Study On The Effect Of Extracellular Polymeric Substances On The Formation And Stabilization Of Aerobic Granular Sludge

Aerobic granulation, a novel environmental biotechnological process, was increasingly drawing interest in the area of biological wastewater treatment, it was exciting research work that explored beyond the limits of aerobic wastewater treatment such as treatment of high strength organic wastewaters, removal of nitrogen, phosphate, and bioremediation of toxic aromatic pollutants. But the formation mechanism of aerobic granule is unclear, and accordant viewpoint is that the formation of granule is related to extracellular polymeric substances (EPS). The effect of EPS on formation and stabilization of aerobic granule were discussed in the paper. Results are as follows:1. Six different physical and chemical methods were chosen for extracting EPS from activated sludge and granular sludge. Through comprehensive comparing extraction efficiency and destruction of cells about different extraction methods, 60℃water-bathing was finally chosen for quantitative analysis of EPS. Analysis for EPS extraction by different methods showed that quantity of EPS in aerobic granules was higher than that of activated sludge, and proteins were more dominant than polysaccharides in all sludge samples.2. The SABR reactor was started under low hydraulic shear stress (StageⅠ). Under the combined regulation of hydraulic and loading selection pressures, the control of Filamentous Sludge Bulking (StageⅡ) and multiplication of aerobic granules (StageⅢ) were effectively realized. Eventually, the tiny and homogeneous aerobic granules were formed when the C/N ratio was adjusted from 16 to 28 (StageⅣ). Aerobic granulation process was accompanied by the improvement of settling ability, increases of biomass concentration, and decrease of sludge loading rate.More aerobic granules were found in stageⅢ, and the EPS content increased significantly, especially PN content which was raised from 61.8 mg·gVSS^-1 to 147.7 mg·gVSS^-1. The process was accompanied with the increasing lanes in the SDS-PAGE. In stageⅣ, the relative content of PS was increased, and complete granulation was obtained in the reactor, with PN/PS decreased. The granular reactor operated very well with the PN/PS ration controlled between 1.0 and 1.4. Thus, it was expected that the increase of PN content and proteins varieties played an important role in the initial formation of the granule, the PN/PS ratio had an great influence on the surface charge and hydrophobicity of aerobic granule, and could effectively regulate aggregation morphology of the microorganism, appropriate PN/PS could enhance structural performance of the granules.3. The hydraulic shear stress had a great influence on the structural stability of aerobic granules and production of EPS. When R1 was operated under lower hydraulic shear stress (surficial gas velocity was 0.5cm/s), the PS content was decreased, however, the PN content was increased, with the ratio of PN/PS significantly improved reaching as high as 3.01, which was accompanied by rapid disintegration of the granules. When R2 and R3 were operated under higher hydraulic shear stress (surficial gas velocity in R2 and R3 were 1.5cm/s and 3.5cm/s, respectively), PN and PS were both increased to some degree, a relatively stable ratio of PN/PS was obtained in the two reactors, achieved 1.39～1.81 and 1.15～1.39 respectively, at the same time the granule structure was maintained in the two reactors. Moreover, compared R2 with R3, it was found that relative higher content of PS , then lower PN/PS ratio in EPS could improve the strength of aerobic granules to ensure that more granules were obtained. Thus, appropriate ratio of PN/PS was important for the maintenance of granule structure, while lower PN/PS ratio was beneficial to the stability and reinforcement of aerobic granules. In a single operation period, PS was significantly consumed under higher hydraulic shear stress. When there was a longer starvation time, the production and consumption of PS might be resulted by microbes' resistance to hostile environmental through controlling their energy metabolism. The production and consumption of PS kept the PN/PS ratio within a particular range during the operation period to realize the stability of granule structure. The in-situ fluorescence staining results showed that PN was distributed in the whole zoogloea, and PS was mainly distributed among the zoogloea. So PN and PS were expected to build the granule structure by jointing together, and formed the backbone of the whole granule with embedded cells to support the mechanical stability of granules.PCR-DGGE analysis indicated that microbial population differed obviously in all three reactors, which operated under different hydraulic shear stress. Phylogenetic analysis indicated that Betaproteobacteria, Alphaproteobacteria, Flavobacteriales were the dominant classes in the three reactors, and the predominant bacterias in low and high hydraulic shear stress conditions were closely related to Chryseobacterium sp. and Balneimonas sp. respectively. It was expected that flocculability of the sludge was closely related to the EPS, which finally determined the maintenance of the granule structure.