The effect of colloidal stability upon activated sludge properties for clarification and dewatering by filtration was investigated through controlled chemical manipulations of sludge in laboratory scale. The manipulations resulted in changes of the surface charge density and the shear sensitivity, the latter indicating a weakening of the sludge floc structure. The shear sensitivity was found to correlate with the residual turbidity after settling as well as with sludge filterability characteristics. The data strongly suggest the importance of blinding to increase with increasing shear sensitivity. The amount of fine particles present in the sludge was found to increase when sludge was exposed to a shear of similar magnitude to the shear in the employed filtration apparatus, indicating erosion of flocs during filtration to be important for blinding. Qualitatively similar results were obtained in full scale investigations of sludge from a treatment plant with varying inlet wastewater composition, indicating the possible importance of colloidal stability for solid/liquid separation processes in full scale wastewater treatment.

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