Study On Detection Of Harmful Gases And Distribution Law In The Mountain City Sewage Pipeline

With the accelerated process of urbanization, the urban population continues toincrease, the burden of the urban drainage system aggravate. A number of sewagepipeline explosion and hydrogen sulfide poisoning have taken place in many cities inChina (such as Beijing、Tianjin、Chongqing、Kunming etc.), it have posed a significantthreat to people’s lives and property safety, and seriously affected social stability. Due tospecial topography and living habits in Chongqing, sewage pipe explosion accidents aremore frequently than other cities. Therefore, it is very necessary to carry out the issue ofcity harmful gases in the sewage pipeline to the safe operation of the city sewagesystem.Paper reviewed the situation about the composition of the harmful gases in the citysewers, the mechanism, the detection methods and the status of the pipeline safety,described the formation of gases in terms of sewage flow characteristics in pipes.According to the city sewage pipeline safety accidents, serious safety problems andothers in Chongqing, a series of portable test instrument were used to detect the gases inthe sewage pipe in some typical areas of Chongqing, and samples were analysed inlaboratory. The paper explore methods suitable for the hazardous gases in mountain citysewers, and study harmful gases distribution and concentration changes in the law incity sewage pipeline, through field test, sampling and laboratory analysis, comparativeanalysis of the relationship of the test data with the surrounding environment,population density and other features. It will provide a theoretical basis for monitoringand warning system of the city sewers.Main research contents and results include:①On the basis of series of simplifying assumptions， the paper analysedsolid-liquid two-phase flow and gas-liquid-solid three-phase flow characteristics of thesewage in the city sewers and the formation of on the basis of sewage flowcharacteristics. In urban sewage pipeline, gases are produced due to pollutants insewage decomposition, oxidation or reduction. Transformation of substances in thesewage involves the conversion of carbonaceous matter, nitrogenous compounds andsulfur-containing substances in the three main types of material, which both oxygen inthe aerobic reaction, and the anaerobic reaction. In the city sewers, CH4、H2S and CO2are produced in the anaerobic digestion process of organic. ②Through three kinds of typical regional sewage pipeline harmful gas field initialdetection in the main District Jiangbei and Shapingba of Chongqing, found the citysewage pipeline of harmful gas is mainly methane, hydrogen sulfide and carbon dioxide,while the concentration of carbon monoxide and ammonia is very low, not even.③Through detection of the scene, it was found that, the concentration of harmfulgases is quite different, especially methane and carbon dioxide, the concentration is verylow at some detecting point, even zero, and some exceed the measuring range of theinstrument, but the concentration of the hydrogen sulfide in the parts per million level.In response to this feature, compared existing detection methods, it is found that the gaschromatography can be very good to meet the large difference between theconcentration of methane and carbon dioxide in demand, and can obtain accurate results.Detection of hydrogen sulfide in the sewage pipe is methylene blue, hydrogen sulfide innatural gas detection method is also applicable to hydrogen sulfide detection in the citysewers and accurate test results.④According to the detection of harmful gases in the urban sewage pipes, the gasconcentration is not stable. The distribution of these gases with different conditions ofenvironment, temperature, inspection well depth, location, etc. showed a certain degreeof regularity. According to the gas generating mechanism and the gas physical, chemicalproperties, found when temperature increased, the gas concentration increased. There isa higher level of methane in the shallow of inspection wells, carbon dioxide andhydrogen sulfide due to their soluble in water, concentration is not necessarily thehighest near the water surface. In the sewage system, the concentration of methane andcarbon dioxide in septic tank was significantly higher than that of the sewage pipeline,hydrogen sulfide is exactly the opposite. In the septic tank, the concentration of methaneand carbon dioxide appeared low to high phenomenon. Along the sewers, theconcentration of each gas has emerged as a lower trend.