Development And Application Of Hazardous Gas Concentration Distribution Model In Sewer

Sewer is an important infrastructure in city,but hazardous gases such as hydrogen sulfide,methane and carbon monoxide in sewer seriously reduce the operating efficiency of sewer.Thus,it is important to develop a mathematical model and calculate the concentration of hazardous gases along sewer.At present,most of the relevant models do not consider gas-liquid mass transfer process in sewer,and can only calculate the concentration of hydrogen sulfide and methane in pressure-flow sewer,not capable of calculating the concentration of hazardous gases in gravity-flow sewer.In addition,carbon monoxide is common in sewer,however,the current models can not calculate its concentration along sewer.Based on the biochemical reaction and mass transfer process in the gravity-flow sewer,the material balance of each component in sewer was carried out,and a comprehensive model named as HGCD(Hazardous Gas Concentration Distribution)model was developed to calculate the concentration of hydrogen sulfide,methane and carbon monoxide along sewer.The HGCD model was calibrated and validated based on measured datas from a series of sewers(4100 m total)in a city.The concentration of hazardous gases in sewer under different hydraulic conditions which are formed by changing sewer’s design parameters is calculated based on HGCD model,and the influence of sewer’s hydraulic conditions on the concentration of hazardous gases is analyzed.The sewer’s design parameters affect the concentration of hydrogen sulfide in sewer,causing different corrosion rates of sewer and thus different sewer’s maintenance costs.In this study,the maintenance cost function of sewer is developed based on the concentration of hydrogen sulfide along sewer,and then sewer’s total cost function is constructed considering the maintenance cost function and construction cost function both.Based on that,the optimal design model of sewer is then developed.The genetic algorithm is selected to realize the sewer’s optimal design model,and the HGCD model is coupled in genetic algorithm to calculate the concentration of hydrogen sulfide along sewer,based on which the maintenance cost is obtained.A case study of optimal design model was carried out using a series of sewers(1380 m total)in a city.The measured datas along sewer were used to calibrate and validate HGCD model.The design parameters of the case sewer were optimized by sewer’s optimal design model realized by genetic algorithm to reduce sewer’s total cost.The main conclusions of this study are as follows:(1)The results of HGCD model are consistent with the measured values along case sewer 1,and the correlavant coefficient value is up to 0.99,which indicates that the accuracy of the HGCD model is high.(2)The trend of hydraulic retention time is opposite to that of specific surface area of sewer.The concentration of hazardous gases decreases first and then increases when hydraulic retention time and specific surface area change.There exists an optimal combination of hydraulic residence time and specific surface area,which contributes to minimize the concentration of hazardous gases in sewer.(3)The total cost of the optimized sewer is 10% lower than that of the original sewer when the case sewer 2 is redesigned by sewer’s optimal design model.