Diffuse pollution, caused by direct discharges from individual houses or groups of houses for which connection to central waste water treatment systems is not feasible, may be significantly reduced by on-site treatment. The performances of 14 different systems for on-site treatment in the range of 5-200 population equivalents (p.e.) in the Netherlands were tested over a two-year period of field research.

Treatment results showed reasonably comparable removal percentages in relation to large systems, the effluent quality however may not be as good due to higher influent concentrations. Significant differences in treatment, operation and maintenance aspects were recorded, causing some systems to be preferred to others. A further evaluation involving costs is currently being carried out.

The feasibility of wide-spread application of small treatment plants may depend more on financial, management and organizational aspects than on technological aspects.

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