Study On Characteristics And Mitigation Strategies Of Greenhouse Gases Emissions From Typical Domestic Wastewater Treatment Processes

Wastewater treatment plant is an important source of greenhouse gas(GHG)emissions.During the wastewater treatment,GHGs composed of the direct emission of fossil-derived CO2,CH4,and N2O and indirect emission due to the consumption of electricity and chemicals.Present studies of total GHG emission inventories and the key sites from wastewater treatment industry are incomplete,The study of different sources of GHG is relatively simple;non-fossil-derived CO2 emission is excluded from the direct emission of GHG inventory,but the fossil-derived CO2 has not been made a quantitative assessment by both Intergovernmental Panel on Climate Change(IPCC)and present studies of our country,which leads an unclear total amount of GHG inventory.In addition,there is a lack of knowledge regarding the mitigation GHG effectively by regulating the operational processes and parameters.This paper gives out GHG emission characteristics of typical domestic wastewater treatment processes of China,reveals the distribution characteristics of fossil-derived carbon and CO2 emissions,and provides technical solutions and recommendations for GHG assessment and mitigation from the aspects of processes selection,operational parameters and policy strategies.In this study,GHG emission characteristics were firstly investigated in both full-scale anoxic/oxic(A/O)and SBR wastewater treatment plants.According to GHG emission(Carbon dioxide equivalent,CO2-eq),the total GHG emission of A/O is 404.93 gCO2-eq/m3 wastewater,which is lower than that of 864.98 gCO2-eq m3 wastewater from SBR,including the direct GHG of 199.12 gCO2-eq m3 wastewater,and 534.93 gCO2-eq m3 wastewater from the A/O and SBR,respectively.SBR has a larger indirect GHG emission than A/O with small range of variation,mainly sourced from energy consumption of the aeration units.In addition,the dominant source of direct GHG emission occurred during the aerobic unit,and was caused by aeration stripping as well as N2O emission(considered as CO2-eq)from nitrification.N2O emitted from aeration units is an important source of total direct GHG emissions from both A/O and SBR processes,accounting for 43.5%and 55.6%of the total GHG emissions,respectively.The amount and fate of fossil-derived carbon and CO2 were detected from four typical wastewater treatment plants(A/O,anoxic/anaerobic/oxic(A2/O),SBR and oxidation ditch),by using the isotopes analyses in the aqueous phase,gaseous phase and waste activated sludge(WAS).Meanwhile,carbon balance were investigated.The results indicated that 26.6-30.8%of total organic carbon(TOC)in the influent and 20.6-34.4%in the effluent were of fossil-derived in four WWTPs.Less content of fossil-derived carbon was detected in WAS phase,which accounted for only 9.0-12.2%of total carbon.As a result,the fossil-derived carbon was more likely to be converted into gaseous CO2,in which 29.59-51.80%of fossil-derived carbon contributed to CO2 emission.Fossil-derived CO2 contribute 24.3-29.0%and 11.1?13.8%of total direct GHG emission in A/O and SBR,respectively.N2O from aeration phase is the main control point in WWTPs.For a better knowledge of the impact factors and mitigation strategies of GHG emission from wastewater treatment plants,this paper studied the influencing factors of the N2O in aeration section.A pilot A2/O equipped with an aeration control system at aerobic tank is used to explore the shifts of N2O emission of various DO.Full-scale study indicated that the N2O emission factor was 0.74%of N-influent.The minimum N2O emission factor was 0.30%of N-influent while DO at the first 1/3 of aeration tank was 1.88mg/L,with the DO in a range of 1.25-2.11 mg/L(aeration rate 3.0-5.0 m3air/m3water/h).it revealed that a proper regulation of DO would greatly mitigate N2O emission.However,excess air could not improve the nutrients removal rates but caused DO inhibition during denitrification,resulting in an increase of N2O production as well as a consequential increase of total CO2-eq.On the other hand,Partial nitrification(PN)conducted by ammonia oxidizing bacteria(AOB)dominated system is a highly efficient ammonia oxidation process.Temperature has a known effect on AOB activities,reducing its ammonia oxidizing rate(AOR)when temperature is lowered and it could further affect the N2O emission rate.In this study,N2o eMission rate was investigated in an AOB enriched PN sequencing batch reactor(PN-SBR)which was operated with a step-wise feeds under 5 different temperatures(30-25-20-15-10?).An exponential decreased of AOR was detected when decreasing the temperature.N2O emissions were also affected by the temperature but only the ones produced during the first aeration of the cycle,when AOB shifted from a period of low activity to a period of high activity.In addition,the nitrite reductase and nitric oxide reductase have high activity while operating at 25?,which might promote nitrifier denitrificaiton to produce more N2O emission.Promoting GHG supervision and management,studding GHG inventories systematically,optimizing the GHG models and operating parameters,improving the efficiency of aeration equipments are the key ways to achieve GHG control and mitigation in wastewater treatment plants.