| In recent years,attention has been turned to developing aerobic granular sludge?AGS?for its regular,strong microbial structure,good settling ability and the ability of withstanding shock loading rate,which has obvious advantages in the biological treatment of wastewater.However,theory on formation and control of AGS is still not completely understood.Aerobic granules have mostly been cultivated in bubble column reactors with the ratio of height to diameter?H/D?of more than 5.Compared with other configurations,the higher hydrodynamic shear force,longer circulation path and higher sludge collision frequency provided by higher ratio of height to diameter making flocs much easier to aggregate.But,reactors with higher ratio of height to diameter has difficulties in engineering application such as high cost and operation management.Most of the related researches were complicated in operation conditions or long in granulation time in recent years.Therefore,it is necessary to study the mechanism of aerobic sludge granulation in reactors with low ratio of height to diameter.In this study,two cylindrical SBRs with low ratio of height to diameter?H/D=1.2?were established to cultivate aerobic granular sludge.The hydraulic shear force was provided by horizontal mechanical stirring?R1?and aeration?R2?,respectively.The characteristics of bioparticles and effect of in granulation process and effect of flow regime on account of different shear conditions were analyzed.And then the average shear rate which mature granules suffered in R1 was analyzed quantitatively.It is expected that the results provided here would be useful for further understanding of the granulation in reactors with lower ratio of height to diameter.The main results and conclusions are made as follows:1.On account of two different shear conditions,granular sludge could be cultivated successfully.In R1,the aerobic sludge was granulated with the mean diameter of 1.12 mm and SVI of 41.96 mL·g-11 on 30th day.In R2,the aerobic sludge was granulated with the mean diameter of 1.67 mm and SVI of 55.78 mL·g-11 on 35th day.But granules disintegrated soon.2.In R1,the average shear rate acted on the surface of mature granules was positively related to rotation speed and superficial gas upflow velocity,but the rotation speed had a more contribution on shear rate.The exponent of rotation speed was 37.48times to the exponent of superficial gas upflow velocity.The average shear rate of mature granules was 27.25s-1,and the shear stress was 3.38×10-2N·m-2.The mean diameter of mature granules was in the dissipation range corresponding to shear stress.It is noted that the shear stress acted on the surface of granules was caused by energy dissipation generated by the eddy flow.3.Due to different size of shear stress during the sludge granulation,characteristics of mature granules formed in R1 and R2 were all different.4.In R1,the eddy secondary flow pattern with appropriate shear force caused by transverse rotation of stirrer was essential to aerobic sludge granulation in the reactor with low ratio of height to diameter.In R2,the position deviation of the aeration sparger optimize the flow regime of bubble column bioreactors with low ratio of height to diameter,which is helpful for the formation of aerobic granules.When the hydrodynamic shear is suitable,the flow regime in the reactor is the key for the aerobic sludge granulation. |
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