Nitrogen removal along the treatment cells of a free-water surface constructed wetland in subtropical Taiwan

The Daniaopi free-water surface constructed wetland (DNCW), which treats municipal wastewater and nitrogen removal, was compared between the warm and cool seasons in northern Taiwan from March 2009 to August 2010. The results showed that the nitrogen removal in terms of area removal rates (ARRs) were significantly positively regressed with the area loading rates (ALRs) for ammonia nitrogen (NH3-N), nitrite and nitrate nitrogen (NOX), particulate organic nitrogen (Org-N), and total nitrogen (TN). The ARR of NH3-N was 1.69 N g m−2 d−1 during the warm season and 1.51 N g m−2 d−1 during the cool season, whereas the concentration removal efficiency (CRE) was approximately 85% for both seasons. The ARR of TN was 1.77 N g m−2 d−1 during the warm season and 1.32 N g m−2 d−1 during the cool season. The CRE of TN of 67% during the warm season was higher than the CRE of TN of 46% during the cool season. The CRE of TN was lower than that of NH3-N, particularly during the cool season, due to the increase in NOX-N and Org-N in the treatment cells. However, the ARR of NH3-N and TN were not significantly different between the cool and warm seasons. Our results show that mass loading was a major factor in the nitrogen removal. Atmospheric gains (precipitation minus evapotranspiration) appear to cause an increase in water flow, thereby reducing the loading rate, which reduces the nitrogen removal rate.