Predicting And Controlling The Hydrogen Sulfide Concentration In Gravity Sewer Under Dynamic State Condition

In the process of operation,the sewer system system will consume oxygen due to the life activities of bacteria,forming an extremely anaerobic environment downstream of the pipeline,thus producing hydrogen sulfide gas（H₂S）.H₂S in sewer not only produces smells affect human body health,but also will cause corrosion to the concrete wall,reduce the useful service life of sewer.The internal environment of sewer is highly dynamic,and usually the H₂S control measures are implemented based on the steady-state prediction,which will lead to inaccurate drug use and waste of drugs.Therefore,it is of great significance to predict the dynamic change of H₂S concentration and optimize the control measures to effectively control the H₂S problem.At present,most related models focus on the pressure sewers,but domestic sewers are basically gravity pipes,so this research takes the gravity sewers as the research object.Using the micro element method to analyze H₂S input and output processes,biochemical reaction and liquid gas mass transfer process,and the unsteady hydrogen sulfide gas prediction model was set up;the finite difference method was used to transform the difference format of the model,and the iterative simulation was carried out in MATLAB software.The experiment,OUR curve method and genetic algorithm were used to determine and optimize the parameters.A field study was conducted on a gravity sewer in a city of Shaanxi province to calibrate and validate the mode,the simulation results were in good agreement with the measured results,and then the influence of temperature,depth ratio and pH in sewer on H₂S concentration was analyzed.Through the analysis of the control effect of iron salt and alkali on H₂S,the reaction of iron salt and alkali and sulfide was coupled into the model based on the prediction model,and the control models of iron salt and alkali were established respectively.After verifying the model,the optimal dosage,location and ratio of H₂S were determined by using the iron-salt control model,and the alkali control model was used to optimize the scheme.The main conclusions of this study are as follows:（1）The correlation coefficient R²of the calibrated H₂S concentration prediction model reached 0.93,which can successfully predict the change of H₂S concentration and the recovery time after ventilation.（2）After H₂S was reduced to about 0ppm by mechanical ventilation,the H₂S concentration will return to the original concentration within 50-60 minutes after the manhole cover was reclosed.（3）The water temperature and depth ratio in the sewer increase,the H₂S maximum concentration will also increase,and the recovery time of concentration after ventilation will be shortened,the effect of pH is just the opposite.（4）When the concentration of iron salt is 5mg L^-1,the sulfide concentration reduction rate and H₂S are 95.7%and 45.3～95.7%respectively.The location of the addition was 251m upstream of the control point.When the concentration of iron salt is 10mg L^-1,the reduction rate of sulfide concentration is99.8%,and that of H₂S is 31.9～99.5%.The location of the addition is 124m upstream of the control point.The combined use of ferric and ferrous salts is better than the single use,and the optimal ratio is 2:1.（5）When Mg（OH）₂is added to the sewer to raise the pH value to 8,the H₂S control requirements can be met.（6）The H₂S control efficiency of iron salt can be improved when alkali is combined with iron salt.