either the farmers or the crop consumers.
WHO GUIDELINES AND RISK MANAGEMENT
The protection of public health can best be achieved by using a multiple barrier approach that interrupts the ow of pathogens from the environment (wastewater, crops, soil, etc.) to people (Figure 1). Human pathogens in the elds 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 (cotton, for example), may prevent pathogens on the crop from affecting the consumer (WHO, 1989; Mara and Cairncross, 1989). The available measures come under ve main categories:
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Table I. Summary of health risks associated with the use of wastewater in irrigation Group exposed Nematode infection Consumers Signicant risks of Ascaris infection for both adults and children with untreated wastewater; no excess risk when wastewater treated to < 1 nematode egg/litre except where conditions favour survival of eggs Signicant risks of Ascaris infection for both adults and children with contact with untreated wastewater; risks remain, especially for children when wastewater treated to < 1 nematode egg/litre; increased risk of hookworm infection to workers Ascaris transmission not studied for sprinkler irrigation but same as above for ood or furrow irrigation with heavy contact Health threats Bacteria/viruses Protozoa
Farm workers and their families
Nearby communities
Cholera, typhoid and shigellosis Evidence of parasitic protozoa outbreaks reported from use of found on wastewater irrigated untreated wastewater; sero-positive vegetable surfaces but no direct responses for Helicobacter evidence of disease transmission pylori (untreated); increase in non-specic diarrhoea when water quality exceeds 104 FC 100 ml1 Increased risk of diarrhoeal disease Risk of Giardia intestinalis infecin young children with wastewater was tion insignicant for contact contact if water quality exceeds with both untreated and treated 104 FC 100 ml1; elevated risk of wastewater, Increased risk of amoebiasis observed with contact Salmonella infection in children with untreated wastewater exposed to untreated wastewater; elevated seroresponse to Norovirus in adults exposed to partially treated wastewater Sprinkler irrigation with poor water No data for transmission of protoquality 1068 TC 100 ml1, and zoan infections during sprinkler irrigation with wastewater high aerosol exposure associated with increased rates of infection; use of partially treated water 1045 FC 100 ml1 or less in sprinkler irrigation is not associated with increased infection rates
Source: Carr et al. (2004).
waste treatment crop restriction irrigation technique human exposure control, and chemotherapy and vaccination
It will often be desirable to apply a combination of several methods. Sometimes partial treatment to a less demanding standard may be sufcient if combined with other measures such as crop restriction, but it may need to be supplemented by additional measures to protect agricultural workers.
Waste treatment
The removal or inactivation of excreted pathogens is the principal objective of wastewater treatment; and treatment to levels proposed by Blumenthal et al. (2000a) should be adequate to protect public health. Conventional wastewater treatment options (primary and secondary treatments) are often better at removing environmental pollutants than removing pathogens, however, and many of these processes may also be difcult and costly to operate properly in developing country situations. Waste stabilization ponds (WSP), when designed and operated properly, are highly effective at removing pathogens and can be operated at low cost where inexpensive land is available. They are designed to use natural processes of biodegradation, disinfection by sunlight, and particle settling under gravity, to purify the water. They form a series of shallow ponds linked together to maximize retention time. However, WSPs should be designed, operated and maintained in such a way as to prevent disease vectors from breeding in the ponds. Where effective treatment is not available, it may be possible to consider other options that improve microbial water quality, such as storage reservoirs to partially treat wastewater or water abstraction from surface waters some
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Crop / Soil
Treatment Application Techniques
Crop Restriction Application Timing Exposure Control Vaccination
Wastewater / Pathogens
Treatment Application Techniques Exposure Control Vaccination
Figure 1. Risk management points
Humans / Animals
distance from wastewater discharges where dilution has already taken place. For example, in Mexico, irrigation with untreated wastewater was frequently associated with Ascaris infections and diarrhoea in farm workers and their families, which could be prevented by using wastewater that had been retained in a series of reservoirs (Cifuentes et al., 2000). If reservoirs are designed specically to increase residence time they can be effective in improving microbial water quality. Reservoirs have the added advantage of storing the wastewater for use in the dry seasonoften a time of peak demand. There is a need for research and development work to improve the helminth-egg removal efcacy of wastewater treatment systems to meet the microbial standards. In some situations, the quality of primary or secondary-treated efuents can be improved by further treatment in a single polishing (maturation) pond of 5 days retention time (Blumenthal et al., 2000b).
Crop restriction
Water of poorer quality can be used to irrigate non-vegetable crops such as cotton or crops that will be cooked before consumption (e.g. potatoes). However, crop restriction may protect the health of consumers but not farm workers and their families. It is therefore not an adequate single control measure, but should be considered within an integrated system of control (Blumenthal et al., 2000b). In Chile the use of crop restriction when implemented with a general hygiene education programme signicantly reduced the transmission of cholera from the consumption of raw vegetables; it has also been used effectively in Mexico and Peru (Blumenthal et al., 2000b; Peasey et al., 2000).
