Efficiency of surface-flow wetlands for finishing treatment of wastewater

The objective of the present studies has been to estimate wastewater treatment efficiency of surface-flow wetlands for finishing treatment of secondary effluent. The studies have been carried out within the reconstructed wastewater treatment facilities in Babenai (Kedainiai district, Lithuania), where, after the initial wastewater treatment in the septic tanks and biological ponds, a plant filter has been set for finishing the wastewater treatment. The removal efficiency of the main pollutants in the filter like BOD5, suspended solids, total N (nitrogen), and total P (phosphorus) has become the focus of the analysis as well. Wastewater treatment efficiency analysis was done on base measurements taken in all seasons of the year within the period 2003-2006 Surface-flow wetlands are characterised by good removal efficiency of the organic pollutants. At the average load of the examined plant filter - 0.9 mg O2 l-1 BOD5 (biochemical oxygen demand), the removal efficiency reached up to 61.2%. Low winter temperatures reduce the rate of removal for BOD and the biological reactions responsible for nitrification and denitrification. During the warm period of the year, the treatment efficiency of BOD5 was 70.8%, and during the cold one – 52.4%. However, the total N constituted up to 16.7% and 9.7% respectively. Due to the anoxic and anaerobic conditions in the filter, wastewater after the treatment on average contains 66.0% of ammonium nitrogen of the total N concentration. After a year, with the growth of plants the phosphorus removal efficiency improved and at the average load of the examined plant filter according to P 0.063 g m-2 d-1, on average 0.018 g m-2 d-1 of the total P was removed, i.e., the removal efficiency constituted 28.6%Vytauto Didžiojo universitetasŽemės ūkio akademij