| Activated sludge dewatering is important in sludge treatment and disposal, and also the bottleneck of the treatment. Dozens of methods have been investigated to enhance the dewaterability. Among them, thermal hydrolysis has been widely focused for its effectiveness and potential application in industry. However, it is still energy-consuming and has a high requirement for equipment. Since some cations, like Ca2+, can enhance the sludge dewaterability markedly through compressing the colloids double layers of sludge and bridging among the negatively charged flocs, CaCl2was adopted to cooperate with thermal hydrolysis to improve the sludge dewaterability and lower the energy consumption in this study. The variation of several sludge properties, which are closely related with the dewatering, were characterized to provide an insight into the mechanism. A pilot experiment of mild thermal treatment with CaCl2and the economy assessment were carried out.The sludge dewaterability was successfully improved by mild thermal treatment (50-90℃) with CaCl2(3.7-1110.0mg/g dry solid (DS)). The sludge dewaterability was deteriorated with single thermal treatment, but significantly promoted in CaCl2solution and advanced further together with mild thermal treatment. An increasing CaCl2dosage 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 CaCl2dosage 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 dewaterability of activated sludge was first deteriorated and then ameliorated when the temperature was raised from100to200℃with a threshold temperature of130℃under hydrothermal treatment. Calcium chloride assisted hydrothermal treatment to improve the dewaterability remarkably under100-200℃, and eliminated the threshold temperature at as less as20.0mg/g DS. An increase in temperature and dosage of CaCl2till60.0mg/g DS allowed a continuous improvement of dewaterability. It is found that the charge neutralization dominated the dewaterability evolution below160℃, while the decomposition of water-binding components played a more important role at higher temperatures. The variation of molecular weight of soluble protein and polysaccharides implies that CaCl2interacted with the component of sludge and altered the constituent during the hydrothermal treatment. The integration of soluble biopolymers into the floc matrix by CaCl2contributed to the compacted floc structure and thus improved the dewaterability.The pilot experiment shows that the solid content of filter cake could be increased to more than30%easily after mild thermal treatment with CaCl2-For example, when the activated sludge was treated at80℃with212.0mg/g DS CaCl2for1.0h and filtered under0.27MPa for4.0h, the solid content of filter cake reached37.7%. The energy consumption was lower than that of hydrothermal treatment, acid and alkaline thermal hydrolysis, thermal drying and electro dewatering. There are alternatives of adopting CaCl2generated from acid-base reaction and increasing the initial solid content of sludge, aiming to reduce the cost of the mild thermal treatment.In general, thermal hydrolysis with CaCl2altered the surface charge, biopolymer constituent, microstructure and inner interaction of flocs, and improved the sludge dewaterability effectively. This method shows good propect for industrial application. |
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