Study On Estimate Of Greenhouse Gas Emission Reduction By Constructed Wetlands

Combating climate change has become one of major issues and challenges for international communities. Both bottom-up and top-down studies indicate that there is high agreement and much evidence of substantial economic potential for the mitigation of global GHG emissions over the coming decades that could offset the projected growth of global emissions or reduce emissions below current levels. Global GHG emission in wastewater sector is approximately 640 million ton CO2 equivalent at year 2005 level. China takes the first place and emits 21 percent of global total emission in this sector.In the peri-urban and rural area where no sewers exist, it is a cost effective strategy to develop decentralized systems instead of centralized one which demands expensive sewer network and intensive energy consumption. Constructed wetland is one of the appropriate technologies in such area. The question is:whether Constructed wetlands mitigate not only water pollutions but also GHG emissions?The emission mechanism in the constructed wetlands is studied. Based on the integration of wastewater treatment plant and wetland ecosystem, a Carbon Mass Balance Model (CMBM) and a method for emission estimation are thus developed. A potential emission factor (K) is used to illustrate the CH4 emision potential in ratio to the total carbon load from both wastewater and plant photosynthesis sources. The K value is estimated from 0 to 0.36 varied upon treatment efficiency and Redox potential indicated by Eh. An emission curve is thus developed to enable rapid estimation.Using methodology of 2006 IPCC GHG inventory guidelines, our study estimates the baseline from wastewater treatment and disposal emissions in Changzhou municipality. A comparison between wastewater treatment plant (WWTP) and a subsurface vertical flow (SSVF) constructed wetland has been made, using LCA technique. The result shows the emissions are different throughout the whole life cycle process, i.e. collection, treatment and disposal. The studied WWTP model emits 116 kg CO2 equvalent a-1 pe-1 in which 37% from anaerobic sludge landfill,33% from anaerobic section in A2O process,28% from indirect emission by energy consumption and 2% from N2O emission, respectively. The studied constructed wetland model emits only 13% and 14.4%, respectively, comparing to WWTP model and rural emission baseline. Thus, SSVF constructed wetland is proved to be an effective measure for GHG emission reduction. Our study also estimates the significance of sludge and rural wastewater emissions. Even all the wastewater are collected and treated in WWTP, the CH4 emission will not be reduced unless the sludge is properly handled and CH4 is sequestrated.A cost benefit analysis has been made for construction and operation of the constructed wetlands. The comparison of three scenarios in Changzhou shows it is both economically and environmentally sustainable to reach the target of 70% rural wastewater being treated by 2030. Scaling up to nation wide, by improved sludge handling and rural wastewater treatment, GHG reduction potential by year 2030 is estimated as 55.6 million ton CO2 equavelent which is approximately 8.7% of global baseline level at year 2005. This will be a significant contribution for China to the mitigation of climate change.