Performance evaluation of hybrid treatment wetland for six years of operation in cold climate.

In Hokkaido, northern Japan, there are 12 hybrid subsurface constructed wetlands (HSCWs) and most of them are treating high concentrated organic wastewater. One of these systems is an HSCW situated in Embetsu, northern Hokkaido, and it has been in operation since November of 2006 to treat dairy milking parlor wastewater. The system is composed of two vertical flow beds and a horizontal flow bed. The influent and the effluent flow rates and pollutant concentrations and loads were extremely variable. Throughout its 6 years of operation, most of the pollutant removals were decently high. Removal efficiencies for COD, BOD5, and SS were ranging in the 90 %. Removal efficiencies for TN, NH4-N, and BOD5 were improving because of the development of the soil ecosystem and the Phragmites australis community. However, the removal efficiencies of TP were decreasing, presumably because of the declining adsorption ability. The accumulation of TP in the first and the second vertical beds had reached its plateau. Vertical beds had high removal efficiencies for TN, COD, BOD5, and SS. These high removal efficiencies of the first vertical bed may be caused from the efficient removal of solid material that is deposited as an organic layer of the first vertical bed. High NH4-N removal efficiencies exerted by the second vertical bed may be due to the recycling of wastewater. In conclusion, the HSCW was working excellently for its 6 years of operation, and it could be concluded that it has not reached its life yet.