Seasonal efficiency of a constructed wetland for treating dairy farm wastewater

A great amount of agricultural wastewater is derived from dairy farms in East Germany. Dairy farm wastewater from one farm in the State of Brandenburg is highly polluted with average concentrations of 3,300 mg 1(-1) COD, 200 mg 1(-1) total nitrogen, 60 mg 1-1 total phosphorus and 460,000 CFU ml(-1) faecal coliforms. This study was designed to show to what extent constructed wetlands are an appropriate technology for cleaning up dairy farm wastewater under changing climatic conditions. During both summer and winter, a horizontal subsurface flow constructed wetland was treated for 10 weeks with dairy farm wastewater with a hydraulic load of 0.013 m(3) m(-2) d(-1) and 0.010 m(3) m(-2) d(-1), respectively. In the summer period mean air temperature was 18.1degreesC and in the winter period 3.1degreesC. Temperature had a great influence on proteolytic, nitrifying and faecal coliform bacteria, which are not adapted to such low temperatures as prevailed during the winter treatment. In contrast to nitrogen mineralisation, decomposition of carbon compounds was not restricted by low temperatures as reflected by CO2 production. At the end of the winter treatment, CO2 production was 1.4 mmol CO2 kg DW-1 h(-1) in the soil depth between 15 cm and 45 cm. During the winter, effluent concentrations were higher for COD and total N. However, related to high influent concentrations, removal rates were even higher in the winter. In the summer period removal rates for COD, total N, total P and faecal coliforms were 89.2%, 83.4%, 75.1% and 99.3%, respectively and in the winter period removal rates were 92.0%, 87.5%, 82.8% and 95.8%, respectively.