| With the rapid development of our country's economy,municipal wastewater production has been increasing year by year,and the waste activated sludge(WAS),as a major byproduct of the treatment process in municipal wastewater treatment plants,also has a large increase in its annual production.The structure of the WAS is complex,containing large amount of organic materials that can be utilized.It is simple and feasible to use anaerobic fermentation to convert the organic matter in the waste activated sludge into volatile fatty acids(Volatile Fatty Acids,VFAs).The key to affecting the yield of VFAs is how to efficiently use the organic matter in the waste activated sludge.Alkaline pretreatment has the advantages of good treatment effect,low cost and simple operation.In this paper,alkaline pretreatment of the waste activated sludge was used for anaerobic fermentation to study the composition and distribution mechanism of organic matter in extracellular polymers during anaerobic fermentation,especially the influence of the distribution and dynamic changes of humus on the yield of VFAs.And the introduction of calcium peroxide combined with alkaline pretreatment can increase the degradation rate of humus in the waste activated sludge,maximize increase the yield of VFAs.The specific research content of this study is as follows:The research content is as follows:(1)Research on the performance of alkaline pretreatment to promote the production of VFAs by anaerobic fermentation of WASThe alkaline pretreatment promoted the cracking of macromolecular organic matter flocs in the WAS and also the conversion of organic matter to VFAs.Protein,polysaccharides,chemical oxygen demand,and VFAs production were used as indicators to explore the relative best p H value of alkaline pretreatment to promote anaerobic fermentation of waste activated sludge.After the alkaline pretreatment,the protein dissolution effect was improved with the increase of alkalinity.The relative optimal p H obtained in the experiment was 12(expressed as A-12).At this time,the protein concentration was reduced from the initial 2415.48 mg/L to743.14 mg/L.The degradation rate was 69.23%,and the protein degradation rate of the raw mud control group was 60.7%;the production of VFAs was also improved.The A-12 group had the best effect for promoting the production of VFAs,reaching 378.59 mg/L,which compared with the control group(185.15 mg/L)had increased by 104.47%.(2)Alkaline pretreatment promotes the degradation mechanism of WAS macromolecule organic matter in the anaerobic fermentation processIn order to explore the specific degradation and utilization of organic matter in the alkaline anaerobic fermentation process of WAS,the digestion broth of the three different digestion stages of WAS was detected,including three-dimensional fluorescence,parallel factor analysis and fourier infrared detection,etc.,used to characterize the distribution of organic components in different extracellular polymer layers of the WAS during anaerobic fermentation.The results of three-dimensional fluorescence detection showed that the fluorescence intensity of group A-12 before anaerobic fermentation reached 15×105,which was 2.14 times that of the original mud(7×105).After the anaerobic fermentation,the organic matter in the loosely bound extracellular polymer in all the experimental groups was completely degraded;parallel factor analysis showed that there were still a large amount of refractory organic matter residues in the fermenter at the late anaerobic stage;fourier infrared detection results show that the A-12 group had a new peak appeared at 1922 cm-1 and the peak at 1362 cm-1 also increased,indicating that the protein content increased and the aromatic structure functional group content increased significantly.The alkaline pretreatment caused the protein structure in the remaining sludge to be destroyed.Model compounds such as bovine serum albumin(BSA),L-glycoside,L-alanine,glucose,etc.,was used to simulate hydrolysis and acidification processes to compare the degradation rate of different organic matter in anaerobic fermentation,and explore the degradation mechanism of different kinds of organic matter in the hydrolysis and acidification stage.The results showed that in the hydrolysis stage,the degradation rate of BSA was as high as 19.51%,and the degradation rate of L-glycoside was 10.56%;in the acidification stage,the amount of VFAs produced in BSA synthesis wastewater was as high as 589.54 mg/L,and the amount of L-glycoside synthesis wastewater was as high as 387.50 mg/L.It indicated that the effect of alkaline pretreatment on anaerobic fermentation of protein organic matter was better than polysaccharides.(3)Calcium peroxide combined with alkaline pretreatment to promote the degradation of refractory organic matter in the WAS to produce VFAsBased on the optimized alkaline pretreatment results,the addition of different concentrations of calcium peroxide(Ca O2)further promoted the degradation of humus in the WAS to produce VFAs.The concentrations of calcium peroxide were 0.01,0.05,0.1,0.2 g/L(expressed as A-12-Ca O2-0.01 g/L,A-12-Ca O2-0.05 g/L,A-12-Ca O2-0.1 g/L,A-12-Ca O2-0.2g/L).The experiment mainly explored the changes in the degradation rate of refractory organic matters,humification,polymerization,and the corresponding changes in the distribution of microbial communities with the production of VFAs in different stages of anaerobic fermentation.The results showed that the A-12-Ca O2-0.1 g/L groups had the highest acid production(964.67 mg/L),which was 421.02%higher than the control group's VFA(185.15mg/L);in addition,the value of SUVA465/SUVA665(SUVA465 represents the absorbance corresponding to the unit total organic carbon wavelength at 465 nm)of A-12-Ca O2-0.1 g/L increased from 0.55 in the initial to 3.6,that is,the degree of humification of the waste activated sludge after the combined pretreatment was significantly reduced;microbial community distribution analysis showed that Acidobacteriota accounted for 17%,and Acetothermia accounted for 8.6%in A-12-Ca O2-0.1 g/L group.Compared with the raw sludge and A-12 groups,the content of these two types of microorganisms increased significantly,and both were beneficial to the increase of VFAs production.The highest VFAs yield in A-12group was 378.59 mg/L,while the highest VFAs yields in A-12-Ca O2-0.1 g/L groups was964.67 mg/L,which was increased by 154.93%compared with A-12 group.The results verified that the combined pretreatment using calcium peroxide and alkaline not only promoted the transformation efficiency of organic matter,while effectively inhibited the growth of methanogens,exhibited synergistic effects,and provided a new idea for the recycling of WAS. |
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