Influences Of Hydraulic Conditions On The Productions Of CH₄ And CO₂ From Different Sewer Sediments

In recent years,as the growth of the urbanization rate and urban drainage system in the sewage pipe quantity is increasing year by year,a large number of organic compounds in wastewater degradation occurred in the process of pipeline transport caused a serious loss of carbon source,to the downstream of sewage treatment plant nutrients and pollutants removal efficiency had a significant negative impact,and formed a large number of CH₄ and CO₂ emissions.Previous studies have shown that the carbon emission of sewage in the pipeline transportation process is an important part of the total carbon emission of sewage treatment system.Carbon emissions from gravity sewer are mainly from sediments,and the physicochemical properties and microbial distribution of sediments are affected by the hydraulic input of exogenous carbon.The hydraulic input conditions mainly include continuous flow,interrupted flow,intermittent flow,pumping process,etc.,which are generally found in the sewers and have an important impact on the carbon emission dynamics and flux of sediments.The formation process of different types of sewer sediments leads to the difference of sediments from different sewers,which is bound to lead to the difference of gas potential.To sum up,it is very necessary to study sewer carbon emissions from the perspective of different hydraulic conditions and different sewer sediments,which is of great significance for improving and refining carbon emissions of drainage systems.In view of the effects of different hydraulic conditions and different sewer sediments on the sewer carbon emissions,this study will study the differences in gas production characteristics of sediments in sewers,storm sewers,and illicit-connected storm sewers,under continuous flow,interrupted flow,intermittent flow,and pumping process from the perspectives of carbon emission flux,organic matter and microorganisms with gas chromatography,three-dimensional fluorescence spectroscopy,Fourier transform infrared spectroscopy,16 S r RNA,metagenomics and other techniques in the form of lab simulated cultivation.The deeper analysis of sediment DOM composition,microbial diversity and metabolic pathways produces the reason for this difference.The research results can provide reliable data for the modeling and prediction of urban pipeline greenhouse gas emissions,thus providing theoretical support for achieving accurate clean-up.The research results improve the theoretical system of sewer CH₄ and CO₂ production from the perspective of hydraulic conditions and different sewer sediments,and provide theoretical support for the formulation of carbon emission reduction measures in the sewers.The main findings are as follows:(1)The gas production rate of the three sediments under continuous flow showed periodic changes with the periodic supplementation of exogenous carbon,and the overall trend was downward.Among them,the CH₄ production was greatly affected by the cyclical supplementation of exogenous carbon,while the CO₂ production was less affected.The sewer sediments cumulative production of CH₄ flux under interrupted conditions is significantly less than that of continuous flow conditions.The CH₄ flux of sewer sediments under interrupted flow experienced a rapid decline after an initial transient increase and gradually became zero.The CO₂ fluxes showed an upward trend in the early stage,then began to decline,and stabilized at the end of the reaction.Under the intermittent flow,the carbon emission flux showed a general downward trend,but there was a certain rebound in each re-supply of the overlying water flow.Compared with CH₄,the response change of CO₂ production and discharge to this working condition is more clear and significant.The pumping progress caused a large amount of gas to escape in the sediment.The CH₄ flux produced a certain increase after the beginning of the river and then rapidly decreased,and gradually became zero after 5 days.Only the sewage pipe sediments remained.A small amount of CH₄ flux produced.The CO₂ fluxes of the three sediments showed a trend of slow decline after the beginning of the simulated pumping station.(2)The degradation of tryptophan in the DOM of the overlaying sewage mainly occurs in the early stage of the reaction and leads to the decrease of exogenous DOM DP,which is the key reason for the shortage of carbon source in the downstream sewage plant,and also the main source of carbon emissions from the sewers.Non-monotonic changes in the composition of the endogenous DOM,it is because the infiltration of exogenous DOM into sediments and the outflow of intracellular proteins caused by microbial death in sediments caused deposition.(3)In the anaerobic fermentation process under stable hydraulic conditions in the sewers,the carboxylic acid and protein synergistically change,and the degree of change is almost the same,the degree of change of polysaccharide is relatively small,and under the condition of strong hydraulic disturbance,the degree of change of carboxylic acid is significantly greater than protein.The sequence of degradation of organic functional groups in pumping progress conditions is also different from the other three conditions,that is,proteins and carboxylic acids are degraded prior to sugars and lipids,which leads to higher degradation efficiency and more carbon.Emissions.(4)The abundance of Methanosaeta(12.68%)in the initial sewer sediments is much higher than the other two sediments(4.29%,2.05%),which causes the sensitivity of the sewer sediments to exogenous carbon.The high proportion of fermenting acid-producing bacteria in the other two sediments has a certain buffering effect on the impact of carbon source shortage.The difference in microbial community composition in sediments from different sources gradually decreased with the increase of culturing time.The differences in microbial community composition in sediments from different pipeline sources tend to be homogeneous with increasing culture time,but some specific species still maintain significant differences throughout the culture process and may affect the carbon of sediments.emission.The characteristic species in the sewage pipeline sediments are Thermotogae(2.11%)and Syntrophomonas(3.62%),and the characteristic species of rainwater pipeline sediments is Acidobacteria(1.18%).(5)Methanobacterium and Methanosaeta are the main functional bacteria of carbon metabolism and methane metabolism in the initial sediment and continuous water flow group,and have higher functions in the first 8 functions of other abundances.Under the condition of interrupted flow,Smithella and Thiobacillus become the main functional bacteria of methane metabolism,and the contribution of acetic acid-type methanogens exceeds that of hydrogen-type methanogens,which may be affected by Smithella.(6)With the passage of time,the relative abundances of key enzymes such as acetyl-Co A synthetase and formyl furan dehydrogenase in the CH₄ pathway of acetic acid and CO₂ produced in pipeline sediments decreased,which is a main reason for carbon emission in the culturing.The decarboxylation of acetyl-Co A to form 5-methyl-H4 MPT is the rate-limiting step in the production of CH₄ from acetic acid.This limitation is particularly manifested in the mixed pipeline deposits of the continuous flow group.The relative change of formate dehydrogenase and formyl furan dehydrogenase in the pathway of Methanogenesis from CO₂/formic acid reduced the utilization rate of CH₄ produced by formic acid under the condition of interrupted flow,and caused more CO₂ emission.