Estimation of N2O emissions from wastewater characteristics in constructed wetlands

Conventional wastewater treatment plants (WWTPs) contribute significantly to the nitrous oxide (N2O) emis-sions. On the other hand, the contribution of constructed wetlands (CWs) to N2O emissions is not clear. In this study, a literature review was initially conducted to study the factors that are correlated with N2O emissions from these systems. Afterwards, a statistical formula was developed using the metaregression method for estimating N2O emission factors (EFs) from CWs. This formula related EFs with influent wastewater characteristics (chemical oxygen demand-COD, total nitrogen-TN, COD/TN) and was applied to 56 small Greek settlements for predicting N2O emissions in an alternate scenario where CWs technology would be applied instead of conven-tional WWTPs. The estimated N2O EFs for the alternate scenario at these settlements ranged between 0.21% and 3.20% (as N2O/TN influent load), while the average value was equal to 0.94%. The specific N2O emission was estimated to 60.82 mg N2O per day and person equivalent. The application of different scenarios with waste-water of different strength showed that the lowest N2O EF in CWs is expected for wastewater with high initial COD and TN concentrations. To compare N2O emissions from CWs and conventional WWTPs, the Ordinary Least Squares multiple regression method was also applied to develop a general statistical equation that relates N2O EFs from conventional WWTPs with wastewater characteristics. According to the results, the estimated N2O emissions from the Greek settlements for conventional wastewater treatment are larger for 77% of the studied cases compared to the emissions found if CW technology had been applied instead.