Controlling Nitrogen Removal Processes in Improved Vertical Flow Constructed Wetland with Hydroponic Materials: Effect of Influent COD/N Ratios

Discharge of nitrogen (N) with wastewater causes eutrophication in surface water. On the other hand, nutrient-rich wastewater can be valuable for agriculture. Tailoring N removal or conservation is important to meet the requirements of different water end uses. Improved vertical flow constructed wetlands with hydroponic materials (CWH) as substrata were developed at lab scale in a greenhouse and studied to optimize N removal in CWH. This study investigated the effect of influent COD/N ratios of 5/1 and 15/1 on the removal or conservation of N in CWHs with Syngonium as vegetation and three substrata, pumice, cocopeat, and mineral wool. CWH with pumice showed the highest TN removal at both COD/N ratios. The Syngonium plant significantly contributed to the additional 50% TN removal in CWH. Nitrification of above 90% was observed at both studied COD/N ratios, indicating sufficient oxygenation due to the vertical pulse flow operated CWH. The denitrification process was enhanced at a higher COD/N ratio of 15/1 compared to 5/1 by around 10–40%. The occurring nitrification and denitrification indicate the coexistence of aerobic and anaerobic conditions in CWH, and balancing these conditions is necessary for future applications to remove N for its specific end use, i.e., irrigation water (high standards) or discharge to surface water (low standards).