Our study analyzed factors limiting nitrogen removal in wetlands constructed for greywater. The treatment plant examined for this study was a two-stage constructed wetland combining a vertical subsurface flow wetland for aerobic treatment and a horizontal subsurface flow wetland for anaerobic treatment. Because of nitrification and denitriflcation effects, the nitrogen removal ratio was about 60% during the three years operation period. Assuming that BOD consumption in the first vertical wetland reflects organic matter decomposition (i.e., C-BOD) and the nitrification of TKN (Total Kjeldahl Nitrogen, i.e., N-BOD), we estimated that over one-third of the total oxygen demand was used for the TKN decomposition. On the other hand, in the second horizontal wetland, it was presumably impossible to obtain a sufficient BOD source which is a limiting factor of denitriflcation. Based on these facts and considerations, treatment operations were improved in August 2017. Widely spraying the entire surface during the first stage improved the nitrification ratio with increasing reaction time. Secondly, to promote denitriflcation, the wetlands were improved to guide wastewater forcibly to the bottom layer of the second stage and at the same time automatically supply the BOD source.
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