Irrigation technique
Because of the formation of aerosols, spray/sprinkler irrigation has the highest potential to spread contamination on crop surfaces and affect nearby communities. Where spray/sprinkler irrigation is used with wastewater it may
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be necessary to set up a buffer zone (e.g. 50100 m from houses and roads) to prevent a health risk for local communities. Farm workers and their families are at the highest risk when ood or furrow irrigation techniques are used, particularly when protective clothing is not worn and earth is moved by hand. Localized irrigation techniques (e.g. bubbler, drip, trickle) offer farm workers the most health protection because the wastewater is applied directly to the plants; but are problematical if the wastewater has suspended material in it which clogs the water emitters. 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). Drip irrigation also improves crop yields and reduces water use. Cessation of irrigation for 12 weeks prior to harvest can be effective in reducing crop contamination. Many vegetables need watering nearly until harvest to increase their market value, but this option may be possible with some fodder crops that do not have to be harvested at the peak of their freshness.
Human exposure control
Four groups of people are at potential risk: agricultural eld workers and their families crop handlers consumers (of crops, meat and milk), and those living near the affected elds.
Agricultural eld workers are at high risk of parasitic infections. Exposure to hookworm infection can be reduced, even eliminated, by the use of less contaminating irrigation methods (as above) and by the use of appropriate protective clothing (i.e. shoes for eld workers and gloves for crop handlers). A rigorous health education programme that targets consumers, farm workers, produce handlers and vendors is needed. Hand washing with soap is a very important behaviour that needs to be emphasized. The simple act of handwashing with soap can reduce the transmission of diarrhoeal disease by up to 33% (WHO, 1993). Field workers should be provided with adequate sanitation facilities and safe water for drinking and hygiene purposes, in order to avoid the consumption of, and contact with, wastewater. Similarly, sanitation facilities and safe water should be provided at markets for washing and freshening produce. Vendors need to practise good personal and food hygiene. Consumers can cook vegetables, meat and milk, and practise good personal and domestic hygiene to protect their health. Meat should be inspected and carcasses infected with tapeworm larvae should be rejected (WHO, 1989; Mara and Cairncross, 1989).
Chemotherapy and vaccination
Anti-helminthic treatment and immunization cannot normally be considered as an adequate strategy to protect farm workers and their families. Immunization against helminthic infections and most diarrhoeal diseases is currently not feasible. However, for highly exposed groups or sensitive subpopulations (e.g. tourists), immunization against typhoid and hepatitis A may be worth considering. Anti-helminthic treatment of intense nematode infections in children and the control of anaemia in both children and adults, especially women and post-menarche girls, is important. Treatments must be reapplied at regular intervals to be effectiveseveral times a year for children living in endemic areas (Blumenthal et al., 2000b; Peasey et al., 2000).
HOW THE GUIDELINES ARE IMPLEMENTED
It is important to consider the health implications of any standards or regulations which are enacted. In countries where low sanitation standards prevail very little of the faecaloral disease may actually be associated with the use of wastewater irrigation. Thus, even enacting stringent standards on the use of wastewater may not have a
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measurable impact on disease transmission. Therefore, an attempt to dene a tolerable risk to society may be necessary for prioritizing the greatest health risks rst. The revised Guidelines include discussion of tolerable risks and methods for prioritizing health threats. Phased implementation of the WHO Guidelines may be necessary as treatment is gradually introduced and improved over a period of time (e.g. 115 years). Implementation of the WHO Guidelines will protect public health most when it is integrated into a comprehensive public health programme that includes other sanitary measures such as personal and domestic hygiene education. For example, it may be possible to link health education and hygiene promotion to agricultural extension activities or other health programmes (e.g. immunization programmes). Guideline implementation will be different in each setting. For example, urban and peri-urban areas are likely to pose challenges because of the dispersed nature of agriculture in these areas and the greater number of small plots. Inspectors may have a hard time ensuring that effective crop restriction is taking place or visiting all of the areas where wastewater is used in agriculture. Crop restriction will be more effective if the types of crops that can be grown are in demand and command an adequate price (e.g. potatoes or maize) in the local market. However, markets may offer additional points of intervention where local authorities may have more control over water supplies, hygiene and sanitation facilities. The key to guideline implementation is setting realistic standards and exibility. WHO Guidelines need to be adapted for 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 signicantly change the background level of disease and/or may be viewed as unachievable and thus ignored entirely. Flexible solutions are needed. Wastewater treatment could be in the form of small, locally developed, decentralized facilities closer to where the wastewater is generated. An initial aim of partial treatmente.g. to meet the WHO helminth guideline valuemay be required, eventually phasing in the ot...