Woodchips as sustained-release carbon source to enhance the nitrogen transformation of low C/N wastewater in a baffle subsurface flow constructed wetland

Abstract

The lack of carbon source in the denitrification process with low C/N wastewater is the main factor limiting the denitrification efficiency in constructed wetlands (CWs). However, the traditional method of adding liquid carbon source is often costly and facing the risk of secondary pollution. This study attempted to solve this problem by adding woodchips sustained-release carbon sources as part of the substrate, together with the dewatered alum sludge as the main wetland substrate to the baffle subsurface flow constructed wetland (BSFCW). The effects of woodchips on denitrification capacity, effluent COD and nitrogen transfer and transformation were studied. Results showed that the removal rates of ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), total nitrogen (TN) and total phosphorus (TP) were 19.0%–75.3%, 63.6%–96.1%, 61.94%–74.4%, 75.0%–98.8%, respectively, when the C/N ratio varies between 0.93 and 1.87. The two-dimensional contour simulation of pollutants concentration in the system indicated that: i) the release rate of COD by woodchips decreased with the time; ii) woodchips reduced the oxidation reduction potential (ORP) of CW system, creating favorable conditions for the denitrification; iii) the closer the woodchips filling position to the inlet, the higher the denitrification efficiency of the system. The combination of dewatered alum sludge and woodchips substrates realized the simultaneous denitrification and dephosphorization of the BSFCW system. This study is of great significance for the treatment of low C/N wastewater containing considerable even high strength P.