WELL STUDIES

Report summary of WELL Task No. 68 (Part 1)

Guidelines for wastewater reuse in agriculture and aquaculture:

Authors: Ursula Blumenthal, Anne Peasey, Guillermo Ruiz-Palacio, and Duncan Mara, June 200

Editor: Darren Saywell

Use of wastewater in agriculture is becoming more important due to increasing water scarcity in dry climate regions of the world. Standards for wastewater reuse in many countries have been influenced by the WHO (1989) Health Guidelines and the USEPA/USAID (1992) Guidelines. Since then, epidemiological studies have been carried out by London School of Hygiene and Tropical Medicine, with colleagues in Mexico and Indonesia, and microbiological studies of crops irrigated with treated wastewater have been carried out by Leeds University, with colleagues in Brazil and Portugal, to assess the validity of these guidelines. The WHO (1989) Guidelines for Wastewater Reuse in Agriculture and Aquaculture are reviewed in the light of these and other recent studies.

There are currently three main approaches for establishing microbiological quality guidelines and standards for treated wastewater reuse in agriculture which have different objectives as their outcome:

(i)ii the absence of faecal indicator organisms in the wastewater,
(ii)i no measurable excess cases in the exposed population, and
(iii) a model generated estimated risk below a defined acceptable risk.

In this review we use approach II, using empirical epidemiological studies supplemented by microbiological studies on pathogen transmission, in conjunction with approach III, using model-based quantitative microbial risk assessment for selected pathogens in coming to our conclusions. Recommendations have also been made for the use of a disease control approach in the setting of country standards.

The results of studies of consumer risks do not provide any evidence to suggest a need to change the WHO faecal coliform guideline of =10 3 FC/100ml for unrestricted irrigation. Epidemiological studies in a situation where enteric infections are endemic suggest that risks of enteric infections are significant, but low, when the guideline is exceeded by a factor of 10. There was no risk associated with the total consumption of raw vegetables and a two-fold increased risk associated with consumption of specific vegetables (e.g. onion). Risk assessment has indicated that the annual risk of enteric virus and bacterial infection from eating lettuce irrigated with water meeting WHO Guideline level ranges from 10 -5 to 10 -9. However, enteric viruses other than the ones studied may outnumber them by possibly an order of magntiude. Since the US microbial standards for drinking water are based on the criteria that human populations should not be subjected to the risk of infection by enteric disease greater than 10 -4 (or I case in 10,000 persons/year), then the WHO Wastewater Reuse Guidelines would appear to offer a similar level of protection. In situations where there are insufficient resources to reach 10 3 FC/100ml, then a more relaxed guideline of 10 4 FC/100ml could be adopted, but should be supplemented by other health protection measures.

The nematode egg guideline of =1 nematode egg/litre for unrestricted irrigation appears to be adequate to protect consumers of cultivated vegetables spray-irrigated with effluent of consistent quality and at high temperatures, but not necessarily consumers of vegetables surface-irrigated with such effluent at lower temperatures. Studies have shown that lettuces spray-irrigated with water of =1 nematode egg/litre were not contaminated or only lightly contaminated at harvest, and any eggs present were not infective. However, since a few eggs on the harvested plants were viable, crops with a long shelf life represent a potential risk to consumers. Children who ate wild vegetables irrigated with water of =1 nematode egg/litre, however, had significantly increased prevalence of Ascaris. It is recommended that a stricter guideline of =0.1 eggs per litre is adopted to prevent transmission of Ascaris infection and to allow for the risks to farm workers involved in cultivating the vegetable crops. A guideline of =1 nematode egg/litre may be adequate where crops with a short shelf life are grown (e.g. salad crops) and wild plants are not eaten, or where the aim is disease control (in this case related to intensity not prevalence of infection), instead of prevention of transmission of infection.

