Study On Formation And Structural Stability Of Aerobic Granules Enhanced By Different Flocculants

Over the past two decades,Aerobic Granules?AGS?technology has received more and more attention and research.Compared with ordinary floc activated sludge,aerobic granular sludge has higher sedimentation capacity to improve reactor operating efficiency and stronger sludge-water separation capacity to ensure effluent water quality and strong bio-adsorption capacity for treating industrial wastewater.The unique layered structure can realize the advantages of simultaneous nitrification and denitrification?SND?.However,the granulation is too time-comsume,and during long-term operation may cause the AGS to disintegrate due to the severe impact of water quality and quantity,the invasion of toxic and harmful substances.So the sedimentation performance deteriorates,eventually leading to the effluent water quality has deteriorated.These two major obstacles prevent the widespread use of aerobic granular sludge technology.In this study,flocculants were added to accelerate the granulation process and improve AGS's structural stability.The selected flocculants include a novel microbial flocculant?MBF?,poly-aluminum chloride?PAC?,and inert carrier granular activated carbon?GAC?.The main research contents are:morphological changes of microbial aggregates during granulation,changes in biomass,changes in sedimentation performance,changes in zeta potential,ability of AGS to remove pollutants,and dissolved oxygen?DO?distribution in granular sludge.Distribution of main components of EPS of granular sludge,resistance to hydraulic shear and hydrolase,and microbial community structure.The formation mechanism of different AGS enhanced by flocculants was summarized.Morphological observation of AGS showed that the mature time in the four reactors?control group,MBF group,PAC group,GAC group?were:52nd day,38th day,22nd day,37th day.After mature,theAGS in the control group and the MBF group showed mesh structure,while the surface structure of the AGS in the PAC group and the GAC group was smooth and compact.All the mature AGS had good sedimentation performance,and the granular size of the four groups were all in the range of 0.6-1.6mm,and there was no significant difference between the groups.The microelectrode probes were used to determine the dissolved DO showing diffusion depth of the aerobic granular sludge in the four groups?control group,MBF group,PAC group,GAC group?were:320?m,240?m,160?m,160?m,respectively.The AGS of the four reactors showed no significant difference in the removal of COD and ammonia,and the effluent water quality could be stabilized to the first grade A standard.Since phosphate promotes the formation of AGS,the TP in the effluent during the granulation process continued to decrease and finally reached Grade B.Before the granulation of the four reactors,the TN in the effluent was about 15 mg/L.After granulation,the microbial aggregates showed obvious simultaneous nitrification and denitrification?SND?phenomenon.The TN in effluent was 10 mg/L,below the first level A standard.In the four reactors,the specific concentration of protein PN in the EPS of the microbial aggregates was increased first after the addition of PAC,from 20 mg PN/g MLVSS in the floc state to 140 mg PN/g MLVSS in the granular state.The second increaseing were the GAC group and the MBF group,and the control group grew the slowest.Fluorescence in situ staining of EPS showed that the extracellular protein?important functional substances?and the extracellular?-polysaccharide?key structural substances?was evenly distributed in the aerobic granular sludge of the control group and MBF group.The PAC group formd a double layer structure of"protein shell-?polysaccharide core".The high density extracellular protein closely surrounded the carrier granular activated carbon GAC.The structural stability test showed that the three flocculants can significantly improve the resistance to shear forece.The lowest relative resistance in the four groups?control group,MBF group,PAC group,GAC group?were:82.1%,87.8%,89.7%,and 88.2%,respectively.The results of resistance to hydrolase test showed that the structural stability of AGS in the control group and MBF group was maintained by extracellular?-polysaccharide.The structural stability of AGS in PAC group was maintained by polyaluminum chloride.In GAC group,the outer layer structure was maintained by extracellular?-polysaccharides,and the inner layer structure was maintained by extracellular proteins.High-throughput sequencing of microorganisms in the four reactors showed the addition of flocculants did not significantly change the diversity of microbial communities at the phylum level.At the genus level,flocculants had the function of selecting different functional microorganisms.The relative abundance of heavier microorganisms?such as Micropruina?which can be improved by PAC.And PAC also can increase the relative abundance of EPS producers.Adding MBF can increase the relative abundance of heavier microorganisms,and increase the genus?such as Flavobacterium?which can hydrolyze complex organic matter,but reduce the relative abundance of EPS producer.GAC only can increase the relative abundance of EPS producer.According to the results and discussion,the formation mechanism of ordinary AGS and MBF-enhanced AGS accorded with the"EPS hypothesis".The formation mechanism of PAC-enhanced AGS was mainly determined by PAC,because of the electric neutralization of Al³⁺and polymer adsorption bridging of PAC.The formation mechanism of GAC-enhanced AGS combined the"crystal nucleus hypothesis"and"EPS hypothesis".In summary,all three flocculants can significantly accelerated the formation of aerobic granules and improved their structural stability.The best efficiency and effect was PAC.Although the new MBF was not as good as PAC,it had certain environmental significance as a new idea and new direction for the treatment and disposal of excess sludge.