During an investigation in 1989 on small low technology treatment systems, particularly reed bed and biological sand filter systems in Denmark, it appeared that biological sand filter systems offer several advantages for the treatment of small wastewater sources. These observations gave rise to a new project concerning systematic registration of all biological sand filter systems in Denmark. Besides this registration the aim of the project was to indicate the reliability of these systems concerning removal ability and to give some general guidelines on different circumstances which seem to have positive or negative effects on the removal of particularly ammonium and phosphorus.

The paper summarizes the results and experiences that have been collected since the beginning of the use of biological sand filter systems in Denmark in the late 70s. Typical purification efficiencies of 90-95% for BI5, 30-45% for nitrogen and 40-60% for phosphorus were observed. The ripening of sand filters to their maximum removal capacity is often short, within a few months, but longer periods of about 6 months may be required to achieve full nitrification. High nitrogen removal depends on the nitrification processes and also of anoxic zones in the filter with denitrification. The best way to achieve good nitrification is to use coarse-grained sand. The removal of phosphorus is indeed affected by the chemical properties of the sand in question. In ferrous enriched sand it is possible to achieve removal efficiencies of 70-90% of the phosphorus at concentrations of 10-15 mg P/l in the inlet.

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