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.