Study On The Control Effect And Mechanism Of Odor Pollution And Volatile Organic Compounds In Sewer System

One negative consequence of the rapid acceleration of urbanization is the expansion of urban drainage systems,which increases the emission of hazardous Volatile Sulfur Compounds(VSCs)and Volatile Organic Compounds(VOCs).These compounds are harmful to humans,endanger the lives of workers undertaking drainage system maintenance,and in general,pollute the immediate surrounding environment.To improve the conditions for human settlement,studies on technologies and methods to control foul odors,especially in urban drainage systems,are important.This study focuses on VSC and VOC produced in drainage pipelines as research objects.Experiments are performed in the laboratory sewer simulation reactor to systematically study the control effects of hydrogen peroxide,nitrate and ferric iron on the release concentration of various odor pollutants,via various liquid phases and gas phases of the reactor during the process of intermittent addition.Through the evaluation of theoretical odor concentration and electronic nose values(of the odor),the control effect of three oxidants to the odor in reactor gas phase is determined.An effective method using electrochemical oxidation to synchronously remove various VSCs and VOCs in liquid phases and gas phases is also proposed.Analysis of the community structure of microorganisms in reactor sewage and sediment reveals the following:(1)In the process of intermittent addition of oxidant,the inhibitory period of nitrate and hydrogen peroxide on sulfide is fairly short,and the concentration of sulfide in sewage increases rapidly after the oxidant is depleted.Besides,the addition of nitrate and hydrogen peroxide also promotes the formation of dimethyl trisulfide(DMTS)in sewage.The control period of ferric iron on sulfide and DMTS in reactor sewage is longer than that of nitrate and hydrogen peroxide.The polysulfides formed during the intermittent addition of oxidant are an important intermediate product which lead to the rapid increase in sulfide concentration and promote the formation of DMTS.When the polysulfides formed in the reactor are precipitated by Fe2+,the rate of increase of sulfide and the rate of formation DMTS will be reduced.The methylation of polysulfides is an important biochemical reaction process of DMTS formation.The addition of oxidant will lead to the increase of the amount of methyltransferase in the reactor,thus enhancing the ability of microorganisms in the methylation of polysulfides.During the subsequent addition of oxidant,thioethers can be oxidized to dimethyl sulfoxide(DMSO),and DMSO will be rapidly reduced to DMTS during the phase of oxidant depletion,which is another important way to form DMTS in the reactor.The formation of DMTS means that other VOSCs may be generated during the intermittent addition of oxidant.(2)Under the control of iron salt,hydrogen peroxide and nitrate,the average removal rates of the total cumulative release concentration of VSC and VOC in drainage pipelines are 38.38%,32.45%and 34.52%,respectively.The average removal rates of VSC are 59.52%,36.25%and 47.64%.In the gas phase,the theoretical odor concentration produced by VSC,which accounts for 29.82%of the total volatile compound concentration,is more than 99.99%of the total OUT,which is the major substance to produce odor.The average removal rates of iron salt,hydrogen peroxide and nitrate to total OUT are 57.88%,26.50%and 42.60%,respectively.By using the electronic nose to detect the odor change after adding oxidant to the reactor,the sensors with high and obvious response values in the gas phase are W5S,W1W,W2S and W2W,respectively.After the addition of iron salt,hydrogen peroxide and nitrate,comparing the response values of the electronic nose to the odor produced by VOSC in the gas phase,only iron salt has a certain control effect within 24 hours.Microorganisms in sediments can produce various VOCs through all kinds of metabolic activities and release them directly into the gas phase.(3)The electrochemical oxidation method has a significant effect on the removal of sulfides in the liquid phase of the sewer simulation reactor.At the voltage of 6V,the average removal rate of sulfide is 87.76%.At the voltage of 8V,the average removal rate of sulfide is 89.19%.At the voltage of 10V,the average removal rate of sulfide is more than 99%.Because when the concentration of chloride ion in sewage is low,the metallic titanium grid platewith ruthenium and iridium coatings,mainly through the synchronization of direct oxidation of electrode surface and indirect oxidation of oxygen produced by electrolytic water,remove the sulfides.After the electrode is electrified,it also stimulates the growth and reproduction of Halothiobacillus and Acidithiobacillus in the sewage,and further removes the sulfides in the water through the oxidation of microorganisms.Using the electrochemical oxidation method to remove various VSCs and VOCs in the gas phase of the sewage simulation reactor,has more obvious advantages than the traditional addition of oxidant in the control effect.At the voltages of 6V,8V and 10V,the removal rates of hydrogen sulfide in the gas phase can reach more than 99%;The removal rates of VOSCs were 92.03%,91.43%and 91.56%,respectively;The removal rates of oxygenated compounds were 74.09%,95.83%and 70.33%;The removal rates of alkane and olefin were 60.63%.94.58%and 88.13%;The removal rates of aromatic compounds were 59.36%,86.13%and 51.56%;The removal rates of halogenated compounds were 59.06%,70.97%and 64.40%.After the electrode is electrified,the surface of the electrode can continuously oxidize various odor pollutants,and the electrode can also produce hydroxyl radicaland oxygen,together generating strong chemical activityto remove various VSCs and VOCs effectively.Yet,because the sediment at the bottom of the drainage pipeline can directly generate various gases for release into the gas phase,and also because the electrode has little effect on the diversity and structure of the microbial community in the bottom sediment,when the electrode is electrified,there will still be small amount of VSCs and VOCs in the gas phase.