Analysis Of Spatial Distribution Difference Of Anaerobic Granular Sludge And Its Fluidization Driving Mechanism

"Calcification of granular sludge"is a technical problem plaguing the waste paper industry.The calcification of granular sludge will lead to the inactivation of granular sludge,aggravate the clumping of granular sludge groups,and cause the hardening of the reactor bed and the collapse of operation.Therefore,it is very important to maintain sufficient fluidization in the bed.In the gas-liquid-solid reactor,the fluidization behavior of granular sludge is not only closely related to its own physical and chemical properties,but also affected by the hydrodynamic conditions in the reactor.To the best of our knowledge,the basic characteristics of granular sludge and the movement of biogas bubbles(endogenous bubble adhesion,secondary bubble wake)are the key factors for the fluidization or sedimentation of granular sludge in the reactor.Therefore,the objectives of this study are to explore the differences between the layers of granular sludge blanket based the influence of granular sludge calcification and studies on the mechanisms driving granular sludge fluidization.The main contents are as follows:(1)By comparing the physical and chemical properties of granular sludge taken from calcified and uncalcified reactors at different heights,the effect of sludge calcification on the characteristics of granular sludge and blanket structure was explored.The results show that the calcification behavior of granular sludge will affect the distribution characteristics of sludge in the reactor,which is mainly manifested in the fact that calcified granular sludge tends to deposit at the bottom of the reactor,and the inhomogeneity of particle size distribution among the layers is obvious.The granular sludge bed has the characteristics of layered distribution in the longitudinal distribution.From bottom to top,it is defined as the expansion area,fluidization area and loss area of the sedimentary particle area,and the layered structure of the bed in the calcification reactor.It is more significant;the boundary density between calcified sludge and uncalcified sludge is preliminarily defined as 1.10×10~3kg/m~3.(2)Based on the balance between the buoyancy driven by endogenous bubbles and the additional gravity caused by calcium precipitation,a variable density model driven by bubble attachment buoyancy is constructed by transforming the bubble size distribution probability in the reactor into a variable density probability,in order to judge the floatability of anaerobic granular sludge in a larger particle density and particle size range.The interception probability behavior of granular sludge predicted by the model is consistent with the actual stratification behavior of granular sludge.The formation of granular sludge bed structure is explained from the perspective of attached bubbles for the first time.(3)The evolution behavior of exogenous secondary bubbles in the reactor and the effect of wake on the sedimentation process of granular sludge were studied.The results show that both the bed height and the superficial gas velocity promote the coalescence of bubbles,and eventually the bubbles coalesce,break up and stabilize in a specific size range.The wake effect of the secondary bubbles with stable size is the dominant factor that determines the sedimentation and loss of particles.The sedimentation process of five kinds of particles was monitored,and obvious bubble wake phenomenon appeared.Quantitative research on the wake effect of secondary bubbles shows that the wake effect has the most significant disturbance to the sedimentation process of low-density and small-sized granular sludge,up to 50%.It shows that flocculent sludge is easier to be eluted in the actual anaerobic reactor.The sludge with high density and large particle size is not easy to be disturbed by bubble wake,so it is easier to settle at the bottom of the reactor.The reason for the formation of bed structure is supported from the perspective of external bubbles.