Reinterpretation of the mechanism of coagulation and its effects in waste activated sludge treatment

Abstract

Previous studies suggest that the coagulation mechanisms in wastewater and sludge treatment are the same. However, this study found that changes of COD in wastewater and sludge supernatant after the addition of coagulants (Fe3+ and Al3+) were opposite, indicating that the mechanisms of coagulation in these two systems were not the same. The dose of coagulant efficiently reduced the pH of sludge, the higher the dosage, the lower the pH, indicating hydrolytic acidification exists in the sludge coagulation process. Coagulation proved superior to acid conditioning in reducing negative charge and improving the floc size of sludge, demonstrating that the mechanisms of double layer compression, charge neutralization, intraparticle bridging and colloid entrapment are suitable for sludge coagulation. Furthermore, the addition of Fe3+ and Al3+ promoted the exchange of Ca2+ and Mg2+ in extracellular polymeric substances (EPS), which weakened cross-linking between EPS and cells, and further accelerated the disintegration and release of EPS. Due to the mechanisms of hydrolytic acidification and ion exchange, the solid organic matter in sludge was dissolved and released as liquid, resulting in a significant increase of COD in the supernatant of sludge coagulation. In addition, changes in sludge extracellular substances do not reflect changes in intracellular substances well. This study contributes an improved understanding of the mechanism of coagulation in the sludge dewatering treatment.