| Currently, much attention has been paid to resource utilization in sludge treatment and disposal study. Alkaline anaerobic fermentation of sewage sludge is a combined method linking anaerobic digestion with alkaline treatment. It can promote the decomposition of internal organics within sludge into small molecular organics, such as organic acids and proteins. These products are able to be utilized by microorganisms. Therefore, this method can supply cheap carbons for wastewater treatment plant of carbon shortage. But a great amount of ammonia and phosphorus was simultaneously existed in fermented liquor. Thus to remove ammonia and phosphorus as well as to abstract internal carbon source effectively were of great significance in practical production.This research studied thoroughly the impact factors of activated sludge alkaline anaerobic fermentation and fermentation mechanism with semi-continuous and sequencing batch operating mode, by which operating conditions of sludge alkaline fermentation were optimized. Furthermore, it revealed the acid fermentation process in the aspect of sludge flocs characteristics and microorganisms changes, which provided fully theory bases to the process optimization. In the study of fermentation mechanism, three methods including ammonia stripping, adding fermented sludge and sterilized fermented sludge were used to strengthen sludge acid fermentation. In addition, considering the dramatically poor dewaterability of fermented sludge, coagulation-sedimentation and ammonia and phosphorus recovery methods for separating fermentation liquid from fermentation sludge were also adopted. These methods might be beneficial to both the abstraction of internal carbon source from fermentation liquid and improvement of fermented sludge dewaterability and disposal.The effects of temperatures (15°C~55°C) on sewage sludge alkaline fermentation were investigated. It showed that the highest soluble chemical oxygen demand (SCOD) yield was obtained at55°C but with a lower volatile fatty acids (VFAs) yield comparing with temperatures at25°C and35°C. The transferring routine of intercellular organic substances study detected that organic substances were firstly transferred from pellet layer to the slime layer of cellular, then could be converted into VFAs via hydrolysis and acidification. But the acidification of soluble organic substances in the slime layer was restricted at55°C, resulting in a lower VFAs production. According to the microbiology perspective, the number of microorganism communities as well as diversity and species richness at25°C and35°C were higher than those at15°C and55°C. It further explained that temperatures at25°C and35°C are relatively suitable conditions for acid fermentation of sewage sludge and also have better sludge dewaterability after fermentation. In addition, Clodtridium and Bacillus in Firmicutes and Gamma proteobacterium in Proteobacteria were the dominant functional bacterial genus for high VFAs accumulation.We further studied the influences of hydraulic retention time (HRT) and alkali types on sewage sludge alkaline fermentation. The results showed that sludge in NaOH system obtained higher organics yields than that in Ca(OH)2system, including protein, carbohydrate, as well as ammonia and phosphate, because of much stronger hydrolytic acidification force of NaOH agent. But Ca(OH)2system was evaluated more economic than NaOH system at the same HRT condition, especially at longer HRTs, in view of alkali cost and VFAs production. So Ca(OH)2was better alkali reagent choice. Besides, great amount of protein and PO43-may precipitate with Ca2+in Ca(OH)2system by analyzing Ca2+concentration, function groups changes, Zeta potential and settleability in both Ca(OH)2and NaOH systems. This is the main reason for a weaker fermentation performance and a better dewaterability of Ca(OH)2than NaOH system. Acetic acid and propionic acid are dominated in both alkali types of systems. Microorganism communities analysis demonstrated that Firmicutes, Proteobacteria and Actinobacteria were the dominant phylas in both two systems and had a higher abundance in NaOH system, which might account for the better acid production performance than Ca(OH)2system.In addition, ammonia stripping, adding fermented sludge or sterilized fermented sludge were also used to strengthen acid production in sludge alkaline fermentation. The maximum hydrolysis and acidification performance was achieved in the NaOH adjustment and ammonia stripping system. Analysis results showed that ammonia stripping could increase VFAs production by decreasing the concentration of NH4+and relieving the inhibition of NH4+on acid-forming bacteria, and enhancing the fermentation of carbohydrate. But the impacts of ammonia stripping on hydrolysis and acidification of soluble proteins were small. Besides, when added fermented sludge or sterilized fermented sludge to batch-mode activated sludge alkaline fermentation system, both two ways could improve sludge hydrolysis and acidification, especially the former process. Mesophilic condition combined with sterilized fermented sludge adding method could not only greatly enhance sludge hydrolysis and acidification, but also keep VFAs yield maintained steadily in a long operation time. Therefore, adding sterilized fermented sludge was a more effective way to enhance VFAs accumulation in alkaline sludge fermentation process.Finally, the experiments that adding the coagulant aid and coagulant to improve the dewaterability of fermentation sludge showed: The optimum scheme for fermented sludge in Ca(OH)2system was adding sole CPAM, the dosage was0.6g:100gTS. It could achieve90%of organics recovery, CST <50s, SV<60%; While the optimum scheme for fermented sludge in NaOH system was FeCl3dosage of25g:100gTS and CPAM dosage of0.9g:100gTS. It could achieved75%of organics recovery, CST <58.9s, SV<56%. In that case both systems achieved relatively higher organics recovery efficiency and favorable sludge dewaterability. However, the NH4+concentration in both fermentation systems were higher and needed ammonia stripping further. Moreover, in consideration of the higher NH4+and PO43-concentration in fermentation sludge in NaOH system, simultaneous recovery of NH4+and PO43-from the alkaline fermentation sludge by struvite precipitation was adopted. Response surface methodology analysis demonstrated the optimal conditions for recovering NH4+and PO43-was n(N):n(P):n(Mg)=1:1.49:2.14at pH=10.5. The result was84%of NH4+and96%of PO43-were recovered. At the same time, the CST of fermented sludge was decreased to211s. Subsequently, adding CPAM in ratio of0.6g:100gTS could achieve good sludge dewaterability. |
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