Efficiency of removal of nitrogen, phosphorus, and zinc from domestic wastewater by a constructed wetland system in rural areas: a case study

The effluent from the combined household wastewater treatment facilities used in unsewered areas of Japan is generally high in nitrogen (N) and phosphorus (P). In Japan, environmental quality standards for zinc (Zn) pollution were enacted recently because of the toxicity of Zn to aquatic ecosystems. In 2004 a fallow paddy field at the Koibuchi College of Agriculture and Nutrition was converted into a surface-water-flow constructed wetland (500m(2)) to clean the effluent from the combined household wastewater treatment facility of a dormitory (100 residents) before discharge to a pond. We evaluated N and P removal efficiencies and the fate of soluble Zn in the wetland from April 2006 to March 2007. Wetland influent contained an average of 18.3mg L-1 total N and 1.86mg L-1 total P. In the effluent from the wetland, average total N concentration was 10.3mg L-1 and average total P was 0.90mg L-1. Average removal rates were 0.37 g m(-2) d(-1) for total N and 0.050 g m(-2) d(-1) for total P (percentage removal rates of 40% and 48%, respectively). Soluble Zn concentration decreased from 0.041 in the influent to 0.023mg L-1 after passing through the wetland. The average Zn removal rate during the year was 0.0007 g m(-2) d(-1) ( percentage removal rate 37%).