The irrigation techniquecan be chosen to reduce the amount of human exposure to the wastewater. In general, health risks are greatest when spray/sprinkler irrigation is used, as this distributes contamination over the surface of crops and exposes nearby population groups to aerosols containing bacteria and viruses (the opposite occurs with nematode eggs, which tend to be washed off during spray irrigation). This technique should be avoided where possible, and if used, stricter effluent standards apply (see table 3). Flood and furrow irrigation exposes field workers to the greatest risk, especially if earth moving is done by hand and without protection. Localized irrigation (inc. drip, trickle and bubbler irrigation) can give the greatest degree of health protection by reducing the exposure of workers to the wastewater. A period without irrigation before harvest (1-2 weeks) can allow die-off of bacteria and viruses such that the quality of irrigated crops improves to levels seen in crops irrigated with fresh water, as shown by Vaz da Costas Vargas et al. (1996). However, it is not practical in unregulated circumstances since farmers will probably not stop irrigation of leafy salad crops 5 days or more before harvest. Replacing partially-treated wastewater with fresh water for a week or so before harvest is not a reliable way of improving crop quality since re-contamination of the crops from the soil has been found to occur. Use of ending of irrigation before harvest is more feasible with fodder crops which do not need to be harvested at their freshest, and could enable the use of lower quality effluents.

Table 3: 1989 WHO guidelines for using treated wastewater in agriculture ^a