Performance comparison of sewage treatment plants before and after their upgradation using emergy evaluation combined with economic analysis: A case from Southwest China

Abstract

Upgradation of sewage treatment plants has been greatly promoted in China, but few studies investigate impact of this practice on synergetic relationship between environmental performance and economic performance of those systems. This study aims to investigate environmental sustainability, economic benefit and greenhouse gasses emission intensity of different sewage treatment schemes and then identify the best one as well as related negative factors needed to be addressed in the future. A comprehensive performance index-Synergetic effect index (SEI), derived from integration of Improved emergy sustainability index (IESI), Net economic benefit ratio (NEBR) and Carbon emission per unit sewage treatment (CEUS), is proposed to address this issue. Firstly, to investigate environmental sustainability of sewage treatment more completely, this approach improved the classic emergy analysis through integrating emissions’ impacts into related classic emergy yield ratio and environmental loading ratio respectively and then built the comprehensive index IESI. Next the indicator NEBR and CEUS were proposed to assess economic benefit and greenhouse gasses emission intensity, respectively. Finally, the three indicators, i.e. IESI and NEBR as well as CEUS, are merged into the index SEI to evaluate comprehensive performance of sewage treatment. A sewage treatment plant, located in Southwest China, as a research case, was investigated using the presented approach from its construction stage to its operation stage. It is found that, after the upgradation, total emergy input increases by 11.96% mainly from extra thermal electricity power consumption, total emission's’ impact decreases by 39.92% mainly from raised removal rate of total phosphorus, emergy yield rate is slightly improved and environmental load rate descends greatly, and thus the sustainability level clearly rises. Meanwhile, environmental emissions significantly affect the environmental load and then the sustainable level. This upgradation improves its economic benefit because rising sewage treatment fee slightly exceeds its added treatment cost. Thermal electricity power consumption in operation stage is the main contributor to total carbon emission. Sensitivity analyses shows that changes of main input categories have not altered the research results. Generally, the upgradation promotes the synergetic relationship between its environmental performance and economic performance by 95.65%. However, indirect environmental emissions from the upgradation should be emphasized. This enterprise should further improve energy performance, optimize the existing processes and promote its circular economy in the future.