Changing profiles of bound water content and distribution in the activated sludge treatment by NaCl addition and pH modification

Currently the dewatering of activated sludge from wastewater treatment plants is a problem not well solved. Extracellular polymeric substances (EPS), including loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS), are highly hydrated biopolymers and play important roles in sludge dewatering. In the present work, two types of treatments, i.e., salt addition and pH modification, were proposed to evaluate the effects of bound water content and its distribution on the dewatering performance of activated sludge. Results show that the bound water content in activated sludge was deeply related to the floc structures and EPS compositions. Both salt addition and acid treatment altered the flocculated matrix and increased the contents of LB-EPS, resulting in the release of trapped water and reduction in sludge volume. In addition, the chemical treatments also affected the distribution of bound water. The internal water content increased upon the dose of NaCl, leading to a decreased water content trapped in EPS. Hence, salt addition caused a slower filtration rate, but did not effectively decrease the total water content, although the EPS were decomposed. Under acidic conditions, cell lysis helped to release the intracellular water and further enhance the sludge dewatering performance. In contrast, conditioning with low-dosage alkali increased the bound water content and deteriorated the sludge dewaterability. Therefore, an in-depth investigation into the mechanisms is useful to optimize the activated sludge dewatering process.