Domestic wastewater treatment by infiltration basins is a process that is becoming common in France. In the absence of accurate operating data, we felt it would be useful to conduct a case study at an existing plant. The Saint Symphorien de Lay plant, serving a population-equivalent of 500, was chosen because the basins are watertight and drained.

The study revealed 85% TSS, COD and Total Phosphorus removal. Nitrogen yields varied over the flooding period, with a high production of nitrates at the beginning of the period. The rate of decontamination (1 to 2 Log unit) was lower than expected.

The infiltration surface should increase from 1 to 1.5 m2/p.e. and spread over three basins. Research is now focusing on finding the biologically active depth of an infiltration basin.

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