Soil Filters

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Courtesy of IWA Publishing

Soil filters (in german: Retentionsbodenfilter) are facilities for the removal of nutrients and/or pollutants; typically on the level of an industrial site, a quarter or a drainage system (sub-)catchment. Usually they are located at the discharge points to surface waterbodies to ensure water quality standards.

Soil filters consist of two parts: The first part is a retention pond or basin for primary treatment (sedimentation) and buffering of discharge peaks. The second part, the soil filter body usually is planted with reed as the plants help to remove nutrients and optimize the soil structure for the filtering process. As the water percolates through the filter body nutrients and pollutants are removed. The filter body can work best, when it gets evenly charged with water from the retention pond. It can get damaged by both hydraulic and sediment overload. Thats why extreme volumes of discharge have to be bypassed and will only retained and cleaned partially. How much water can be retained by the soil filter varies very much with the demand for nutrient and pollutant removal. As a big part of the pollutant freight gets transported by the frequent rainfall events and associated run-off processes soil filters usually don't have large retention capacities.

After all soil filters are built and optimized to reduce nutrient / pollutant loads and not for the retention of run-off. Nevertheless their retaining effect can be taken into account for flood risk mananagement. The construction of soil filters for the reduction of flooding should only be considered if storm-water run-off in existing settlements already causes water quality problems that have to be tackled anyway.

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