Sedimentation of Activated Sludge Under Different Shear Conditions

Laboratory experiments were carried out using five identical sequencing batch reactors (SBR) to investigate the effect of shear conditions on sedimentation of activated sludge in wastewater treatment. All of the aerated stirred bioreactors, which volume was 2L each and were seeded with activated sludge, were fed at the same rate with synthetic wastewater. Shear conditions were changed by adjustment of stirring paddle in SBR and were analyzed by the technology ofPIV (Particle Image Velocimetry). Based on analysis, the turbulent dissipation rate of specific area was used to evaluate the shear conditions in aerated stirred bioreactors. Attempts were made to relate the settlement characteristics of activated sludge with turbulent dissipation rate. The results showed that using the turbulent dissipation rate in specific area to evaluate the shear conditions in stirred bioreactors was effective. Sedimentation of activated sludge in bioreactors is sensitive to variable shear condition. As the stirring speed rose, the turbulent energy dissipation of different parts in bioreactor increased gradually, the settleability of activated sludge was improved first and then deteriorated The best indexes of turbulent dissipation rate in the experiment were 2.45 times 10^-3 m²/s³ of main part and 7.75 times 10^-6 m²/s³ of other part. Settleability of activated sludge can be improved in practice by declining the turbulent dissipation rate of main part and raising that of main part.