Research On The Greenhouse Gas Emissions From Full-scale Municipal Wastewater Treatment Plants

With the accelerating of global warming, the reduction of greenhouse gas (GHG) emissions has aroused international attention. Nitrous oxide (N2O) and methane (CH4) are two important greenhouse gases, and their global warming potentials (GWPs) are 298 and 23 times higher than CO2, respectively. It had been reported that biological wastewater treatment process is an important anthropogenic source of N2O and CH4. Therefore, it is very important to study the production, emission and reduction control of N2O and CH4 during biological wastewater treatment processes.This study selected typical municipal wastewater treatment plants (WWTPs) where different biological wastewater treatment processes are used. By long-term systematically field sampling, on-site monitoring in these WWTPs combined with subsequent laboratory analyses, the GHGs production and emission status and variation trends in the entire wastewater/sludge treatment processes in these WWTPs were investigated. The GHGs emission ranges and emission factors (EFs) were calculated based on the experimental data. By comparison between the GHGs emissions of different WWTPs using different wastewater treatment processes, the effect of biological treatment process on GHG emissions were investigated to determine the lowest GHG emission wastewater treatment process. The factors which can significantly influence GHG emissions in WWTPs were identified by both in-situ measurement in WWTPs and laboratory analysis. A simple mathematical model was established to estimate the CH4 emissions and tested by actual experimental data. The main research conclusions are as follows:(1) The N2O emissions from 3 WWTPs using typical A/A/O process were measured systematically in long-term experimental period and the No.1 WWTP in Jinan city was selected for N2O emissions analysis as a typical A/A/O WWTP. Results showed the main N2O emission sources are the oxic tanks, which account for more than 80% of total emissions. The per capita EFs and flow based EFs are 1.69～2.34gperson-1yr-1 and 2.32×10-5～3.21×10-5gL-1. The dissolved oxygen (DO) concentration and nitrite concentration in the oxic tanks are the dominant factors influencing N2O emissions. The lower water temperature range in winter could promote N2O emissions in these WWTPs. Total removed nitrogen in influent wastewater is an important factor influencing the proportion of emitted N2O in the T-N removed from this WWTP, the proportion of N2O in the removed T-N was 0.119～0.195% taking these 3 A/A/O WWTPs into account.(2) The CH4 emissions from 3 WWTPs using typical A/A/O process were measured systematically in long-term experimental period and the No.1 WWTP in Jinan city was selected for CH4 emission analysis as a typical A/A/O WWTP. Results showed the main CH4 emission sources are the anaerobic tanks and oxic tanks, which account for 40% and 32% of total emissions. The per capita EFs and flow based EFs are 8.17～17.66gperson-1 and 1.12×10-4～2.42×10-4gL-1. The DO concentration in oxic tanks and aerated grit tanks and the water temperature in high density settler tanks are the dominant factors influencing CH4 emissions. The higher water temperature range in summer could promote CH4 emissions in these WWTPs. A simple mathematical model was established to estimate the CH4 emissions in WWTPs and the test for model accuracy by actual experimental data showed this model can better reflect the CH4 emissions from these WWTPs.(3) Based on the research of GHG emissions from 3 A/A/O WWTPs, long-term comparative studies were conducted on the GHG emissions from three other WWTPs where anaerobic oxidation ditch, preanoxic A/A/O and reversed A/A/O processes were used respectively. The GHG emission fluxes and sources were determined, the emission variation trends were investigated, and the EFs were figured out. Correlation analyses between N2O and CH4 emissions and water quality parameters were performed to determine the dominant factors influencing GHG emissions from different WWTPs using different biological wastewater treatment processes. Results showed nitrite concentration and oxidation-reduction potential were the factors significantly influencing GHG emissions from these 3 WWTPs. By comparison between the GHG emissions of these WWTPs using 4 different treatment processes, the effect of biological treatment process on GHG emissions were researched, and the lowest GHG emission wastewater treatment process was determined to be the reversed A/A/O process, which has both the lowest N2O/(removed T-N) conversion ratio and CH4/(removed COD) conversion ratio among these 4 studied treatment processes(anaerobic oxidation ditch, conventional A/A/O, preanoxic A/A/O and reversed A/A/O processes).