The Comparison Study On Environmental Sustainability Between A CASS And An Orbal Oxidation Ditch Process In Town Wastewater Treatment Based On Emergy Theory

With the rapid growth of population and economy,the environmental loading on water body is growing fast.In 2015,more than 35.543 billion tons of water was drawn from the centralized drinking water source water places in 338 countries in China,which served population of 332 million.Among them,the surface drinking water sources places had 557,of which 92.6%can satisfy the related standard,and the main pollution was derived from total phosphorus,DO and BOD5;the underground drinking water source places had 358,of which 86.6%can meet the related standard,and the main pollution came from Mn,Fe and NH3-N.In order to preserve the water environment,a lot of wastewater treatment plants have been constructed and in operation in China in recent years.The number of urban sewage treatment plants in China has ascended from 1521 in 2008 to 4436 in 2014,with the annual growth rate of 19.5%.However,sewage treatment processes have extra adverse impact on resources and the environment due to a lot of resources and energy consumption during the course of their construction and operation.Therefore,the active contribution and the adverse impact should be concerned simultaneously so as to promote the sustainable development of the wastewater treatment industry.This research carried out a comparison study of the sustainability of two town sewage treatment systems(sewage treatment plant A:Orbaloxidationditch process;sewage treatment B:CASS)located in Chengdu,Sichuan Province,China,using emergy approach.Based on emissions' impact,the targeted emergy indicator system was found for sewage treatment systems.Meanwhile,the influences of lifetime and treatment capacity were evaluated using scenario analysis.The main conclusions are summarized as follows.(1)According to the designed lifetime and treatment capacity,the proportions of renewable resources and nonrenewable resources of sewage treatment plant A are 37.91%and 62.09%respectively;the proportions of renewable resources and nonrenewable resources of sewage treatment plant B are 65.65%and 34.35%respectively;(2)Based on the designed lifetime and treatment capacity,for the two sewage treatment plants,SO2 and NH3-N are the main contributor to ecosystem services;NOx takes the main responsibility for emergy loss;ecological services contribute to more than 96%to the emissions' impact;(3)According to the designed lifetime and treatment capacity,these indicator values of CUPE,RPO,R/N and SI for sewage treatment plant B are larger than those for the sewage treatment plant A,i.e.the sewage treatment plant B has lower treatment cost,higher positive output efficiency,more reasonable resource mix and high comprehensive performance than the sewage treatment plant A;(4)The total emergy input obviously ascends with the extension of the lifetime.And the treatment capacity has different influence on the total emergy input:it has no obvious effect on the total emergy input when the design treatment capacity is very small;it has obviously effect on the total emergy input when the design treatment capacity is larger,i.e.the total emergy input takes on a downtrend with the decrease of the treatment capacity due to reduced purchased nonrenewable inputs;(5)The lifetime doesn't affect emergy loss obviously;the emergy loss takes on downtrend with decrease of the treatment capacity;(6)With the extension of the lifetime,these indicator values of RPO,SI,R/N and ELP increase,while those indicator values of CUPE and ELR decrease;with increase of treatment capacity,these indicator values of RPO,ELR,SI and ELP are enhanced while those indicator values of CUPE and R/N are reduced;(7)Under the condition of the different lifetimes and treatment capacities,these indicator values of R/N,ELP,RPO,and SI of sewage treatment plant B are higher than those of sewage treatment plant A,these indicator values of CUPE and ELR of wastewater treatment plant B are smaller than those of the sewage treatment plant A,i.e.sewage treatment plant A has more renewable resource mix,higher share of external diseconomies,low treatment cost,better positive output efficiency,lower environmental loading and comprehensive performance;(8)With increase of the treatment capacity and extension of the life cycle,the positive output efficiency and the sustainability level also show rising trends.Comparatively speaking,the sustainability level of sewage treatment plant B is larger than sewage treatment plant A,which means that sewage treatment plant B has better comprehensive performance than sewage treatment plant A.