Simulation Research On Pollutant Transformation In Urban Sewer Networks

Being an important part of urban water infrastructure,the sewer networks have drawn a great deal of attention as a bioreactor.Thus,in light of the topological relationship and biochemical reactions involved in the sewer networks,the Sewer Wastewater Transformation Model(SWTM)was firstly developed to simulate a variety of biochemical processes including hydrolysis,acidogenesis,methanogenesis,ammonification,and sulfate reducing.Based on the SWTM model,the complex topological relationship and the flow behavior in sewer,the level identification algorithm was further used to build a pollutants transformation model for various scenes in sewer networks with confluence.Then,Morris screening method was used to find out high-sensitive parameters in the model.According to the screening results,dominant parameters was determined and calibrated.Based on this,through the simulation of the pilot-scale sewer device and the simple sewer system,the contaminants transformation trend both along the sewer and in junctions was revealed.Finally,the model was applied to a complex sewer network to analyze both the change of pollutants in the networks and the distribution of CH4 and H2S.The research results were of great significance to the prediction of influent quality in wastewater treatment plants,the prevention and control of H2S and CH4 in the sewer,and the maintenance of urban sewer networks.The main research contents and conclusions of this paper are as follows:(1)Based on pollutants transformation processes such as hydrolysis,acidogenesis,methanogenesis,ammonification,and sulfate reducing presented in the sewer,a SWTM model was developed based on Monod equation in the form of matrix model.By simulating the growth and metabolism process of fermentation bacteria,hydrogen-producing acetogens,methanogens,ammonifying bacteria and sulfate reducing bacteria(SRB)and other microorganisms in the biofilm,the model clearly depicted 27components in 26 processes.On this basis,combined with the water routing algorithm,the level identification algorithm,and the flow velocity algorithm of confluence,the pollutants transformation model is established for sewer networks with changeable condition.(2)The sensitivity analysis showed that the composition of pollutants exerted a great influence on the screening results of high sensitivity parameters of the model.For COD and ammonia nitrogen,the pollutants composition in sewage was different due to contaminants transformation in sewer.The high sensitivity parameters at the beginning and the end of the sewer were different as well.On the basis of sensitivity analysis and with the aid of pilot experiment system,the high sensitivity parameters were adjusted accordingly.After calibration,the goodness of fit of the model was significantly higher than that in similar sewer networks.(3)The simulation results of pollutants transformation showed that acetic acid plays a different role in various processes in sewer.In the process of methane production,the methane produced by CO2 and H2 metabolism accounted for 25.3±1.4%of the total methane.Besides,the methane produced by methyl substrates accounted for less than 3.7±0.5%.In addition,the rest of the methane originated from the HAc.However,in the sulfate reduction process,the metabolic rate of propionic acid was much higher than that of acetic acid,and the continuously accumulated acetic acid did not become the main metabolite in the sulfate reducing process.As a result,the half-saturation coefficients and maximum uptake rate for acetic acid were much lower than those for propionic acid.Under the influence of the half-saturation coefficients and yield coefficient of acetic acid,propionic acid and butyric acid,acetic acid-based SRB bacteria grew slowly.(4)The simulation results showed that the composition of pollutants tended to be homogenized along the sewer due to the continuous transformation of carbon source from macromolecules to small molecules.This trend led to a continuous increase in the proportion of VFA and acetic acid in total COD.This further led to a smaller decrease in COD concentration along the branch sewer with an obvious change in the composition of carbon source and a greater decrease in COD concentration along the main sewer with a little change in the composition of carbon source.(5)The model was applied to a complex sewer network in Xi'an.The results showed that,under the influence of homogenization of pollutants composition,not only the percentage of small molecular organic substances such as acetate in total COD increased continuously,but also the proportion of COD to N tended to be 5:1 along the sewer,and these results led to the fact that wastewater at the end of the sewer was more likely to be degraded in the wastewater treatment plant.Besides,due to homogenization of pollutants composition,CH₄ was mainly concentrated in the main sewer.In addition,H₂S concentration presented the trend of lateral sewers>main sewers>branch sewers and the average concentration of H₂S in main and lateral sewer exceeded 10 mg/m³ with the highest value being as high as 23 mg/m³.