Mechanism Of Greenhouse Gas Generation In Pipe Network During Urban Sewage Transportation Process

The fact that an elevated discharge in greenhouse gas concentration results in global warming by human activities,defined as greenhouse effect,has been a toppest issue.The suite of Sewage Sludge treatment is considered as an important source of greenhouse gas discharge.The amount of microorganism lived in sewage-transpoted biofilms could not only change water quality,but also discharge a mass of greenhouse gas in urban sewage pipe network.However,few studies have focused on quantitative evaluation of sewage-transported greenhouse gas in urban sewage pipe network.In this study,we monitored the variation of greenhouse gas concentration in urban sewage pipe network?the total length of 839km?in Xi'an from 2014 to 2017,and conducted a quantative evaluation of greenhouse gas discharge based upon carbon footprint analysis.The results show that average CO₂ concentrations in branch pipes,main pines and leading pipes are 5916 mg/L,2871 mg/L and 1830 mg/L,respectively,and average CH₄concentrations are 2937 mg/L,1445 mg/L and 914 mg/L,respectively.The total lengths in branch pipes,main pipes and leading pipes are 247521 m,219506 m and 371873 m,respectively,and the diameters are 1000-2000 mm and 400-800 mm and 250-500 mm,respectively,and the average flow velositiesare 0.0027 m³/s,0.027 m³/s and 0.045 m³/s,respectively.Therefore,the total amount of CO₂ discharge is 15.07 t/d,the total amount of CH₄ is 7.50 t/d in urban sewage pipe network in X'an.Combined with the influence factors of CO₂ and CH₄ on the greenhouse effect?1:23?,so the total CO₂ discharge is187.59 t/d.In order to further explore the specific sources of greenhouse gases generation in urban sewage pipe network,we analyzed the variations in greenhouse gases discharge in the sub-pipes with microorganism species along different class pipes.The CO₂ generation is associated with three organic macromolecules including glucoses,proteins,and oils etc.Four pathways to produce CO₂ in glucosedegradation:1)glucose degrades to ethanol and CO₂;2)glucose is degraded to lactic acid;further lactic acid can degrade acetic acid and CO₂;3)glucose is degraded to acetic acid,propionic acid and CO₂,propionic acid is further degraded into acetic acid and CO₂;4)glucose is degraded to butyric acid and CO₂.The dominant bacteria are Tolumonas,Flaiobacterium,Trichococus,Lactobacillus,Veillonella,Dechloromonas,Anaerolinea,Aeromonas and Enterobacter.Three pathways to produce CO₂ in protein degradation:1)urea degradation to NH₃ and CO₂;2)protein degradation to amino acids,and the formation of CO₂;3)amino acids are degraded to acetic acid,propionic acid,butyric acid and CO₂,propionic acid and butyric acid are further converted into acetic acid and CO₂.The dominant bacteria were Escherichia coli,Staphylococcus,Mold,Trichoderma,Klebsiella,Streptococcus,Dechloromonas and Anaerolinea.Three pathways to degrade CO₂ from oils:1)glycerol degrades to acetic acid and CO₂;2)the glycerol can be degraded to ethanol and CO₂;3)fatty acids can be degraded to produce methanol and methanol can be further decomposed to produce CO₂.The dominant bacteria are Propionibacterium,Dechloromonas,Anaerolinea,Escherichia coli,Kluyvera Farmer,Klebsiella,Bacillus,Methanogens.There are four pathways to produce CH₄:conversion of acetic acid to methane,conversion of formic acid to methane,conversion of methanol to methane,and conversion of methylamine to methane.The dominant bacteria are Methanogenic bacterias.The amount of greenhouse gas produced in the pipe network is calculated.The calculation and analysis of the greenhouse gas are found to be similar to the monitoring value?Monitor:187.59 t/d,Calculation:199.25 t/d?.Thus,it can be proved that the greenhouse gas directly discharged from the urban sewage pipe network is produced by a series of biochemical reactions in the pipe network.The differences among various cities occur,the greenhouse gases discharge is high in the eastern,central China,and Sichuan Basin,but a relatively low in the western China.Based upon the population in different cities,we can make a contrast in total greenhouse gas discharge among different industries,the results show that greenhouse gas emissions from urban sewerage networks are large and need attention.