Water Engineering and Development Centre
Series: WELL Studies in Water, Sanitation and Environmental Health Task 068/2
Wastewater is a valuable resource, however without a properly developed framework policy, safe and efficient management of this resource can not be achieved. Wastewater reuse standards for agriculture are reviewed in this document in the light of recent epidemiological and microbiological studies, with particular emphasis on wastewater reuse policy and standards in Mexico.
Contact with wastewater
Results of the studies presented indicate the need for a bacterial guideline of 103 FC/100ml to protect adults and especially children, in direct contact with wastewater, as proposed within Mexican wastewater reuse standards. There is an increasing risk of diarrhoeal disease among 5-14 year olds and of Calicivirus-Mx among adults in contact with this partially treated irrigation water. Studies also showed that farmworkers and their families, particularly children, continued to be at risk when in contact with wastewater containing <1 ova/litre. A nematode egg guideline of 0.1 ova/litre was proposed. However, in situations where there are insufficient resources to reach 103 FC/100ml, then a more relaxed guideline of 104 FC/100ml could be adopted, but should be supplemented by other health protection measures. Where children are not in direct contact with wastewater, the nematode egg guideline could be relaxed to 1.0 ova/litre.
Consumption of crops
There is a risk to the health of consumers of raw crops irrigated with water whose microbiological quality is one order of magnitude above the WHO guideline level. Consumption of certain raw crops is associated with slight bacterial risk of enteric infection, while other crops appear not to pose a threat. Therefore, a bacterial standard of 103 FC/100ml would seem adequate for restricted irrigation. A slight risk of Ascaris infection was associated with the consumption of wild vegetables irrigated with effluent containing <1 ova/litre. Microbiological studies of lettuces irrigated with water <1 ova/litre found a few viable though not infective eggs at harvest, suggesting a potential risk to consumers. A nematode egg guideline of 0.1 ova/litre is therefore recommended. However, where crops have a short shelf life and where workers are not in direct contact with wastewater, a nematode egg standard of <1 ova/litre would appear adequate. For policy makers in Mexico, it may also be important to consider the sanitary regulations of the markers where produce will be sold (eg USA), stricter standards may be warranted in such situations, to protect the export market.
When developing wastewater reuse policy, it is important that policy makers consider all possible health protection measures, not just wastewater treatment, and so create a realistic wastewater reuse policy which ensures that those in contact with irrigation water are genuinely protected.