Study On Enhanced Pretreatment Technology And Mechanism Of Anaerobic Digestion Of Sludge With Low Organic Matter And High Solid Content

In a large number of urban sewage plants,the anaerobic digestion efficiency of low organic sludge is low,which becomes the bottleneck in the recycling of sludge.This study focused on the low organic sludge in the urban sewage treatment plants,put forward the ultrasonic-alkali synchronous and thermal alkali-biological synergistic enhanced pretreatment technology for low organic sludge,by which to improve the efficiency of sludge hydrolytic acidification.This study comprehensively analyzed the basic properties,sludge particle size,surface charge,components&structure of organic matter and BMP of low organic matter sludge by SEM-EDS,3D-EEM and FRI.The comprehensive effects of pH,contact time,power density,sludge concentration and synergistic mode on the efficiency of alkali ultrasonic synchronous pretreatment system were examined.The key operating parameters were obtained as a result.Besides,the transformation law of the systematic organic matter and the effects of sludge on the efficiency and microbial population structure of the anaerobic digestion system after alkali-ultrasonic synchronous pretreatment were also explored.In combination with the phenomenon of a large number of hydrolytic acidification bacteria in an environment of high temperature(55?)and high alkalinity(pH=10?12)found in the study,the effects of pH,SRT and solid content on the construction and efficiency of the thermal alkali-biological synergistic pretreatment system for low organic sludge were systematically studied.On this basis,in order to further improve the efficiency of the system,the study focused on the exploration of the thermal alkali-biological pretreatment system for high solid content and low organic sludge,investigated the influence of pH value and microbial growth mode on the system construction and efficiency,and finally determined the key operating condition parameters and methods of system construction.At the same time,using FTIR,FISH,3D-EEM and FRI technology,the study explored the organic transformation law of the system,analyzed the effect of sludges with different enhanced pretreatment methods on the distribution of bacteria and archaea in the anaerobic digestion system.Through the 16S r RNA high-throughput sequencing technology,the microbial population structure and functional genera in each enhanced pretreatment system were analyzed,and the microbial mechanism of the system was revealed as a result.The main conclusions of the study are as follows:In an typical low organic sludge of one urban sewage plant,the VSS/TSS is 35.6%,the proportion of chemical elements C and Si on the sludge surface is 35.42%and 15.06%respectively.The surface charge of the sludge was-0.031 meq/g MLSS,which was 2.29times lower than that of the high organic sludge in the control group.The EPS content of the sludge is 50.18 mg/g VSS.The EPS/TCOD ratio(2.66%)is significantly lower than that in the control group(26.93%).The content of LB-EPS is 2.93 mg/g VSS respectively,the proportion of easily degradable organic matter is 62.71%?78.79%,significantly lower than that in the control group(82.19%?87.40%).At the same time,the proportion of soluble protein and polysaccharide in the low organic sludge is 84.71%?85.10%and 4.78%?5.44%,respectively,with their biodegradabilities of 52.24%and39.70%,respectively.Result analysis of SEM-EDS and FTIR shows that:The material composition of low organic sludge is basically the same as that of high organic sludge,and the main composition and structure of inorganic sand in it are Si O₂ and Si-O.However,the N-H bond,C=O bond and-CO-NH bond in low organic sludge have higher wave crest number.Inorganic sand and protein organic matter combine to form inorganic-biological flocs,where the proteinaceous substance is difficult to be dissolved.BMP test results show that:The methane yield of low organic sludge is 34.6%lower than that of the control group,and the recycling potential of sludge is obviously inhibited by inorganic sand.pH,contact time,ultrasonic frequency,power density and sludge concentration have significant effects on the dissolution of organic matter in the low organic sludge alkali-ultrasonic synchronous pretreatment system;the treatment efficiency is in the order of alkali-ultrasonic synchronous treatment>ultrasonic-alkali step-by-step treatment>alkali-ultrasonic step-by-step treatment>ultrasonic treatment>alkali treatment.As indicated in the results of orthogonal test,the comprehensive effects of factors on the efficiency of alkali-ultrasonic synchronous pretreatment are ranked as follows:pH>contact time>power density>sludge concentration.Under the conditions of 28 k Hz,pH=10,power density=0.4 w/ml,sludge concentration=80 g/L and contact time=15 min,the SCOD of low organic sludge pretreated by alkali-ultrasonic synchronous system increases by 4.31 times compared with the control group(original sludge),and its methane production increases by 74.2%compared with that of the control group.The total fluorescence intensity of the sludge supernatant increased to24.58×107(a.u.)after alkali-ultrasonic synchronous pretreatment of low organic sludge,and the proportion of the fluorescence intensity of the area having easily biodegradable substances reached 87.35%,19.1%higher than the control group.The low organic sludge subject to the alkaline-ultrasonic synchronous pretreatmenthas significantly lowerd the diversity of microbe populations in the anaerobic digestion