Nutrient recovery in a small scale wastewater treatment plant in cold climate
An onsite wastewater treatment plant at Brändön, north of Luleå, receiving primarily treated wastewater from a village, was operated and investigated during one year. The wastewater flow was 0.5 m3/d. The main treatment steps were a prefilter, mainly to distribute the flow, a vegetation filter consisting of two different clones of Salix and two phosphorus filters with Filtralite-P and blast furnace slag (BF slag) operated in parallel. The willow bed reduction of BOD7 was in average about 80 % and of phosphorus (P) and nitrogen (N) 20–30 %. The main mechanism was believed to be filtration in the bed. The reduction through plant uptake was minor. The Filtralite-P filter reduced BOD7, P and N with 67 %, 72 % and 20 %, respectively. The BF slag filter reduced P and N with 53 % and 3 %, respectively. The release of sulphuric compounds from the BF slag filter increased largely the BOD7 content in the effluent. The Filtralite-P system achieved the requirements of the normal protection level given by the Swedish Environmental Protection Agency but not all of the requirements of the high protection level. The BF slag system did not fulfil the requirements of the two protection levels.