Irrigation with foul water
Irrigating vegetable crops for local consumption / sale, using foul water as the main source of water
Key organisation involved: The Nehemiah Project seeks to assist those whose families are impacted by AIDS in Zimbabwe. This project is based in Bulawayo. One of the areas of work for this charity is to help commercial sex workers to have an alternative way of providing for themselves and their children. One part of this is to encourage them to grow some of their own food to provide a balanced diet. Full details of technical support request: There is a proposal from this charity to irrigate a patch of land for vegetable production. The issue is that the only source of irrigation water is an open sewer running alongside, that carries foul water to a local council treatment centre. So, my question is - assuming that we receive the go-ahead to abstract this foul water from the local council - how could we use / treat this (relatively small flow of) sewage before spreading it over food crops. My concern is not just the residue being left on the crops, but the very idea that adults and/or children would be handling raw sewage to an extent (if we go for surface irrigation) or any manual intervention in the application of the sewage to the crops. There is a local idea that drip-irrigation could prevent any manual intervention, but I have my doubts that a drip system would last any length of time at all before it gets choked up with excrement or plant based materials.
Lots on the internet on this from WHO, FAO and others as it is a widespread practice. Yes there is a risk from those working in the fields - aerosol and direct contact - so good that they are thinking about this. Issue with crops depends very much on delay between last irrigation with dirty water and harvesting, also how food is prepared (uncooked salad is obviously more dangerous than boiled potatoes). Munich sewage works used to at least grow fish in some of their tanks....
Here is the URL for the WHO guidelines now in their third edition https://www.who.int/water_sanitation_health/wastewater/gsuww/en/index.html They can be downloaded for free in PDF format.
This is a complicated issue, and one that is becoming increasingly important as more and more people look at recycling and reusing their wastewater.
The textbook answer is that the use of untreated effluent in agriculture is potentially hazardous and requires careful handling plus restrictions on the type of crops irrigated and the use of the crop products (as per the revised 2005 WHO Guidelines for the safe use of wastewater, excreta and greywater. Volume 2: Wastewater use in agriculture). I have extracted a few of the relevant sections from this long and wordy document, and copied them below this email.
The WHO note that the greatest health risks are associated with crops that are eaten raw - for example salad crops, especially if they are root crops (e.g. radishes and onions) - or crops that grow close to the soil (e.g. lettuce, courgettes). In addition, crops that have irregular surface properties (e.g. hairy, sticky, creviced, or rough) protect pathogens from exposure to radiation and make them more difficult to wash off with rain or post-harvest washing. The amount of water each crop holds is also an important factor: one study found that lettuce retains 10.8ml of irrigation water, whereas a cucumber retains only 0.36ml (i.e. only 3% the volume retained by the lettuce). Obviously, the more irrigation water retained the more likely the contamination.
A practical response is that untreated effluent is a valuable and nutrient-rich commodity in many developing countries, and that few poor communities (or individuals) are willing to follow the restrictive guidelines advocated by the WHO (and few local authorities have the resources to treat wastewater or enforce environmental regulations). In particular, those coming into contact with raw wastewater are usually reluctant to wear the required protective boots, gloves and clothing, particularly in hot climates; and smallholder farmers are often unwilling to stop growing the high value fruit and vegetables that are most at risk of contamination.
Relevant approaches (as detailed in the attached Water Policy Briefing from the International Water Management Institute) include:
Improved crop selection and irrigation practices Selecting crops that are suitable for wastewater irrigation (often low value grains and fodder crops) and using irrigation techniques that prevent direct contact with untreated wastewater (e.g. drip irrigation; 'bed and furrow' cultivation).
Preventive medical care programs Provide anti-helminthic (deworming) medication to farmers (and their families) exposed to untreated wastewater.
Improved post-harvest handling Washing and improved storage of harvested vegetables.
Conjunctive management of wastewater and other water supplies Because wastewater has a low/negligible cost, farmers tend to overwater, which needlessly increases the environmental and health risks (as well as wasting valuable nutrients). Where alternative water sources are available, the blending of wastewater and other water supplies allows expansion of the irrigated area (potentially allowing more people to benefit) and more efficient use of the nutrient-rich wastewater.
Educating farmers and awareness-raising among consumers and authorities Farmers (and others) should be aware of the risks that they face, and would benefit from ...
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