In the WHO Guidelines (1989) there was no faecal coliform guideline for restricted irrigation due to the lack of evidence of a risk of bacterial and viral infections to farm workers and nearby residents. Recent evidence of enteric infections in farming families in direct contact with partially treated wastewater and in populations living nearby sprinkler irrigated fields, when the water quality exceeds 10 6 FC/100ml, suggests that a faecal coliform guideline should now be added. However, where adults and school-aged rural children are in direct contact with the partially treated wastewater originating in an urban area, there may still be at risk of diarrhoeal disease and Human Norwalk-like Virus/Mexico at a level of 10 3 -10 4 FC/100ml. A reduced guideline level of =10 3 FC/100ml would be safer where adults are involved in flood/furrow irrigation and children are regularly exposed (through farm work or play). This would also help reduce the risks from epidemic infections which could then be transmitted to effluent-irrigating communities from an outbreak in the source community. Where there are insufficient resources to provide treatment to reach this stricter guideline, a guideline of 10 5 FC/100ml should be supplemented by other health protection measures for children.

The nematode egg guideline of 1 egg per litre is adequate if no children are exposed, but a revised guideline of 0.1 egg per litre is recommended if children are in contact with the wastewater through irrigation or play. Children in contact with effluent from a storage reservoir which met WHO Guidelines had increased prevalence and intensity of Ascaris infection, but when the effluent had been stored in two reservoirs and no nematode eggs were detectable, there was very little excess Ascaris infection in any age group. A stricter guideline of =0.1 eggs per litre is recommended where children are exposed to irrigation water. Alternatively, a country with limited resources aiming at disease control could adopt a less strict guideline and adopt additional health protection measures; such as, human exposure control and chemotherapeutic intervention.

The evidence reviewed did not support the need for a separate guideline to specifically protect against viral infection, but there were insufficient data to evaluate the need for a specific guideline for protozoa. Regarding wastewater use in aquaculture, evidence from epidemiological studies shows that the faecal coliform guideline needs to be below 10 4 FC/100ml. There appears to be sufficient evidence to suggest that the tentative faecal coliform guideline of =10 3 FC/100ml for the fishpond water is the right order of magnitude, and insufficient data to warrant a reduction of this level to 10 2 FC/100ml or a relaxation to 10 4 FC/100ml. This implies that the quality of the feed water can be around 10 4 -10 5 FC/100ml, depending on the size of the fishpond and the amount of dilution that occurs. In future, it would be useful to consider adding a bacterial guideline for the quality of the wastewater (SPC/100ml) and for the quality of fish (SPC/g). This will address concerns over the adequacy of faecal coliforms as indicators of health risks from waste-fed aquaculture.

Wastewater treatment technologies suitable for meeting the revised microbiological guidelines for agriculture include the use of waste stabilisation ponds (WSP), wastewater storage and treatment reservoirs (WSTR), or conventional treatment processes. When using WSP, the revised guidelines usually require the use of 1 or more maturation ponds after the anaerobic and facultative ponds. Use of sequential batch-fed storage and treatment reservoirs can be designed to meet the guidelines for unrestricted and restricted irrigation. When conventional treatment processes are used secondary treatment, filtration and disinfection are often needed to meet the revised guidelines. The cost and difficulty in operating and maintaining conventional treatment plants to the level needed to meet the guidelines means that they are not recommended where WSP and WSTR can be used.

A range of health protection measures including crop restriction, irrigation technique, human exposure control and chemotherapeutic intervention should all be considered in conjunction with partial wastewater treatment. In some cases, community interventions using health promotion programmes and/or regular chemotherapy programmes could be considered, in particular where no wastewater treatment is provided or where there is a time delay before treatment plants can be built.

In order to meet the faecal coliform guideline for aquaculture, wastewater (or excreta/septage) needs to undergo some form of treatment before it can be used in fishponds. Where WSP are used, effluent from the facultative pond or first maturation pond can be discharged into the fishpond (depending on the effluent quality and size of the fishpond). Where effluent from conventional secondary treatment plants is used, the quality of the effluent may need to be improved by use of a polishing pond prior to the effluent being discharged into a fishpond.

FULL REPORT.....BACK TO TOP

Home > Resources > WELL studies > Guidelines for wastewater reuse in agriculture and aquaculture