A review on the main affecting factors of greenhouse gases emission in constructed wetlands

Constructed wetlands (CWs) are natural-like systems for wastewater treatment capable to remove both pollutants and nutrients without additional energy demand. In these systems, gaseous compounds are released into the atmosphere through microbial processes. Among these gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are the most dangerous because they act as greenhouse gases (GHGs) and are well known as contributory factors to cause global warming. In this paper we reviewed 224 scientific articles (from 1980 to 2016) from the literature in order to analyze the most important factors that drive the quantity and type of GHGs production and emission from different CWs systems. Wastewater flow and composition, hydroperiod, environmental conditions and plant presence and species used to vegetate CWs have been considered. CWs typologies influence GHGs fluxes with lower CH4 emissions from subsurface flow CWs than free water surface (FWS) ones and higher N2O emissions from vertical subsurface flow (VSSF) CWs than FWS ones. The inlet wastewater COD/N ratio of 5:1 has been found as the best ratio to obtain in the same time the lowest N2O emission and the highest nitrogen removal in FWS CWs. The inlet wastewater C/N ratio of 5:1 allows to obtain the lowest CO2 and CH4 emissions in VSSF CW treatment. Intermittent CWs bed wastewater loading decreases CH4 and promote CO2 and N2O emissions. Temperature is positively correlated with CO2, CH4 and N2O emissions and solar radiation with CO2 and CH4 emissions. GHGs flux is affected by plant presence and species, and it is influenced both by the phenology and density of vegetation. Plant presence significantly increases the CO2 emission respect to unvegetated condition in all CWs types, and increases N2O and CH4 emissions in VSSF CWs. Considering the HSSF CWs plant presence significantly reduce the CH4 emissions. Plant species richness effect on CH4 emission has been investigates in a limited number of papers with not unique results, probably due to the different plant species and number used by authors, which may have influenced the CWs microbial population and activity. Considering plant species Zizania latifolia determine significant higher CH4 and N2O emissions than Phragmites australis. No significant different CH4 and N2O emissions have been found between P. australis and Typha latifolia. Significant lower N2O emissions determine the T. angustifolia than P. australis. Although plant presence, in some case, increases CW GHG emissions respect to unvegetated situation, the vegetation fixes atmospheric carbon by photosynthesis; as a consequence CWs act, in most cases, as sink of CO2(eq).