Effect of spatiotemporal patterns of hydrophytes on N and P removal in a multistage ponds-wetlands treatment system

The effects of variations in hydrophytes on nitrogen and phosphorus removal were evaluated in different units of a com- bined pond-wetland wastewater treatment facility, based on the heterogeneity of spatiotemporal patterns of hydrophytes in these sys- tems. Variations in species and biomass of hydrophytes led to periodic variations in nitrogen and phosphorus removal in each unit. Hydrophytes affected the removal of NH3, NOx , total phosphorus and organic nitrogen through influencing rates of various mecha- - nisms, especially nitrification-denitrification, chemical precipitation and metabolism of algae. Different mechanisms involved in nitrogen and phosphate removal determined that different species of nitrogen and phosphate are mainly removed in different unit. Briefly, the removal of NH3 and total phosphorus mainly occurred in aeration-fish ponds (29.5%, 30.1%), fish ponds (16.9%, 17.8%) and hydrophyte ponds (24.5%, 19.4%); the removal of NOx mainly occurred in the constructed wetland (0.4 mg/L in final effluent); - and the removal of organic nitrogen mainly occurred in hybrid facultative ponds (32.3%) and fish ponds (28.1%). Additionally, con- structed wetland exhibits relatively low potential of removing NH3 and total phosphate, less than 8.5% and 11.5% respectively, ow- ing to its high capacity of filtrating algae and subsequent mineralization of algae biomass.