| Aerobic granular sludge(AGS)is a kind of microbial aggregates containing a variety of functional bacteria.Microorganisms are critical to its formation,structural stability and pollutant removal efficiency.A little research is conducted on AGS at the microbial level,which limits our understanding and application of AGS.In this thesis,the impact of microorganisms on the formation,growth and destabilization of aerobic sludge aggregates was explored at the microscopic level.Firstly,the mechanism of microbial community assembly during aerobic granulation was investigated under two influent conditions;Then,the changes of the microbesto-microbes interaction and aggregation performance,together with the microbes-to-surface interaction and the viscoelasticity of the adhesion layer during the growth process of AGS were investigated,this for further understanding the growth and stability of the AGS provides a theoretical basis.The main research contents and conclusions are as follows:(1)Microbial community assembly during aerobic granulation.AGS was cultured under two influent conditions:sodium acetate as single-substrate(R1)and glucose,sodium acetate,sodium propionate,peptone and starch as the mixed-substrate(R2).The result showed that granulation rate of sludge reached 80%on day 65(R2)and 80(R1),respectively.The mixedsubstrate as influent,not only significantly increased the diversity and richness of the microbial community,but also stimulated the secretion of extracellular polymeric substances(EPS),which facilitated the rapid formation of granules in R2.As shown by the co-occurrence network,the species competition was increased and species diversity was decreased in the R1 during the granulation phase,which may have contributed to the slow granulation of sludge.According to the module connectivity(Zi)and among-module connectivity(Pi),in the two groups of reactors,rare taxa accounted for 53.57%in the keystone taxa,indicating that some low-abundance bacteria also play a key role in maintaining system stability during granulation.Infer community assembly mechanisms by phylogenetic-bin-based null model and neutral community model revealed that deterministic and stochastic processes jointly shape the microbial community structure during sludge granulation,in which homogeneous selection(52.83-72.88%)was the most important deterministic process,drift(18.89-34.07%)was the most important stochastic process.There were 51 and 56 groups of microorganisms in R1 and R2,respectively,were regulated by deterministic processes and contributed 28.87%and 36.22%to aerobic sludge granulation,respectively,and were the main contributors to the sludge granulation.Therefore,it is of great significance to select such microbes through deterministic processes,which are conducive to the formation of AGS.This study is helpful to deepen our understanding of microbial ecology in the AGS system and lay a foundation for further research on community regulation strategies during granulation.(2)Aggregation and adhesion behavior of microbes in aerobic granular sludge.In order to simulate the different stages of granules growth,the aerobic sludge was classified into five types of sludge according to its granules size:extra-small(XS),small(S),medium(M),large(L)and super-large(XL).The results showed that from XS to M,the sludge strength and the settling performance improved.Moreover,with the increase of hydrophobicity and the decrease of negative charge and repulsive potential of bacteria,the aggregation performance among bacteria was gradually enhanced,facilitating the growth of granules.At this stage,the EPS content increased significantly,which enhanced the adhesion between microbes and inert material.The internal inorganic core structure was compact and intact,and the adhesion layers of microbes-to-surface became more rigid,making granules exhibit better stability to resist external disturbance.From M to XL,the granules began to crack,or even completely disintegrated.However,during this process,the decrease of hydrophilic functional groups in the sludge EPS and the increase of Lewis acid-base interaction forces lead to the improvement of the microbe-to-microbe aggregation performance.Therefore,with the increase of granules size,the disintegration of AGS was not caused by the decrease of microbial aggregation performance.Moreover,the atomic force microscope and quartz crystal microbalance with dissipation monitoring equipment indicated that the adhesion between bacteria and inorganic kernel is reduced during this growth process,leading to the formation of a soft adhesion layer,which were not conducive to resisting external shear forces and tend to disintegrate.This study confirms the influence of inorganic kernel on the structural stability of AGS,and provides a basis for further understanding the growth or instability of AGS. |
