Treatment of Groundwater with Slow Sand Filtration

The purification process and techniques of the slow sand filtration method for treatment of groundwater was studied on the basis of pilot plant and full scale tests and studies of waterworks, to obtain guidelines for construction and maintenance. The purification process consists in general of two principal phases which are pre-treatment and slow sand filtration. Both are biological filters. The main purpose of the pre-treatment is to reduce the iron content of raw water, in order to slow down the clogging of the slow sand filters. Different types of biofilters have proved very effective in the pre-treatment phase, with reduction of total iron from 50 % to over 80 %. During the treatment, the oxidation reduction conditions gradually change becoming suitable for chemical and biological precipitation of iron, manganese and for oxidation of ammonium. Suitable environmental conditions are crucial in the oxidation of manganese and ammonium which, according to these studies, mainly occurs in slow sand filters, at the end of the process. Low water temperature in winter does not seem to prevent the biological activities connected with the removal of iron, manganese and ammonium, the chief properties necessitating treatment of groundwater in Finland.