Enhanced Production Of Short-chain Fatty Acids From Waste Activated Sludge Pretreated By Free Ammonia Combined With Peracetic Acid And Utilization Of Fermentation Liquid

As the scale of urban sewage treatment continues to expand,excess sludge,as the associated waste in the biological treatment section,has increasingly serious treatment and disposal problems.Anaerobic fermentation technology can not only promote sludge reduction,but also produce Short Chain Fatty Acids（SCFAs）,which can be used as low-cost carbon source for biosynthesis of degradable plastics（Polyhydroxyalkanoate,PHA）.Aiming at the problems of sludge disintegration,slow hydrolysis rate and difficult accumulation of SCFAs in traditional anaerobic fermentation,a pretreatment of peracetic acid（PAA）combined with free ammonia（FA）was proposed to enhance acid production from sludge and used in Research ideas for the synthesis of PHA by mixed flora.Firstly,the optimal parameters of PAA combined with FA pretreatment to promote acid production from sludge fermentation were studied,and then the mechanism of this method was expounded from the aspects of sludge disintegration,hydrolysis,acidification,methane production,and changes of DOM and EPS in fermentation broth.The microbial community structure was analyzed by high-throughput sequencing technology,and the feasibility of this method to obtain acid-producing fermentation broth for the synthesis of PHA by mixed flora was explored.The main conclusions are as follows:（1）The optimum conditions for PAA combined with FA pretreatment for sludge fermentation to produce acid are PAA and then FA treatment.The dosage of PAA is 0.1 g/g VSS and the concentration of FA is 70 mg/L.On the 5th day of fermentation,the highest acid production rate was 472.17±1.27 mg COD/g VSS,which was 19.7,3.3,and 2.4 times higher than that of the blank group,the single FA group and the single PAA group,respectively,and the highest proportion of acetic acid was 78.8%.Compared with the single treatment,the combined pretreatment further promoted the dissolution of NH4+-N in the sludge;and the reduction rate of sludge organic matter reached 27.45±0.7%after fermentation for 10 days,which was helpful for sludge reduction.（2）PAA combined with FA pretreatment enhanced the effect of sludge fermentation and acid production by promoting sludge dissolution,improving substrate biodegradability,enhancing hydrolase activity and inhibiting methane production.Compared with the single treatment,the combined pretreatment further promoted the dissolution of particulate organic matter in the sludge,and the SCOD,carbohydrate and protein concentrations all increased significantly;SEM and particle size analysis proved its effective cracking of sludge cells and EPS.The DOM and EPS experiments further confirmed that the combined pretreatment stripped and fell off the organic matter in the sludge TB-EPS and transferred it to DOM and LB-EPS,and its release effect on carbohydrates was stronger than that of proteins;3D-EEM analysis found that it significantly improved the biodegradability of sludge DOM and EPS.PAA combined with FA pretreatment promotes the release of hydrolase and inhibits sludge hydrolysis,acidification and methanogenesis,and its inhibition of methanogenesis is much greater than that of hydrolysis and acidification,which leads to the continuous accumulation of SCFAs in anaerobic fermentation.（3）Microbial community analysis showed that PAA combined with FA pretreatment significantly reduced the abundance and diversity of microbial species in sludge,and changed the structure of functional flora.The functional flora analysis showed that it significantly enriched the flora related to hydrolysis and acid production and made it the dominant bacteria,while reducing the abundance of bacteria related to SCFAs consumption.At the phylum level,Firmicutes and Bacteroidetes were significantly enhanced,especially Firmicutes,whose relative abundance increased from 3.61%（blank group）to 63.87%;at the same time,Proteobacteria,Planctomycetes and Chloroflexi.At the class level,Clostridia and Bacteroidia dominate,accounting for 59.61%and 10.60%,respectively;while three classes of Proteobacteria and Anaerolineae are dominant.The bacterial flora that consumes SCFAs was reduced by only 6.70%and 1.37%.At the genus level,a total of 6 fermentative acidogenic bacteria genera were effectively enriched,especially Proteiniclasticum,accounting for 45.55%,much higher than the other three groups（all less than 2.5%）;meanwhile,the relative abundances of Petrimonas and Macellibacteroides were also respectively reached 4.04%and4.57%.（4）The acidification degree of PAA combined with FA pretreatment sludge acid-producing fermentation broth is higher than 85%.When the diluted fermentation broth is used as the substrate,the optimal feeding times under the condition of synthesizing PHA by mixed flora is three times,and its PHA production is the third.The five batches reached 83.63 wt%,and the proportion of HB was 91.89%.In each batch cycle,DO,p H and carbohydrate concentrations all increased,while NH₄⁺-N,PO₄^3--P,SCFAs and protein concentrations all decreased,and the consumption rate of SCFAs was:acetic acid>propionic acid>Butyric acid>valeric acid.The high-load carbon source fermentation broth directly used as a substrate had a certain inhibitory effect on the synthesis of PHA by the mixed flora.In the fifth batch,the highest PHA yield was55.05 wt%,and the proportion of HB was 78.11%.