Study On Production Emissions Caculation Of Exhaust Gas From Sewer System

Consisting of septic tanks, pipes, various inspection wells, pump stations and domestic sewage treatment plants, the urban sewer system is the fundamental municipal facility to provide important support for maintaining the city environment. As the system of collecting, transporting and treating domestic sewage, it serves as a huge biological reactor, from which the exhaust gases like CH4and H2S will be produced, threatening the work and lives of municipal workers and endangering city people’s lives and health. Monitoring and controlling these harmful and toxic exhaust gases have aroused great concern from the government and public. Researches on this topic began abroad quite earlier than domestic researches. Although many domestic studies on exhaust gases from sewage treatment plants have been carried out, concerns on exhaust gases from collecting and transporting sewage system are quite few, especially it’s still blank in the research on generation and emission coefficients. Centered on the generation and emission of exhaust gases from Kunming domestic sewage system, including septic tanks and various inspection wells, and joining the exhaust gases generation and emission coefficients of Guangzhou and Lanzhou, the studies carried out are as follows:(1) Based on the wide range of literature review, expert consultancy and the preliminary on-the-spot monitoring, the study determines the pollution factors of exhaust gases from sewage system. With researching the rules of generation and emission of exhaust gas from sewage system, we systematically analyzed the composition and affecting factors of exhaust gases.(2) By the laboratory simulation, the statistics of pollutant degradation and exhaust gases of varied functional zones in different situations are attained. Rules of their dynamics are studied by means of model analysis software.(3)Combining on-the-spot monitoring with the laboratory simulation, generation and emission coefficients based on the available statistic units (such as population, sewage flow, pipeline length or unit pollution load etc.) are attained, calculating method of exhaust gases quantity is worked out, and the major exhaust gases quantity is estimated.By on-the-spot monitoring and the laboratory simulation in Kunming urban sewage system, the generation and emission rules of exhaust gases are researched, the emissions system and emissions rules were researched. The quantity of exhaust gases generation and emission is calculated. The conclusions are come up with as follows:(1) The generation and emission of exhaust gases occur in different time, that is, the peak and off peak time of using water as well as weekdays and weekends in a week. During the peak time the generation and emission quantity is evidently more than that of off peak time. Rainfall influences the generation and emission from the combined flowing system but the split flow system is not affected. High turbulent flow of sewage of drop manholes in sewage system is advantageous to gas diffusion from liquid phase to gas phase, increasing the generation and emission quantity.(2) The generation and emission of exhaust gas from drainage system are mainly concentrated in the septic tank and various inspection wells, especially the generation and emission of septic tank account for the major proportion. Usually only H2S and CO2gas can be detected in inspection wells (sand sediment traps, water wells of clay and drop wells), and in septic tank H2S, CH4and CO2can be detected. Compared with inspection wells, septic tanks have higher anaerobic degree. CO2concentration in septic tanks is higher than that in inspection wells because the hydraulic retention time in the septic tank is generally12-24h. Long retention time benefits organic compounds degradation of sewage anaerobic reaction and increase the accumulation of exhaust gases. Due to the long-term accumulation, the bottom of septic tank will deposit sediment greatly, which enhances the production of CH4.(3)The characteristics of exhaust gases emission from various sewage areas are different, because the sewage flow and concentration is different. COD concentration of catering sewage is evidently higher than that of other three kinds of sewage. H2S and CO2emission rate of catering sewage is higher than the residential sewage.(4)By simulating experiments, main affecting factors of biochemical reaction in sewage system are attained, such as Hydraulic Retention Time (HRT), organic concentration, pH value and temperature. Major factor which influences exhaust gases discharging into the atmosphere is the degree of hydraulic turbulence. Exhaust gases emission from different sewage systems gives express to rising emission quantity because of the increasing depth ratio of combined sewage system in raining time. (5)According to the simulation experiments, the relationship of COD and CH4concentrations can be expressed by one-level dynamic formula-ln(Xo/Xt)=0.0164t-0.21, and its degradation coefficient KH=0.0164h-1.When pH=6.9, final CH4concentration is5.80g/m3. When temperature is25℃, the final CH4concentration of residential sewage is9.18g/m3.(6)By comparison of CH4generation and emission coefficients attained from researching Chinese cities and the recommended CH4value of IPCC Greenhouse Gas Inventories, conclusions can be drawn that the maximal value of CH4generation coefficient provided by IPCC Guidelines is0.25gCH4/gCOD, and CH4emission coefficient of sewage septic tanks is0.125gCH4/gCOD. This generation coefficient is3.6times of that of Lanzhou city, and emission coefficient is10.7times of Lanzhou. The IPCC commending CH4emission coefficient value is43times of Kunming and33times of Guangzhou. In2011, H2S generation quantity and emission quantity estimated from Kunming sewage are7.96t/a and1.46t/a, and its emission proportion is18.3%. CH4generation quantity and emission quantity are109.52t/a and44.63t/a, and its emission proportion is40.74%. CO2generation quantity and emission quantity are957.69t/a and300.13t/a, and its emission proportion is31.00%.The results of this research not only provide exhaust gases generation and emission coefficients of Kunming sewage system, but also prove that CH4emission coefficient of sewage system given by IPCC guidance is bigger. That is significant because the research provides basic data for calculating greenhouse gas. Partial research results have been applied to the management of Kunming sewage system, which can be served as guidance and reference for calculating and controlling exhaust gases in other Chinese cities.