The Seasonal Dynamics of Removal of Nitrogen in Free Water Surface Constructed Wetlands in cold climate

Abstract: In Sweden and other Nordic countries, Free Water Surface (FWS) constructed wetlands are widely used for advanced sewage treatment. This study was performed in an experimental wetlands system in order to research the seasonal dynamics and interferences in N removal under cold climate condition. According to the result of study, as expected, N removal in FWS wetlands is strong related to temperature, the removal rates are higher in late summer and autumn than in spring and early summer. Removal at similar temperatures are quite different when they are in different months. For instance, in the study, the average N removal in June is significant different from N removal in September (P＜0.001), indicating that there are other factors affecting the N removal, different treatment wetlands have different situations. N removal in two different periods (March to July & August to December) were extracted for covariance analysis, it indicated that they are significant different. Besides that, the r2-value of correlation test showed that total N removal rate in FWS constructed wetlands is higher in relation to temperature in autumn and winter (r2-value is 0.4449) than in spring and summer (r2-value is 0.3857). Generally speaking, the dynamics of N removal in FWS wetlands is not steady and variable even at similar temperatures. Finally, I find the temperature of excess heating from district heating which has been used by the residents’ house in Sweden is high enough to heat up the temperature in FWS wetlands. That is a valuable improvement to be put forward.