Nitrogen Removal by a Combined Subsurface Vertical Down-Flow and Up-Flow Constructed Wetland System

A subsurface vertical up-flow followed by down-flow constructed wetland system was used to treat anaerobic digester effluent from pig-farm wastewater. The system consists of two beds connected in series and have equal size of 1.2 x 1.2 x 0.6 m (W x L x D). The first bed was planted with Typha angustifolia and the second bed was planted with Cyperus alternifolius L. The wastewater with NH3-N and chemical oxygen demand (COD) concentration of approximately 300 mg 1(-1) was fed to the first bed which performed as a vertical up-flow bed and then passed to the second vertical down-flow bed and recirculated to the first bed with the recycle ratio of 1: 1 to enhance nitrogen removal. The system was operated with the hydraulic loading rate (HLR) of 10, 20, 30 and 40cm day(-1) including recycled water to evaluate the capability of the system under high HLR. The results showed that the removal efficiencies of N and COD decreased when the system operated with higher HLR. The removal efficiency for NH3-N and COD was maximum at 10cm day(-1) (79 and 74% respectively). Nitrification was significant in vertical down-flow bed, denitrification in the up-flow bed at every HLR. The two-stage constructed wetland removed 96-98% of incoming total suspended solids (TSS) and the removal was not affected by high HLR. The removal efficiency of total phosphorus (TP) was low (24-27%) at all HLR.