Effects of plants and temperature on nitrogen removal and microbiology in pilot-scale horizontal subsurface flow constructed wetlands treating domestic wastewater

Two pilot-scale intermittently operated horizontal subsurface-flow constructed wetlands (HSSFCWs), one planted with Acorus calamus L. and one with Phragmites australis (Cav.) Trin. ex Steud., were implemented for the polishing treatment of domestic wastewater from Huazhong Agricultural University dormitories, Hubei Province, China. The characteristics of nitrogen removal between the plant rhizosphere and non-rhizosphere were assessed from 19 September to 12 December 2012 to address the effect of plants. The mean removal efficiency of total nitrogen (TN) was 45.2% for the two HSSFCWs with a hydraulic loading rate of 0.15 m3 m−2 d−1. In both pilot-scale HSSFCWs, the nitrification intensity and numbers of nitrite-oxidizing bacteria and ammonia-oxidizing bacteria in the rhizosphere were significantly higher than in the non-rhizosphere, associated with oxygen release from plant roots. In contrast, the denitrification intensity and number of denitrifying bacteria were higher in the non-rhizosphere. Although significantly higher dissolved oxygen concentration at the root surface and greater numbers of nitrogen-processing bacteria in the rhizosphere occurred for Acorus calamus compared to Phragmites australis, very similar nitrogen removal efficiencies were observed in the two HSSFCWs, probably due to the relatively low N concentration of the wastewater treated in the wetlands. In addition, the removal efficiencies of NH4+-N and NO3−-N were significantly positively correlated with water temperature in both HSSFCWs.