Study On The Influence Of Canceling Septic Tank On Drainage System

The septic tank has been used for more than 100 years,which originated in Europe in the 19 th century.However,in recent years,with the continuous improvement of urban drainage network and sewage treatment plant construction,the problems caused by septic tanks have become increasingly prominent,and the abolition or transformation of septic tanks or the general trend.In this study,the residential area in the main urban area of Chongqing was selected for field investigation.The sewage quality,flow rate,H₂S and other indicators of the inlet and outlet of the septic tank in the residential area were measured,and the impact of the abolition of the septic tank on the drainage system and the surrounding environment was analyzed.One of the residential areas was selected to model the sewage pipe network and analyze the deposition and flow rate in the sewage pipe after the septic tank was cancelled.The influent water quality of sewage treatment plants in and outside the city was investigated,and the carbon emissions of septic tanks were calculated to analyze the impact of the abolition of septic tanks on sewage treatment plants and carbon emissions.Finally,according to the research results,the guarantee measures for the cancellation of septic tanks in Chongqing are proposed.The main research results are as follows:（1）According to the results of field investigation,the residential areas in the main urban area of Chongqing are basically equipped with septic tanks.Under the premise of reasonable cleaning,there is no obvious deposition in the inspection wells before and after the septic tanks.According to the measured results of the community flow,there is little difference in the flow before and after the septic tank on weekdays or weekends,and the cancellation of the septic tank has little effect on the total amount of sewage entering the municipal pipe network.According to the measured results of water quality in the district,the average removal rates of COD,BOD₅,NH₃-N,TN,TP and SS in the septic tank were 41%,40%,2%,8%,14%and 48.5%,respectively.The results showed that the septic tank had a higher reduction rate for COD,BOD₅and SS.The average decline rate of COD/TN is 43%.The septic tank is indeed one of the reasons for the lack of carbon source in the influent of the sewage plant.The average decline rate of COD/TP is 38%.The COD/TP values before and after the septic tank meet the requirements of biological phosphorus removal in the sewage treatment plant.The COD/TP value is not less than 50,the average growth rate of BOD₅/COD is 7.5%,and the biodegradability of the sewage after the septic tank is cancelled.According to the measured results of H₂S in the community,the average concentration of H₂S in the septic tank is 5.39ppm,and the average concentration of H₂S in the sewage pipeline is1.77ppm.Canceling the septic tank can effectively reduce the risk of odor emission that can be obviously perceived by human senses,and has a positive effect on improving the air quality of the community.（2）According to the model simulation results,under the condition of no septic tank,the pipeline deposition mainly occurs in the pipe section of the inlet of the sewage discharge of the residential building,and the pipe section where deposition occurs accounts for about 20%of all pipe sections.The average deposition depth of the sewage pipeline varies from 1.83×10^-4to 0.17m,and the deposition depth of all the sewage pipelines where deposition occurs is up to 0.24m.Pipeline deposition generally occurs in areas where the slope is gentle and the hydraulic conditions change greatly.Generally,the sediment depth increases linearly with time.At the same time,as the downstream pipeline is blocked by deposition,the risk of deposition blockage in the upstream pipeline will increase sharply.The slope of the sewage pipeline in the community varies from-0.058 to 0.094,and the sewage flow rate varies from 0 to 1.384m/s.Except for the pipe section near the inlet of the sewage discharge of the residential building,the sewage flow rate of the other pipe sections after the intersection basically meets the requirement of 0.6m/s greater than the non-silting flow rate.The sewage flow rate of the downstream pipeline connecting the septic tank was compared and analyzed under the two conditions of setting the septic tank and not setting the septic tank.The results showed that the sewage flow rate before and after the cancellation of septic tank 1changed in the range of 0.4～0.8m/s,and the sewage flow rate before and after the cancellation of septic tank 2 changed in the range of 0.9～1.5m/s,and the change rule was similar.The cancellation of septic tank had no effect on the flow rate in the subsequent pipeline.（3）Based on the investigation and analysis of the influent water quality of sewage treatment plants in and outside the city,the influence of cancelling the septic tank on the influent water quality of sewage treatment plants within the service scope of sewage treatment plants was analyzed.The results showed that the average change rates of COD,BOD₅,NH₃-N,TN and TP in the influent water quality of sewage treatment plants in Qingdao were 1.04,0.86,0.39,0.44 and 0.46,respectively.The average change rates of COD/TN,COD/TP and BOD₅/COD were 0.42,0.39 and-0.07,respectively.Combined with the measured water quality,the increase rate of each index in the influent of the sewage plant after the removal of the septic tank was estimated.The calculation results showed that the COD increase rate was 2.26%-31.57%.The increase rate of BOD₅was 2.20%～30.80%.The increase rate of NH₃-N was 0.11%-1.54%.The increase rate of TN was 0.44%-6.16%.The increase rate of TP is 0.77%～10.78%,which can alleviate the problem of lack of carbon source in the influent of the sewage plant to a certain extent and improve the economic benefits of the sewage plant.The calculation results of the carbon emission of the septic tank show that the total annual CH₄emission of the septic tank is 5.55×107kg/a,and the carbon emission equivalent of the septic tank is similar to that of the sewage treatment plant.If the septic tank is cancelled on a large scale,it can avoid the greenhouse gas generated when the sewage stays in the septic tank,reduce the overall carbon emission in the sewage discharge treatment process,and have a very positive impact on carbon emission reduction.（4）The main starting point of the safeguard measures after the cancellation of the septic tank is the pipe network layout of the new community,the cancellation of the existing septic tank and the operation and maintenance management.In the design of the new sewage pipe network,the factors such as pipe diameter and slope should be considered comprehensively,so that the designed flow rate of sewage in the pipeline can meet the requirements of not less than 0.6 m/s.The grid grit chamber can be installed at the exit of the community to avoid the discharge of large solid waste into the pipe network.For the disposal of existing septic tanks that can be cancelled,it is necessary to consider the geographical location of the septic tank,combined with the process of dismantling the septic tank and the impact on the surrounding environment after dismantling,select the appropriate demolition method;in the aspect of operation and maintenance management,the intelligent management system can be installed to monitor the operation status of the pipeline in real time.In addition,the relevant management functions and powers should be strictly implemented,and the problematic pipeline segments should be dealt with in time to avoid shirking their responsibilities when problems arise.