Vertical oxygen distribution trend and oxygen source analysis for vertical-flow constructed wetlands treating domestic wastewater

To improve nitrogen removal performance in vertical-flow constructed wetlands (VFCWs) and promote their application in domestic wastewater treatment in China, it is important to identify the reoxygenation sources, their contribution weightings and the oxygen distribution profiles in the VFCW substrates. This study indicated that dissolved oxygen (DO) levels rise at first and then drop in the VFCWs vertically from top to bottom in the VFCWs treating domestic wastewater. There were other sources of oxygen supply to the wastewater in the VFCWs, especially in the upper portion of the VFCWs. In treating domestic wastewater, atmospheric reoxygenation is the main oxygen source for the VFCWs, contributing more than 99.9% of the total oxygen supply. Approximately 50% of atmospheric reoxygenation is supplied to 0–10 cm below the wastewater distribution system in the VFCWs. Over 99.8% of the oxygen consumed is used for organics degradation and nitrification. In this study, oxygen was mainly consumed in organics degradation in the upper 0–40 cm of the VFCWs and nitrification was dominant in the lower part.