Study On The Effects Of Extracellular Polysaccharides On The Stability Of Aerobic Granular Sludge

Aerobic granular sludge（AGS）is difficult to maintain structural stability during long-term operation,which has become one of the main obstacles hindering the practical engineering application of AGS.Extracellular polysaccharides is one of the important components of aerobic granular sludge,as the gel component and structural skeleton of AGS,it is of great significance to the internal mechanism of AGS formation and stability.The thesis uses three modes of rapid water feed direct aeration（R1）,rapid water feed anaerobic stirring（R2）and anaerobic slow plug-flow（R3,R4）to start the AGS reactor.Taking limited-bulking sludge R1,normal sludge R2 and bulking sludge R3 on the 36th day as the research objects,analyze the sludge characteristics,microbial diversity,and extracellular polysaccharide characteristics,Explore the mechanism of viscous bulking of the sludge.Taking the successfully granulated AGS in R1,R2 and R4 as the research objects,analyze the sludge structural stability,microbial diversity and the characteristics of extracellular polysaccharides,and explore the influence of extracellular polysaccharides on the structural stability of AGS under different water intake modes.Carry out enzymatic hydrolysis experiments to explore the influence of extracellular polysaccharides and proteins on the structural stability of AGS.The research helps to understand the influence of extracellular polysaccharides on the stability of AGS and provides theoretical support for the application of AGS process.This paper got the following main conclusions:（1）Different influent modes have an impact on the morphology of aerobic granular sludge.R1,R2 and R4 were successfully granulated within 84 days.MLSS was 4.6 g/L,5.8 g/L and 9.9 g/L in the stable phase,and the granulation rate was close to 90%.The AGS of rapid water direct aeration mode R1 has a large particle size and a large number of filamentous bacteria on the surface.The AGS of fast-inflow anaerobic stirring mode R2 is relatively small,and a large number of cocci can be seen on the surface.The AGS of the anaerobic slow plug-flow mode R4 has a large particle size,the surface is full of bacillus,and it is covered by a large amount of EPS.The COD removal rate at the mature stage of R1,R2 and R4 can reach more than 86%,the removal rate of NH₄⁺-N is close to 100%,and the removal rate of TN is up to 60%～70%.（2）During the start-up period,the sludge characteristics,microbial community diversity and exopolysaccharide characteristics of viscous bulking sludge and normal sludge are different.As the sludge viscous bulking deepens,the sludge aggregation capacity is weakened,the flocculation efficiency becomes worse,and the electronegativity and hydrophilicity increase.The content of exopolysaccharides in the viscous bulking sludge was significantly increased,and the hydrophobicity of exopolysaccharides was weakened.The abundance of the polysaccharide-secreting bacteria Flavobacterium was much higher than that of normal sludge.The analysis of exopolysaccharide functional group found that the O-H deformation vibration between the exopolysaccharide and the bound water in the bulking sludge is enhanced,and the exopolysaccharide can absorb a large amount of bound water to enhance the water retention of the sludge.The viscous bulking of the sludge is caused by the excessive secretion of the hydrophilic exopolysaccharide.Correlation analysis shows that the higher sludge load F/M in the reactor will lead to the enrichment of Flavobacterium,which secretes excessive exopolysaccharides,which in turn reduces the hydrophobicity,increases the electronegativity of the sludge,the aggregation and sedimentation performance deteriorates,and the sludge occur to viscous bulking.（3）The structural stability of aerobic granular sludge cultivated with different water intake modes is different.The AGS of anaerobic slow push-flow inlet mode R4 has the strongest structure stability,the structure stability of AGS of fast water inlet anaerobic stirring mode R2 takes the second place,and the structure stability of AGS in fast water inlet direct aeration mode R1 is the weakest.AGS surface characteristics and XDLVO analysis show that the stronger the hydrophobicity and the weaker the electronegativity of the sludge surface is,the smaller free energy of interfacial adsorption?₍(6（9?）and the total potential energy W_TOT energy of the system is,the easier the sludge is to aggregate,and it exhibits stronger structural stability.The microbial composition of AGS is different under different water inlet modes.The dominant genus of rapid water direct aeration mode R1 is filamentous bacteria Neomegalonema,and the dominant genus of rapid water anaerobic stirring mode R2 are glycan bacteria Candidatus＿Competibacter and unclassified＿f＿Comamonadaceae.The dominant bacteria in the anaerobic slow push-flow inlet mode R4 is glycan bacteria Candidatus＿Competibacter,which grows slowly and can secrete exopolysaccharides with strong gel properties,which helps maintain the stability of AGS.The properties of exopolysaccharides have an impact on the structural stability of AGS.The content of exopolysaccharides is significantly positively correlated with the structural strength of AGS.The microscopic observation of polysaccharides shows that the degree of branching and gel properties of exopolysaccharides help maintain the stability of AGS.Functional group analysis found that the hydrogen bond cross-linking,gel properties,α-configuration bridging and hydrophobicity of exopolysaccharides help to improve the structural stability of AGS.（4）Enzymatic hydrolysis of the exopolysaccharide and protein components of AGS,found that the structural strength and storage modulus of AGS were significantly reduced after the exopolysaccharide was hydrolyzed,indicating that exopolysaccharide played an important role in the structural stability of AGS.The rheological properties test showed that the gel properties of AGS were derived from exopolysaccharides rather than extracellular proteins.The contact angle of sludge decreased significantly after exopolysaccharide hydrolysis,indicating that exopolysaccharides can maintain the structural stability of AGS through hydrophobicity.After exopolysaccharide and protein hydrolyzed,the free energy of interface adsorption?₍(6（9?）and the total potential energy W_TOT decreased,indicating that both exopolysaccharide and protein contribute to the aggregation performance of AGS.EPS hydrogen bond analysis showed that exopolysaccharides dominate the intermolecular hydrogen bonds in EPS and make a major contribution to the gel properties of AGS.Protein secondary structure analysis showed that extracellular proteins mainly affect the aggregation ability of sludge through hydrophobicity and thus contribute to the stability of AGS.