In temperate climates facultative ponds and rock filters (either unaerated or, if ammonia removal is required, aerated) are a low-cost but high-performance treatment system for small rural communities. Effluent quality is suitable for surface water discharge or, in summer, for restricted crop irrigation. In tropical climates anaerobic and facultative ponds and either unaerated rock filters or, if ammonia reduction is required, subsurface horizontal-flow or vertical-flow constructed wetland, can be used if the effluents are discharged to surface waters. However, if the treated wastewater is to be used for crop irrigation, then a 3-log unit pathogen reduction by treatment in anaerobic, facultative and single maturation ponds is required for both restricted and unrestricted irrigation, provided that, in the case of unrestricted irrigation, there are in place post-treatment health-protection control measures that together provide a further 4-log unit pathogen reduction.

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