PURIFICATION EFFICIENCY OF DANISH BIOLOGICAL SAND FILTER SYSTEMS

During an investigation in 1989 on small low technology treatment syst ems, particularly reed bed and biological sand filter systems in Denma rk, it appeared that biological sand filter systems offer several adva ntages for the treatment of small wastewater sources. These observatio ns gave rise to a new project concerning systematic registration of al l biological sand filter systems in Denmark. Besides this registration the aim of the project was to indicate the reliability of these syste ms concerning removal ability and to give some general guidelines on d ifferent circumstances which seem to have positive or negative effects on the removal of particularly ammonium and phosphorus. The paper sum marizes 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, wit hin a few months, but longer periods of about 6 months may be required to achieve full nitrification. High nitrogen removal depends on the n itrification processes and also of anoxic zones in the filter with den itrification. The best way to achieve good nitrification is to use coa rse-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 phosph orus at concentrations of 10-15 mg P/l in the inlet.