Mechanisms And Mathematical Modeling Of Activated Sludge Granulation

Aerobic granulation is a process that granular sludge replaces the sludge flocsas the main sludge form. Due to the advantages such as denser structure, bettersettleability and higher biomass concentration, aerobic granulation is considered tobe a promising wastewater treatment. Therefore, fast and stable granulation of thesludge in a biological wastewater treatment reactor is a new direction in the field ofbiological wastewater treatment in recent years. In this study, aerobic granules werecultivated in a sequencing batch reactor fed with sucrose synthetic wastewater.Mean diameter of the sludge increased from71μm to860μm after70daysoperation, and the steady system was composed mainly of granular sludge， with asmall amount of flocs.Granules and flocs were used in the competitive growth experiment. Flocsconsumed substrate faster and grew more under the same conditions withoutsettlement selection. Floc sludge had the advantages of substrate degradation andgrowth compared to the aerobic granular sludge.The intensified breakages of aerobic granules were studied under shear forceand strong acid. It took15hours of the aerobic granules to break up by stirring, andthe mean diameter of the sludge decreased from1230μm to320μm. The resultswere similar to the breakage in strong acid, but the breakage time was110hours. Itwas found that the both the external shear force and the reducing activity of theinternal structure led to the breakdown and disintegration. The breakage process ofaerobic granules could be divided into four phase: log phase, gentle phase,secondary log phase and steady phase.With the increasing of settling height, the concentration of suspended solids,the specific gravity and the number of granules increased. In the lower layer of thereactor, the roundness of granules was higher, and the complexity was lower, thatwas, the granules at lower layer were more regular.According to the mechanisms of granule growth and settlement mechanism,the mathematical model was developed. Based on the model sensitivity analysisand parameter estimation, the results of model were calculated as follows: Thesettling time of the reactor was calibrated as110seconds when the settling heightof the reactor was40cm, respectively. It was also found that when the organic loading was determined to be8kg COD/m³d, the maximum growth rate wascalibrated as0.22h^-1, and the mean diameter of the sludge would reach1.32mmwhen stable granulation was achieved by300cycles’ operation of the reactor. Themodel was verified with experimental data and provided guidance on controllingthe characteristics of granular sludge and achieving sludge granulation.