| For the past one hundred years,in order to tackle the problems of hygiene,health and environment from sewerage in urban city,Municipal Wastewater Treatment Plants(WWTPs)have been under significant development.Traditional wastewater treatment process,however,requires considerable energy and chemicals consumption,and generates greenhouse gas(GHG)emissions including CH4,N2 O and CO2.In developed countries,energy consumption from WWTPs accounts for around 3% of total energy consumption,and GHG emissions contribute to 1-2% of total emissions.Therefore,in some developed countries,wastewater treatment sector has been covered in the national carbon emissions reduction program.From the perspective of sustainable wastewater treatment,this study focuses on accounting the GHG emissions of different treamtent process by utilizing the framework of Life Cycle Assessment(LCA).The system boundary starts with the wastewater flowing into the WWTPs and ends with treated water entering into water body and sludge going into final disposal.The function unit was defined as treating one tonne of wastewater daily.Meanwhile,in order to analyse the charateristics of various wastewater treatment processes and the emissions trend,four scenarios were set for the study: 1)conventional wastewater treatment processes,2)wastewater energy recovery,3)water reclaimation from wastewater,and 4)phosphorous recovery from wastewater.Besides,remote sensing technology was applied for semi-quantitative study on WWTPs' heat island effect.Under the conventional wastewater treatment scenario,inventory data of energy consumption,chemicals consumption and GHG emissions were accounted for five conventional wastewater treament processes schemes(AAO,AO,Oxygen Ditch,SBR,and CASS)with five treatment volumes,three discharge standards(Class I B,Class I A,quasi-IV Class surface water),and two sludge disposal processes(landfill and composting).Then energy consumption and chemicals consumption were converted into GHG emissions,and finally sixty wastewater treatment process schemes were assessed.Results showed that electricity-related GHG emissions account for 30-50% of total GHG emisisons,and direct GHG emissions(CO2 and N2O)from biological treatment contribute to 20-45%,and emissions from sludge disposal could reach up to 30%.With characteristic analysis of different treatment process schemes,it concluded that different opportunities of emissions reduction for different treatment process,e.g.lowering electricity consumption for AAO,AO and Oxygen Ditch,and controlling N2 O generation for SBR and CASS.To further study the characteristics of GHG emsisions of energy and resources recovery from wastewater,sludge anaerobic digestion with biogas recovery,water reclaimed for industrial cooling system,struvite recovery from digested sludge were selected for three wastewater wastewater energy and resources recovery scenarios,with comparisions with GHG emisisons from conventional wastewater treatment scenario.Overall,energy and resources recovery is favorable for GHG emisisons reduction.Among the three recovery technologies,water reclaimation yields the highest emisison reductions(18-31%),and struvite recovery and sludge anaerobic digestion with biogas recovery generates the lower emission reductions(7-18%).In the study of WWTPs' heat island effect,it was found that WWTPs had a limited heat island effect,with the results of WHII were significantly lower than UHII.When WWTPs were under construction,heat island effect would apprea due to the change of land use type and thermal properties of surface,whereas,when under operation,heat island effect would be significantly reduced due to large area of wastewater surface.Besides,based on the statistical analysis it was found that wastewater treatment process and discharge standard have a significant correlation with heat island intensity of WWTPs. |
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