Wastewater and Excreta Reuse

Wastewater is increasingly used for agriculture especially in areas of water scarcity, population increase and related demands for food. There is also a growing recognition of the value of wastewater as a source of both water and nutrients. Wastewater can be a "reliable" source of water for irrigation throughout the year. However, wastewater is always a public health risk. WHO Guidelines pay attention to the Stockholm Framework which is an integrated approach that combines both risk assessment and risk management to control water-related diseases.

Hazards associated with wastewater irrigated products include excreta-related pathogens and some toxic chemicals. The following health protection measures can protect the product consumers: (edited from the WHO Guidelines Vol 2):

Need for wastewater treatment: While treatment of wastewater prior to its use in irrigation is always desirable, it will often be difficult to provide the level of treatment required by the WHO Guidelines. These guidelines indicate a detention time of about 22 days in a waste stabilization pond (WSP) system for unrestricted irrigation, or 11 days retention in a WSP system or equivalent for restricted irrigation. Unrestricted irrigation includes irrigation of vegetable and salad crops that might be eaten uncooked. Restricted irrigation means irrigation that is limited to cereal, industrial and fodder crops, pasture and trees.

Wastewater application methods: Contamination of crops and risks to farm workers vary depending on the method used to apply irrigation water. The most hazardous option is spray irrigation, followed by general inundation of the area to be irrigated. Ridge and furrow irrigation reduces risks to some extent but the best irrigation option from a health point of view is allow water to drip from pipes laid along the ground. Drip irrigation is also advantageous in that it minimizes the amount of irrigation water required. Drip irrigation systems discharge water through small holes and this means that they will only be viable if pre-treatment is provided to remove solids. This pre-treatment might be by septic tanks, baffled reactors, upward flow anaerobic filters or simple pond systems.

Irrigation timing: Health risks to users can be reduced by stopping irrigation two weeks before crops are harvested. However, this option does not protect the health of farm workers. Choice of crops / crop restriction: The dangers to consumers will be negligible if wastewater is only used to irrigate crops that are cooked before eating. However, this will not reduce the risks to farm workers. In practice, it may be difficult to persuade farmers to forego high value salad crops.

Health and hygiene promotion: including hygienic practices at food markets and during the food preparation, produce washing, disinfection and proper cooking.

Health protection measures are also needed for the workers and their families. Use of personal protective clothing, such as rubber gloves and waterproof shoes, could be recommended, but in practice the workers may be reluctant to use them where they are hot and restrict their movement. Health and hygiene promotion, disease vector and intermediate host control, health check ups, and reduced vector contact are amongst the other measures to consider to protect workers., Basic access to safe drinking water and sanitation (including handwashing!) facilities at farms represent a basic investment in worker protection.

Health protection measures are further needed for the communities which may be at similar risk as those workers who work directly at the farms. Measures to protect the surrounding communities include wastewater treatment, restricted access to irrigated fields and hydraulic structures, access to safe drinking water sources and recreational water, and health and hygiene promotion. All these should be specific attention to the children.