system,the hydrolytic acidizing bacteria are fully gathered.the abundance of Firmicutes and Synergistetes increased by 1.26 times and 6.15 times,respectively,compared with that of the control group(with no pretreatment);the abundance of 7 hydrolytic acidizing bacteria including vadin HA17 and Peptoclostridium is 31.81%,increased by 1.07 times compared with that of the control group;quantities of archaeon such as Methanosaeta and Methanosarcina are in an absolute advantage.pH,SRT and solid content have a significant effect on the construction and efficiency of the thermal alkali-biological synergistic pretreatment system for low organic sludge.The system has a high pretreatment efficiency under the conditions of pH10,SRT 5d and solid content of 8%.In a system of pH12,the efficiency of organic matter dissolution and sludge VSS reduction is higher.While,in a system of pH10,the acidification efficiency is higher,with the VFAs reaching 3081.22 mg/L,and the proportion of acetic acid is 61.10%.When SRT=5d,the system organic matter dissolution and sludge VSS reduction efficiency are higher,;when SRT=3d,the acidification efficiency is higher,containing 2964.78 mg/L of VFAs,with 51.27%of acetic acid.When the solid content is 8%,the efficiency of organic matter dissolution,hydrolysis acidification is high,VFAs production is 135.82 mg/g VSS,with 42.52%of acetic acid.The pH has a significant effect on the construction and efficiency of high solid content(8%)and low organic sludge thermal alkali-biological synergistic pretreatment system.Under the conditions of 55?and SRT=3d,the hydrolysis efficiency in the system increases with the increase of pH value,and the acidification efficiency first increases(pH increases from 6.8 to 10)and then decreases(pH increases from 10 to 12)with the increase of pH value.When pH10,the efficiency of the pretreatment system reaches the best,VFAs/SCOD(?a)reaches 32.15%,and the VFAs concentration is 8.68times higher than that in the control group of conventional hydrolytic acidification.After treatment under the condition of pH10,the methane yield of sludge is 1.08 times higher than that in the control group,the average particle size of sludge is 24.60%lower than that of the original sludge,and the surface charge(SC)is 3.32 times higher than that of the original sludge.The result of the 3D-EEM analysis indicates that the proportion of biodegradable organic matter in sludge DOM increases from 61.78%to77.60%after treatment in the pH 10 system.The fluorescence indexes of FI,BIX and HIX all show that DOM comes from autogenic sources such as microorganisms,which indicates that microorganisms play an important role in the enhanced hydrolytic acidification system.The 16S r RNA high-throughput sequencing results show that thermophilic hydrolytic acidification functional bacteria of the pH10 system mainly include Tepidimicrobium(29.50%),Coprothermobacter(5.23%),Candidatus?Microthrix(4.69%),Aquabacterium(4.32%),Tepidanaerobacter(4.23%)and Proteiniclasticum(1.28%).The microbial growth mode has a significant effect on the construction and efficiency of high solid content and low organic sludge thermal alkali-biological synergistic pretreatment system.The microbial activity,sludge solubility,biodegradability and methanogenic efficiency of the thermal alkali-biomembrane synergistic pretreatment system are significantly improved compared with those of the thermal alkali-activated sludge pretreatment system.The dissolved concentrations of SCOD and VFAs in the biomembrane pretreatment system are 22692.29 mg/L and7373.05 mg COD/L,respectively,which is 9.87%and 11.04%higher than that in the activated sludge system.The protease activity is 1,273.98 U/g DS,which is 2.73 times of that in the activated sludge system;methanogenic efficiency is 252.92 m L/g VSS,with an improvement of 1.4 times compared with the unpretreatment system,and an improvement of 15.46%compared with the alkali-activated sludge system;the proportion of biodegradable substances in DOM of biomembrane system is 79.41%;and the VSS reduction is 31.16%higher than that in the activated sludge system.The16S r RNA high-throughput sequencing results show that the dominant hydrolytic acidizing functional bacteria genera in the thermal alkali-biomembrane pretreatment system mainly include Tepidimicrobium(27.65%),Coprothermobacter(11.09%),Candidatus?Microthrix(6.02%),Aquabacterium(5.78%),Tepidanaerobacter(4.23%),Proteiniclasticum(1.19%),Hydrogenispora(0.27%)and Caldicoprobacter(0.01%).Their abundances are significantly improved than that in the control group.The FISH analysis results indicate that:The proportion of total archaea in the anaerobic digestion system of sludge pretreated by thermal alkali-biomembrane system,thermal alkali-activated sludge system and thermal alkali system is 95.07%,87.26%and 78.70%,respectively,and the methane production rate in the anaerobic digestion system is 53.52%,38.54%and 17.10%higher than that in the control group.After subjecting to thermal alkali-biological synergistic pretreatment,the methane yield of the low organic sludge are as followed:the thermal alkali-biomembrane synergistic pretreatment system(276.34 m L/g VSS),thermal alkali-activated sludge system(249.37m L/g VSS),thermal alkali system(210.78 m L/g VSS),all of which are higher than the unpretreatment system(180 m L/g VSS).Thermal alkali and biological action contribute 85%-93%and 7.07%-17.63%,respectively,to the dissolution of organic matters.Hot alkali mainly plays the role of hydrolysis,while biology mainly plays the role of acidification.