Nitrate removal from water by using Cyperus alternifolius plants in surface flow constructed wetlands

In this research, nine systems were constructed and the flow of urban water was continuously maintained from May to November 2018. Three of these systems were considered as an emergent, three were set as floating canals and three other systems were unplanted and porous media. The nitrate inlet was 20 mg/L and hydraulic retention times (HRTs) were 1, 3, and 5 d. The experimental design consisted of a factorial split-plot design. The analysis of variance showed that the efficiency of nitrate removal was affected by the type of constructed wetland, HRT, temperature changes, and the reciprocal effects between these parameters (P <= 0.01). In HRT of 1 d, the average efficiency of nitrate removal by the emergent system, floating canal system, and unplanted system were 14.34%, 12.09%, and 10.51%, respectively. With an HRT of 3 d, these average efficiencies became 17.62%, 15.76%, and 13.54%, respectively. With the HRT of 5 d, these percentages increased to 17.75%, 17.66%, and 16.08%, respectively. The comparison of mean values pertaining to the effect of temperature on nitrate removal showed significant differences between the efficiencies of nitrate removal in some months of the year (P <= 0.05). The reciprocal showed that the highest nitrate removal efficiency was 17.75% by the HRT of 5 d in the emergent system and in the month of June.