Study On Calculating Carbon Footprint And Assessing The Total Environmental Impact Of Wastewater Treatment

Generally,current wastewater treatment paradigm is a process of destroying energy by energy and resultant pollution transfer.It means that the protection of aquatic environment is achieved at the cost of atmospheric pollution and land depletion.Although most of the organic matters(COD)in wastewater is biogenic is not included in carbon emissions,it is objectively associated with direct or indirect energy consumption processes,which inevitably makes wastewater treatment plants(WWTPs)a potential large carbon emission source.Therefore,it is of great significance to carry out carbon emission accounting for WWTPs and guide the emission reduction pathway.The present study was to evaluate the entire environmental impact of a typical WWTP in China with life cycle assessment(LCA)tool to reveal the connotation of environmental impact of sewage treatment,propose impact improvement measures,and conduct LCA quantitative evaluation.The main work of this present study include: 1)there is a certain amount of fossil carbon accounting for the COD in wastewater,which needs to be analyzed and quantified in the traditional steady-state carbon emission model;2)localization of relevant factors involved in existing LCA methods;3)a multi objective programming(MOP)approach was introduced into the LCA model to provide a multifactor optimization strategy;4)using the LCA-MOP model to evaluate carbon footprint and total environmental impact,including discharge standard of “Class I-A” and “Quasi-IV” of environmental quality standards for surface water of a conventional wastewater treatment plant(WWTP),a energy/resource recovery WWTP,and an underground WWTP.Based on the LCA model after supplement,modification,and upgrade,the environmental impact of two full-scale wastewater treatment processes was evaluated and compared.The LCA-MOP evaluation showed that the conventional wastewater treatment processes had the problem of “pollution transfer”.Although it helped improve the water environment,the overall environmental benefit was negative.From “Class I-A” to “Quasi-IV”effluent standards,the total environmental impact of the case WWTP increased;the total impact index values of the two processes increased from 9.85E-03 and 1.08E-02 to 1.25E-02 and 1.40E-02,with an increase of 26.9% and 32.0% respectively.Pollution transfer is an environmental problem commonly faced by traditional WWTPs.In order to deal with this problem,wastewater treatment should consider resource/energy recovery to offset the negative impact on the overall environment.Resources/energy recovery of wastewater treatment includes not only conventional water reuse,but also phosphorus recovery,and organic/thermal energy.The LCA-MOP evaluation of the proposed resource recovery process showed that water reuse,phosphorus recovery,and organic energy use were still difficult to achieve a “Net-zero” on the total environmental impact.It was only possible to achieve a “Net-zero” total environmental impact by including thermal energy recovery and utilization.The evaluation of the aboveground and underground WWTPs using the MBR process by the established LCA-MOP model showed that the total environmental impact of the underground WWTP was higher,with an increase of 1.3%.The results indicated that underground WWTPs were not beneficial to the overall environment.In the construction of WWTPs in the future,we must not only consider the benefits of human beings,but also the impact on the total ecological environment.