Influential factors of nitrogen removal efficiency by the integrated vertical-flow and horizontal-flow constructed wetlands

Influential factors of ammonia nitrogen($NH_{4}^{+}$-N) and total nitrogen(TN) removal efficiency by the integrated vertical-horizontal flow wetlands mainly include hydraulic loading rate, dissolved oxygen, plant and season, etc. The research results showed that the removal efficiency of TN and $NH_{4}^{+}$-N by the constructed wetland under high, mid and low-loads hydraulic loading rate were 58.28%, 61.71%, 63.94%, and 71.71%, 59.74%, 68.37%, respectively. Removal efficiency of $NH_{4}^{+}$-N and TN under well-oxygenated conditions were obviously improved from 40%-50% to 80% and 30% to 70%, respectively. Despite the 3%-4% higher removal of $NH_{4}^{+}$-N and TN from the influent sewage in the system vegetated with plants, no significant differences in performance were observed between units with or without plants (P>0.05). $NH_{4}^{+}$-N removal efficiency in summer and autumn was significantly higher than spring and winter at different retention time (P<0.05). TN removal efficiency in the wetland system was also remarkably different in different seasons (P<0.05), which reached the highest value in summer(86.62%) and the lowest value in spring(about 30%-40%). The temperature and dissolved oxygen were the most important limiting factors for $NH_{4}^{+}$-N and TN removal in the integrated vertical-horizontal flow constructed wetland.