The Potential Of Mitigating Greenhouse Gas Emissions From Urban Domestic Water Systems

Water is an indispensable resource for modern economies and national security.The urban domestic water system（UDWS）,including the comprehensive management of urban domestic water supply and wastewater treatment,is responsible for providing residents with the water resources,and removing pollutants from wastewater.During the daily operation of UDWS,greenhouse gases（GHGs）are directly and indirectly generated.Socioeconomic development level has changed residents’living style,affecting the features of domestic water consumption and wastewater discharge,which in turn leads to uncertainty in the GHG emissions of the UDWS.Therefore,based on the analysis of the uncertainty features of GHG emissions in UDWS through engineering and non-engineering measures,it is of great significance to scientifically and reasonably optimize wastewater treatment technologies and energy structures in UDWS to achieve multiple goals such as GHG mitigation and socioeconomic development.This paper focused on the following aspects:First,based on the Life Cycle Assessment（LCA）method,the GHG emissions of the whole processes of the UDWS were analyzed.The UDWS mainly focused on the following stages with significant GHG emissions:water transportation,treatment,and distribution,as well as domestic wastewater treatment and sludge disposal;Second,based on Copula functions,the correlation between socioeconomic development and the amount of domestic water consumption or wastewater discharge was identified,and the GHG emission features of the UDWS were predicted by forecasting the domestic water consumption and wastewater discharge from 2020 to 2030.Finally,based on the violation risk of GHG mitigation goals and the Interval linear programming model,an uncertainty optimization model for the UDWS under the background of GHG emission mitigation was constructed.The objective of the model was to minimize the cost in wastewater treatment and electricity generation.The decision variable was the amount of wastewater treated through the multiple technologie.Constrains were established based on the goals for GHG mitigation and capacities of wastewater treatment plants,seeking the optimum strategies of technologies for wastewater treatment and energy mix for electricity generation,and providing decision-making support for GHG mitigation strategies in the UDWSs.To verify the applicability of the method,Shenzhen city in Guangdong Province was chosen to analyze the potential of the UDWS to mitigate GHG emissions.The innovation of this method lied in proposing a hybrid approach,incorporating correlation analysis and a programming model in a LCA framework,in order to support comprehensive assessment of GHG mitigation potential in the UDWSs.The results showed that the amount of GHG emissions from the UDWS in Shenzhen was747.82 kt CO₂ eq in 2020.The results of the correlation analysis indicated a strong correlation between the domestic water consumption and wastewater discharge with economic development in Shenzhen.Domestic water consumption would increase by 11.35%and 18.67%in 2025 and 2030,respectively,and wastewater discharge would increase by 12.88%and19.73%in 2025 and 2030,respectively.Compared with that in 2020,the domestic water consumption and wastewater discharge of Baoan District in 2025 and 2030 would vary to the smallest extent,with an average annual increase of 1.37%and 1.56%,respectively.In contrast,the annual average domestic water consumption and sewage discharge in Longhua District would increase by 37.59%and 34.85%respectively from 2020 to 2030.Two scenarios were proposed in this study.Infrastructure and energy mix for electricity generation would maintain the current status in future under scenario 1（S1）,while optimum strategies of technologies for wastewater treatment and energy mix for electricity generation were considered under scenario2（S2）.The results showed that of UDWS in Shenzhen under S1 would not achieve the GHG mitigation goals.Optimum strategies of technologies for wastewater treatment and energy mix for electricity generation were obtained under S2,with consideration of GHG mitigation goals in the future.As energy consumption contributed more than 25%of GHG emissions in the USDWs of Shenzhen,the contributions of new energy sources for electricity production（i.e.,offshore wind,solar photovoltaic,biomass,and nuclear）would increase by 0.58-1.02 times and 0.68-1.20 times from 2020-2025 and 2025-2030,respectively.The capacity of wastewater treatment would increase by 53.33%-80.00%and 27.44%-29.82%of the SBR and AAO technologies,respectively.Based on these results,the following conclusions of this study included:（1）a hybrid approach,incorporating correlation analysis and a programming model into a LCA framework,can obtain the desired strategies for GHG mitigation in the UDWSs;（2）the UDWS of Shenzhen is facing significant GHG mitigation violation risks in the future;and（3）upgrading projects for wastewater treatment and electricity generation have a significant effect in GHG mitigation in the UDWS.