Interactions And Formation Mechanisms Of Microbial Aggregates

Biological treatment is one of the most widely used wastewater treatment processes.The microbial aggregates,including flocs,biofilm and granules,are the key element in the bioreactors.Their structure,shape,function and surface characteristics may significantly influent the stability,efficiency and effluent quality of bioreactors,and play a crucial role in wastewater biological treatments.However, the formation of microbial aggregates is a time-consuming process,and can be influenced by many factors.Therefore,study on the interactions among the microbial aggregates is essential to understand the microbial aggregation process.In this work, based on experimental results and innovational analytical methods,the original and extended DLVO theories are employed for characterizing the surface characteristics, the microbial adhesion and flocculation behavior of microbial aggregates in biological wastewater treatment.The roles of extracellular polymeric substances(EPS)in microbial aggregates are also explored and quantified.Main contents and results are as follows:1.Based on the microbial surface thermodynamic approach,which reflects the bacterial surface characteristics,a simple and effective method for the evaluation and determination of microbial adhesion and flocculation was developed.Taking as a photosynthetic bacterium Rhodopseudomonas acidophila as example,the relationship between its surface characteristics and flocculability was evaluated.The appropriate electrolyte concentration and pH for the strain were found to be 0.1 mol L^-1NaCl solution and pH 7.0,respectively.The addition of Ca²⁺could improve its flocculability.In addition,the EPS produced by the cell were observed to have a significant effect on its flocculation.The effective Hamaker constant and the repulsive total interfacial free energy seemed to be responsible for the poor flocculability of the strain.2.The microbial surface and flocculability were quantitatively characterized through the combination of the surface thermodynamic and the extended DLVO approaches,with Ralstonia eutropha,a polyhydroxybutyrate-producing bacterium,as an example.The total interfacial free energy(â–³G_adh)was changed from -80 mJ m^-2to 28.5 mJ m^-2,and the ratio ofγ_B^LW/γ_B^ABdecreased significantly in its entire growth process.This suggests that the bacterial surface changed from hydrophobic into hydrophilic.As a result,the stability ratio of suspensions increased with the increasing cultivation time.The Lewis acid-base interaction(W_AB)in the extended-DLVO approach could provide an additional asset to the increase or decrease in the total energy barrier,and,therefore,seems play an important role on bacterial suspensions.3.The different cultivation conditions for R.eutropha could be optimized and evaluated based on the extended DLVO theory.It was found that glucose of 30 g/L and pH 8.0 was the proper cultivation condition.The strain also had a good flocculability under the conditions.The energy barrier levels with different substrates were highly related to the substrate type.This could provide a feasible and useful method to qualitatively describe the microbial flocculability based on its substrate type.4.As a rapid,sensitive and selective analytical method,3-dimensional excitation-emission matrix fluorescence spectroscopy was applied to characterize the EPS extracted from the aerobic and anaerobic sludge in wastewater treatment reactors. Two fluorescence peaks were identified at(excitation/emission)280-285/340-350 nm and 340-350/430-450 nm in the aerobic sludge EPS fluorescence spectra.The two peaks were attributed to the protein-like and NADH fluorophores,respectively.The anaerobic sludge EPS fluorescence spectra also had two fluorescence peaks.One peak was similar to that for the aerobic sludge EPS and attributed to the proteins.Another peak corresponded to the F₄₂₀fluorophore,which was the characteristic substance of the methanogens.The differences in the EPS fluorescence parameters,e.g.,peak locations,intensities and ratios of various peak intensities,indicate the difference in the chemical structures of the EPS from various origins.5.The change of sludge flocculability and the role of EPS were evaluated from the EPS extraction experimental results.Both.the loosely bound EPS EPS(LB-EPS) and tightly bound EPS(TB-EPS)could provide an effective contribution on the sludge flocculability.After the extraction of the LB-EPS and TB-EPS,the sludge flocculability decreased significantly.Thus,based on the DLVO theory,the quantification method on the role of EPS in the flocculation was proposed.The results shows the TB-EPS in the actived sludge played a more significant role in the microbial flocculation.6.A global optimization model,based on the molecular mechanics(MM)method, was developed and constructed to simulate the three-dimensional structural morphology of microbial aggregates at a microscopic scale.The DLVO theory, usually used to describe the interactions between particles in colloidal suspensions,is modified to explain the interactions among microbial particles.With the constructed model,some putative aggregate micro-motifs with the microbial particles from 3 up to 100 was acquired.Results suggest that the microbial clusters were built up with the same growth pattern,which were arranged with a multi-shell structural packing style. This model offers a rational platform to investigate the microbial aggregate formation process.