Investigation On Sludge Dewaterability And Biogas Generation During Anaerobic Digestion Through Regulating The Changes Of Nitrogenous Compounds In Municipal Sludge

Municipal sludge is composed of aggregates of EPS?Extracellular Polymeric Substances?secreted by various functional microorganisms.Due to the hydrophilic components presented in EPS,sludge is typically difficult to be dewatered.Simultaneously,the efficiency of anaerobic digestion is limited by the low hydrolysis efficiency and the low biodegradability of sludge.Protein is one of the main components of sludge EPS,and it plays a critical role in sludge dewatering and anaerobic digestion.However,the influences of protein type,secondary structure,molecular weight and content on sludge dewaterability are still unclear.In addition,there is still no clear understanding of the migration and transformation of proteinaceous compounds?e.g.,protein and amino acids?during sludge anaerobic digestion.Therefore,this thesis proposed a new sludge dewatering method based on targeted precipitation of the hydrophilic protein.In addition,this study investigated the migration and transformation of nitrogenous substances during sludge anaerobic digestion using different pretreatment methods.The main contents include:1.Enhanced sludge dewaterability based on the targeted precipitation of the hydrophilic proteinAmmonium sulfate??NH₄?₂SO₄?was used to enhance sludge dewaterability through precipitation of the proteinaceous compounds.Compared with raw sludge,87.59%reduction of capillary suction time?CST?and 88.17%reduction of specific resistance to filtration?SRF?were achieved with the addition of 60%saturation of?NH₄?₂SO₄,and 38.49%reduction of protein content in the liquid phase was also noted.With the addition of?NH₄?₂SO₄,the content of?-helix was increased and the content of?-sheet was decreased in the liquid phase.In the solid phase,the content of protein-N was increased and larger protein aggregates were formed after the addition of?NH₄?₂SO₄.The?NH₄?₂SO₄ addition might destroy the hydration layer of proteins and enhance the hydrophobicity of sludge solid phase.A sludge dewatering mechanism can be proposed:?1?The added?NH₄?₂SO₄ may destroy the cell membrane structure,thus leading to cell lysis and the release of bound water;?2?The proteinaceous matters,particularly protein type of aromatic protein II,can be precipitated by?NH₄?₂SO₄,thus resulting in the shift of proteinaceous compounds from liquid phase to solid phase.Therefore,sludge dewaterability can be significantly enhanced by using ammonium sulfate at proper concentrations.2.Effects of different pretreatments on the changes of nitrogenous substances and biogas production during anaerobic digestionThe changes of the content,distribution and species variation of nitrogenous substances in sludge were investigated by different pretreatment methods?e.g.,ultrasonic,acid,alkali and thermal pretreatment?.Subsequently,the pretreated sludge was subjected to anaerobic digestion for 30 days to investigate the variations of biogas production during anaerobic sludge digestion.Finally,Pearson's correlation analysis was carried out to analyze the relationship between characteristics of nitrogenous substances in sludge and the cumulative biogas production of sludge anaerobic digestion.Hence,the key nitrogen-containing substances that affect the anaerobic digestion performance of sludge can be identified.The results showed that the TOC?Total Organic Carbon?,SCOD?Soluble Chemical Oxygen Demand?and protein content in the sludge S-EPS?Soluble-EPS?after pretreatment all showed significant positive correlations with the cumulative biogas production.In addition,the biogas production of sludge showed a strong positive correlation with tryptophan protein,aromatic protein I and fulvic acid in sludge S-EPS.This indicated that the above three types of nitrogen-containing substances played more important roles during the anaerobic digestion of sludge.The cumulative biogas production of sludge also showed a positive correlation with aspartic acid,threonine and cysteine content.Among them,the correlation between anaerobic digestion biogas production and cysteine content was the strongest,and its R²?correlation coefficient?was 0.77?p<0.01?.3.Transformation of nitrogenous substances and biogas production in anaerobic digestion of sludgeThe characteristics of nitrogenous substances in the whole process of sludge anaerobic digestion were investigated.Pearson correlation analysis and principal component analysis were carried out on the characteristics of nitrogenous materials and sludge biogas production,and a SEM?Structural Equation Model?was constructed.The results showed that the total protein,tryptophan protein,aromatic protein I and fulvic acid content in the sludge before and after pretreatment showed a strong negative correlation with the biogas production of sludge,indicating that the degradation rate of above substances in the process of sludge anaerobic digestion can directly affect the production of biogas.Therefore,it is necessary to accelerate the degradation rate of the above organic matter.Through the analysis of 17 amino acids,it was found that the content of cysteine showed a significant positive correlation with the biogas production of sludge.This indicates that the accumulation of cysteine in the anaerobic digestion process has a certain promoting effect on the biogas production.According to the results of SEM,the protein content and cysteine content in the sludge anaerobic digestion can directly affect the final biogas production of the sludge?R²=-0.428,p<0.001,R²=0.752,p<0.001?,playing an important role in the anaerobic digestion and gas production process.