Buried filters were investigated experimentally in pilot and full scale as typical on-site treatment for small wastewater flows. The filters were operated by intermittent flushing which causes the water and the pollutant transport through the unsaturated media to be of a highly dynamic nature. Water transport and tracer studies at low and high hydraulic flush loads revealed frequent flushing at low loads to be superior to less frequent flushing at high loads for treatment of the same daily amount of wastewater. These findings were confirmed in a full scale plant through monitoring of the dynamic washout of unoxidized matter in terms of COD and NH4+ after application of different hydraulic loads. The moisture retention capacity of the filter media correlated to the grain size distribution was found to be an important parameter. COD-removal and nitrification rates depend strongly on the oxygen supply to the media. In general, the oxygen diffusion into the media and the air exchange, induced by intermittent flushing, are sufficient. However, when applying relatively large hydraulic loads and coarse filter grains, especially in the range above 1 mm, buried filters tend to larger breakthroughs of unoxidized matter due to short retention times and instantaneous lack of oxygen. Experiments on average treatment performance were carried out and showed that under optimized conditions even wastewaters containing relatively high ammonia contents (150 gNH4+-N/m3) can be fully nitrified when limestone type filter material is used. Full scale operation revealed further that careful pre-treatment (e.g. septic tank) for the removal of most of the suspended solids is necessary to guarantee safe operation.

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