Improvement of activated sludge dewaterability by mild thermal treatment in CaCl2 solution

Activated sludge dewatering is of great importance in sludge treatment and disposal. To enhance the dewaterability, a novel method was performed by treating the sludge under mild temperature (50–90 °C) in CaCl2 solution (3.7–1110.0 mg/g dry sludge). The capillary suction time, zeta potential, Fourier-transformed infrared spectra, concentration of soluble protein and carbohydrates were employed to characterize the dewaterability and influencing mechanism. The sludge dewaterability was deteriorated with single thermal treatment, but significantly promoted in CaCl2 solution and advanced further together with thermal treatment. An increasing CaCl2 dosage reduced the surface charge remarkably, and a higher temperature could strengthen this impact. The spectra indicate that Ca2+ could interact with the protein, phenols and O–H functional group in the flocs. The thermal treatment could cause the solubilization of protein and carbohydrates, providing more binding sites for Ca2+ to establish a strong bridging among the flocs. As CaCl2 dosage elevated, the soluble carbohydrates showed a reduction trend, while the soluble protein lowered firstly and then bounced back except that remained unchanged at room temperature. A bridging equilibrium is presumed to exist between Ca2+ and the soluble protein. And the bridging between Ca2+ and the soluble carbohydrates plays a more important role in the dewatering. The sludge dewaterability was successfully and economically improved by thermal treatment in CaCl2 solution.

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► Sludge dewaterability was improved by mild thermal treatment in CaCl2 solution.
► Protein and carbohydrates were solubilized to expose negative surfaces.
► Calcium ions interacted with functional groups to neutralize flocs charges.
► Thermal treatment generated much more binding sites for Ca2+ to bridge with flocs.