Nitrogen and phosphorus removal in a wetland treating sludge dewatering effluent

This study was performed in Katrineholm, South Sweden, where, since 1990, a constructed wetland system has been used for treatment of sludge dewatering effluent, containing 731 mg L-1, 26 mg L-1, and 944 mg L-1 of tot-N, tot-P and TSS, respectively. Concentrating on the first (2.7 ha) of two wetlands, the aim was to investigate the annual nutrient removal efficiency. Water level, inflow and outflow were recorded and grab water samples taken at 11 locations along the wetland. From April - October, there was no outflow but the water level rose. Including possible leakage through the earth dam in the outflow figures, a tot-N balance indicated a net loss of nitrogen from the wetland during winter, but a considerable removal during the rest of the year. The mean removal in 1996 was 0.36 g m-2d(-1) or 33% of the load with a further 0.43 g m(-2) d(-1) retained in the wetland water due to the temporary water level increase. Water quality analyses indicated that both ammonia volatilization and denitrification were important processes. For P, the annual removal amounted to 0.024 g m(-2) d(-1) or 63%, mainly due to sedimentation. More than half of the P removed was retrieved from the wetland when the inlet section was desludged.