Hydrogen And Methane Production From Excess Sludge In MEC And The Analysis Of Microbial Structure

Large amount of waste activated sludge (WAS) is produced in urban sewage treatment plant and timely treatment and disposal is needed. The WAS, if not well utilized, would be wasted and contaminate the environment. The disposal and reutilization of WAS would be the development direction of sludge treatment technologies. This study employed the optimized WAS pretreatment method and treatment WAS with MEC systems and MECs inoculated with special microbial communities, overcoming the inefficient utilization of proteins in anaerobic digestion (AD) and the inhibition of digestion process from propionic acid accumulation. Microbial community structures were also analyzed. This approach made great progress in fuel gas production, proposing a novel way of sludge reutilization.In order to improve the soluble substance content in WAS and further to promote the effect of WAS disposal, this study made comparisons among several single pretreatment methods (alkaline pretreatment, ultrasonic pretreatment, thermal pretreatment and frozen pretreatment) and combined pretreatment (ultrasonic+alkaline pretreatment and alkaline+ ultrasonic pretreatment). Then series of analysis methods were applied in WAS pretreated by different methods, like COD, protein, volatile fatty acids (VFAs), solid concentration, LIVE/DEAD Baclight and three Dimensional excitation-emission fluorescence matrix (3D-EEM). The results suggested alkaline pretreatment as the chosen method in the following experiments because of its excellent effect and low cost. The optimized dosage of alkali was 7.27±0.35 mg NaOH/g VSS (pH:12). Then anaerobic fermentation with raw WAS (R-WAS) and alkaline pretreated WAS (A-WAS) was conducted. The results indicated that the highest methane yield and production rate of A-WAS were 362.2 mL/L·d and 68.7% respectively, notably higher than that of R-WAS of 251.2 mL/L·d and 62.2% respectively. Meanwhile, after alkaline pretreatment, VSS removal rate rised from 61.5±2.0% to 76.4±0.5%, protein removal increased from 4830±480 mg/L to 6530±430 mg/L, the effect of sludge treatment improved significantly. There existed big difference between A-WAS and R-WAS in microbial community structure. According to taxonomic classification at bacterial phyla level, the dominant phylum in R-WAS was Nitrospirae, followed by Proteobacteria and Firmicutes, while Chloroflexi and Firmicutes were in the first and second places of A-WAS.According to taxonomic classification at bacterial genus level, lamia and Rhodobacter were in the first and second places of R-WAS,while the first two genus of A-WAS were lamia and Petrimonas.The microbial community of Archaea was quite different.Methanosaeta was the most with nearly half proportion of the total quantity in R-WAS while Methanosarcina made up more than 50% of A-WAS Archaea. The variation between microbial communities of R-WAS and A-WAS resulted in their different fermentation effects.Plenty of protein-like organics could not be digested and utilized in sludge treated with traditional fermentation. This study employed MECs to treat WAS with comparison between A-WAS and R-WAS.WAS and PBS buffer were mixed into sludge of different concentrations for the following treatment and comparison. At concentration of 16 COD/L, the hydrogen yield was 14.22±0.39 mg-H2/g-VSS in MECs fed with A-WAS,and the hydrogen production rate was 0.47±0.02 m3-H2/m3d. Hydrogen made up 57.38% of the total gas quantity in MECs with A-WAS.The methane yield was 92.38±2.63 mg-CH4/g-VSS in MECs fed with A-WAS,and the methane production rate was 0.235±0.005 m3-CH4/m3·d. At the optimal WAS concentration of 16 g COD/L, hydrogen yield and methane yield had been greatly improved in MEC system combined with traditional hydrogen fermentation with A-WAS. At the phyla level of bacterial community, Geobacter and Petrimonas are the dominant phyla. At the genus level of Archaea, Methanocorpusculum of Methanomicrobia was the most in every experimental group, and then was the Methanosaeta. Methanocorpusculum took up more in A-WAS, which indicated that this Archaea was suitable to live in alkaline condition.In order to obtain good disposal effect of WAS, MEC system was inoculated by the sludge in the reactor when the best methane production was achieved with traditional anaerobic fermentation, with the further enhancement of sludge from anaerobic reactor of propionic acid synthesis sewage treated with UASB process. At WAS concentration of 15 g COD/L in MECs fed with A-WAS, hydrogen yield was 33.40±1.30 ng-H2/g-VSS, and methane yield was 309.43±0.85 mg-CH4/g-VSS, both were higher han that of MECs not inoculated. Inoculated MEC obtained high hydrogen and methane yield and better treatment effects in removals of TCOD, protein and every other solid index than MEC not inoculated did. Proteobacteria, Firmicutes and Bacteroidetes were the dominant bacterial phyla, and Geobacter and Desulfuromonas of Deltaproteobacteria were the main genus. Theses microorganisms were related with sludge reduction and electricity generation in bioelectrochemical system. Methanosaeta and Methanocorpusculum were the dominant Archaea in Archaea community, indicating hydrogenotrophic methanogens produced methane in indirect electron transfer pathway.