| Anaerobic digestion method for sludge treatment has long digestion cycle and low methane yield.The rate-limiting step is Hydrolysis acidification.In order to increase the rate of sludge hydrolysis and enhance the efficiency of anaerobic digestion.In this project,the feasibility study of ultrasonic combined with free nitrous acid pretreatment to improve methane production by anaerobic digestion of sludge was carried out.Analyzed the mechanism analysis of combined pretreatment to improve the anaerobic digestion and methanation of sludge.The main conclusions were as follows:(1)Ultrasound pretreatment was performed through different ultrasonic intensities and different ultrasonic times.Under the action of ultrasound with ultrasound intensity of 1.0 W/m L and ultrasound time of 15 min,the average particle size of the sludge was reduced to 17.8 μm.SCOD concentration reached the maximum value of 1387.4 mg/L.Dissolution amounts of protein and polysaccharides were 648.4 mg/L and 120.3 mg/L.The DNA content increased to 171.4 mg/L.The main mechanism of ultrasonic pretreatment was to promote the deflocculation of sludge and the exfoliation of extracellular polymer,which had little effect on cell destruction.The best ultrasonic pretreatment conditions in this topic were: ultrasonic intensity 1.0 W/m L and ultrasonic time 15 min.(2)Free nitrous acid pretreatment was performed through with different p H and different reaction time.The release of SCOD,dissolution of protein and polysaccharides,and dissolution of DNA at p H 5.5 were better than those at p H6.5.The increase of the reaction time was also conducive to the dissolution of SCOD,protein and polysaccharides,and the release of DNA.The main mechanism of free nitrous acid pretreatment was sludge cell cracking and inactivation,which had limited effect on sludge floc structure destruction and extracellular polymer stripping.The best free nitrous acid pretreatment conditions in this subject were: p H 5.5 and reaction time 5 h.(3)Combined with the best process conditions,the ultrasonic combined with free nitrous acid pretreatment was carried out,and the influence of the combined pretreatment on the sludge dissolution stage was analyzed.The particle size distribution of the sludge showed that the combined pretreatment destroyed the sludge flocs mainly by ultrasound.The combined pretreatment released more organic matter.Under the combined pretreatment condition of 1.0W/m L+3.55 mg FNA/L,SCOD dissolution reached 2134.5 mg/L.The maximum protein content in S-EPS reached 713.5 mg/L.The maximum polysaccharide content in S-EPS increased to 410.7 mg/L.Protein content in L-EPS increased to215.3 mg/L.Polysaccharide content in L-EPS increased to 83.0 mg/L.Combined pretreatment significantly enhanced the release of proteins and polysaccharides from T-EPS to the outer layer(L-EPS,S-EPS).Maximum DNA content reached 5193.6 mg/L.Free nitrous acid-ultrasound further enhanced the release of organic matter in the sludge pretreatment process.(4)The effect of ultrasound combined with free nitrous acid pretreatment on sludge anaerobic digestion was studied.Under the condition of 1.0W/m L+2.84 mg FNA/L,the maximum methane production was 155.3 m L/g VS,which was 37.1% higher than the control group.Methanogenesis kinetics showed that the addition of free nitrous acid caused the kinetic inhibition in the early stage of fermentation.Alpha diversity analysis found that free nitrous acid was the main factor affecting microbial diversity.The dominant flora of this subject included Firmicutes,Bacteroidota,Proteobacteria,Chloroflexi and Actinobacteriota.Methanogens were dominated by auxotrophic methanogens(Methanosaeta).The addition of free nitrous acid led to the inhibition of microorganisms required for the hydrolysis and acidification process,and the hydrolysis and acidification process slowed down.The hydrogen accumulated in the process of hydrolysis and acidification provided a stable metabolic substrate for the hydrogenophilic methanogen(Methanobacterium),and the metabolic pathway of hydrogenophilic methanogen was improved. |
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