COMBINED NITROGEN AND PHOSPHORUS REMOVAL IN A FULL-SCALE CONTINUOUS UP-FLOW SAND FILTER

An attractive method for post-denitrification may be the use of sand f ilters. In this paper, a description and evaluation are given of full scale studies of the use of a continuous sand filter for the combined removal of suspended solids, phosphorus and nitrogen. Experiments were performed using methanol as a carbon source for denitrification and f erric chloride for an improved phosphorus removal. The studied continu ous sand filter was a DynaSand filter marketed by Nordic Water Product s AB. The filter has a surface area of 4.7 m(2) and a maximum possible bed height of 6 m. The bed consisted of sand with 1.2-2 mm grain size . The tested bed heights were 3.5-4.8 m, and the hydraulic load varied between 5.4 and 24.5 m/h. The effluent from the Loudden treatment pla nt in Stockholm was supplied to the filter. Influent nitrate concentra tions up to 20 mg N/l were tested and they decreased to 0.5-2 mg N/l. The methanol dosage was controlled by the measurement of nitrate conti nuously in the effluent by a dr Lange meter. The denitrification rate followed a half order reaction down to low values of nitrate. Results showed that an effluent concentration of 0.15 mg P/l could easily be o btained. It was found that the influence of phosphate concentration is small on the denitrification rate if the phosphate concentration is a bove 0.1 mg P/l. The reject has a low sludge index which is favourable if the reject is returned to a sedimentation basin. The emission of n itrous gas (N2O) is very low. The installation makes it possible to us e space efficiently, since polishing, phosphorus removal and denitrifi cation can take place in the same unit. Already the phosphorus removal process reduces the need for process volume by 80 % compared to a con ventional process with flocculation and sedimentation basins.