Study On Characteristics And Reduction Strategies Of N₂O Emission From Typical Urban Wastewater Treatment Processes

As the increase of urban water usage and wastewater treatment efficiency, the emission of N2O from wastewater treatment plants (WWTPs) will get an increase gradually. By now, there is a great defect not only in the basic data but also in N2O reduction method on N2O emission from WWTPs in China. This research has given out the emission characteristics of N2O from urban WWTPs using typical treatment processes, studied the influencing factors on N2O emission from each process, constructed the predicting models of N2O emission from typical processes, forecasted the emission of N2O from all the urban WWTPs in the future10years in China, and proposed the reduction strategies of N2O emission from urban WWTPs, which is suitable for our national conditions. These works can offer a certain data reference for constituting the reduction policy and environmental management for N2O emission from WWTPs in China, and supply some technical support for the diplomatic negotiations and the implementation of international conventions.N2O emission from the non-aerated and aerated units and the dissolved N2O concentration in the typical treatment processes were monitored using the static chamber technique, the gas bag technique and the static headspace technique respectively. Results showed that, for the effect of insufficient O2supply during nitrification and O2-inhibition during denitrification, N2O could be produced in both the oxic and the anoxic treatment units (periods), where could be determined as the main control points for N2O production. Because the N2O dissolved in the oxic treatment units (periods) would be stripped out to the atmosphere through aeration in the four typical wastewater treatment processes, the emission of N2O from the A2/0-oxic tank, the A/O-oxic tank, the oxidation ditch tank and the SBR-feeding and aeration period accounted for99.14%of A2/O process,98.6%of A/O process,86.7%of the OD process and99.9%of the SBR process, which showed as the main control points for N2O emission. Pilot experiments were designed to study the influence of DO and the utilization efficiency of carbon source in influent during denitrification on N2O emission from A/O, OD and SBR processes through adjusting some critical operating parameters. Results showed that, a proper adjustment of the aeration rate in A/O process, a pre-denitrificaiton operating mode with a higher aeration rate in the OD and SBR processes could lead to a effective control of DO concentration during nitrification and denitrificaiton and enhacing the utilization of influent carbon during denitrification, which were favorable to reduce the production and emission of N2O from different processes.A back propogation (BP) artificial neural network basing on the optimization by genetic algorithm was established for the prediction of N2O emission from WWTPs and the emission of N2O from urban WWTPs in China in the future10years (from2014to2023) was predicted. The results showed that, the correlation coefficient (r) between the predicted values got by the GA-BP models and the monitored values of N2O emission from WWTPs were0.91,0.95,0.98and0.99, which indicated that the models constructed in our work could realize an accurate prediction of N2O emission from urban WWTPs in China. The forecasting results showed that, the total emission of N2O would increased from (1.32～4.67)×104ton in2014to that of (2.17～7.64)×104ton in2023as the continuous increase of China’s urban sewage treatment.According to the research results of N2O emission characteristics and influencing factors in typical urban wastewater treatment processes, technical strategies suitable for our national conditions were proposed to reduce N2O emission from both national and urban WWTPs’aspects. The national technical strategy controlling N2O emission included selecting the OD process as the optimal treatment process and recommending some newer technologies for wastewater treatment, et al. The WWTPs’included raising the DO concentration during nitrification properly and enhancing the utilization efficiency of carbon source in influent during denitrification, et al.