| The output of waste activated sludge isincreasing,as the increasing social investment in sewage treatment,and the disposal of waste activated sludge has become one of the most important problems all over the world.Among various waste activated sludge treatment technologies,anaerobic digestion technology is widely used because of its low energy consumption,low operating cost,and realizing energy recovery.However,lower energy conversion limits the further development of anaerobic digestion.Studies have shown that some pretreatment methods and the addition of exogenous substances could promote anaerobic digestion efficiency.However,these studies mainly focused on single factor,which have limited effect on the promotion of anaerobic digestion.In order to achieve better anaerobic digestion,this study combined the thermalalkali pretreatment technology with the iron-carbon combined technology to carry out the research on enhanced anaerobic digestion.The experiment pretreated the sludge by setting different temperature and pH.Then evaluated the cracking of the sludge based on the water quality and the sludge index,and explored the optimum combination of temperature and pH for sludge cracking.After that,the strengthening effect of the combination on sludge anaerobic digestion was explored by adding iron and carbon.Meanwhile,combined with biotechnologies,such as high-throughput sequencing(miseq),the microbial community structure of sludge after anaerobic digestion was analyzed to reveal the mechanism of microbial function and the mechanism of combination of iron and carbon.Finally,the applicability of the combined technology was explored by changing the solids content of the waste activated sludge,which provided a new idea for the recycling and reduction of the waste activated sludge.This study finally came to the following conclusions:(1)The content of SCOD increased with the increase of temperature and pH after thermal-alkali pretreatment.The dissolution rates of SCOD in experimental group B3(70℃,pH 12),C2(90℃,pH 11)and C3(90℃,pH 12)were the highest,51.79%,53.87%and 51.29%,respectively.The increase of pretreatment temperature and pH was beneficial to the dissolution of protein and polysaccharide.The contents of soluble protein and soluble polysaccharide were the highest in B3,C2 and C3,which were 3704.5 mg/L and 1379.5 mg/L,3557.1 mg/L and 1438.2 mg/L,3647.8 mg/L and 1456.2 mg/L,respectively.In this study,the influence of pH on the dissolution of proteins and polysaccharides was greater than that of temperature.From the perspective of waste activated sludge reduction,the removal rate of VSS increased with the increase of temperature and pH.The results of anaerobic digestion showed that the highest cumulative methane production in anaerobic digestion were B3,C2,C3,and the increase rate were 64.1%,71.3%and 78.2%,compared with the control group.Considering the resource utilization and economy,the optimum conditions are 70℃and a pH of 12.(2)After pretreatment of waste activated sludge under optimum conditions,the experiments of hydrolytic acidification and anaerobic digestion with addition of iron and carbon were carried out.Hydrolysis acidification experiment showed that the highest degradation rate of SCOD were P2(Fe 10 g/L,GAC 40 g/L)and P4(Fe 20 g/L,GAC 40 g/L),which were 23.3%and 24.1%respectively.The group with the highest acetic acid contents were P2 and P4,and the contents were 2392.7 mg/L and 2529.2 mg/L,accounting for 49.7%and 50.4%of VFAs,respectively.The combined addition of iron and carbon promoted the hydrolysis and acidification process of the waste activated sludge more effectively than the separate addition of iron or carbon.The promotion was further enhanced by the addition of carbon,although the amount of iron added decreaaed.In addition,the separate addition of carbon does not significantly promote the hydrolysis acidification.The results of anaerobic digestion showed that the process of methane production was enhanced by the combination of iron and carbon.The cumulative amount of methane produced by P2 was the highest,reaching 258.7 ml/g-VSS,which was 35.0%higher than the control group.Meanwhile,P2 completed methane production faster than other groups,and the methane content was also the highest,reaching 61.4%,which was 9.7%higher than the control group.On the other hand,the lowest cumulative CO2 production was P2 and P4,with yields of 148.6 ml/gVSS and 141.3 ml/g-VSS,respectively,accounting for 35.8%and 33.5%of the total gas production,respectively.These results indicated that the combined addition of iron and carbon has a significant promoting effect on anaerobic digestion.Moreover,the promotion was the best when the amount of iron and carbon added was 10 g/L and 40 g/L,respectively.The combined addition of iron and carbon increased the dissolution of iron and strengthened the effect of iron.The dissolution of iron ion in P2 was greater than that of P3,indicating that the addition of carbon was helpful to save iron consumption.The best reduction of VSS were achieved in P2 and P4 groups,with the removal rate reaching 54.7%and 56.3%,respectively.Microbiological analysis showed that iron-carbon combination promoted hydrolysis acidification towards acetic acid and thereby increasing acetic acid yield.The growth of Methanosaeta was promoted,and the abundances of Methanosaeta in P2 and P4 were 43.6%and 52.2%,respectively.In addition,due to the combination of iron and carbon,the role of iron was further exerted,enriching the abundance of hydrogen-consuming methanogens.(3)The results of thermal-alkali pretreatment experiment showed that appropriate increase of solid content was conductive to the dissolution of SCOD,protein and polysaccharide in the pretreatment of waste activated sludge.Sludge with 6%solid content had the best decomposition while the dissolution of SCOD was 23257.6 mg/L,which was increased by 28.6 times.The growth ratio of soluble protein and soluble polysaccharide was increased by 26.9 and 30.5 times,respectively,and the sum of the content accounted for the highest proportion of SCOD,which was 69.1%.The results of gas production of anaerobic digestion showed that the group with a solid content of 6%had the highest cumulative methane production(301.1 mL/g-VSS)and the highest methane content(57.7%).The data showed that the solid content of 6%was a turning point.When the solid content was lower than this solid content,the cumulative methane production would increase with the solid content,and when the solid content was further increased,the cumulative methane production would decrease.From the perspective of completion time,the increase of solids content would prolong the completion time of anaerobic digestion.When the solid content of waste activated sludge was less than 6%,the increase of solid content contributed to the dissolution of iron,however,the higher solid content was not conducive to the dissolution of iron.The increase of the solid content of waste activated sludge was not contribute to the enrichment of Methanoseata,and the highest abundance of Methanoseata was found in the group with a solid content of 4%,which abundance was 50.7%.The sludge with a solid content of 4%-6%would be more beneficial to methane-consuming methanogens enrichment(the combined abundance of Methanobacterium and Methanolinea reached a peak of 44.9%),and it would be more beneficial to the joint effect of iron and carbon. |
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