Microwave-acid pretreatment: A potential process for enhancing sludge dewaterability

Activated sludge is hard to be dewatered due to the highly water bounded in sludge flocs. This study investigated the hybrid treatment of microwave irradiation and acidification on sludge dewaterability as well as its mechanism. Results showed that the combined microwave-acid treatment (T = 100 °C, initial pH = 2.5) was effective for improving sludge dewaterability, e.g. capillary suction time (CST) decreased from 37.7 s to 9.2 s, bound water content decreased from 1.96 ± 0.19 g/g Dry Sludge (DS) to 0.88 ± 0.24 g/g DS. The treated sludge showed more fluidity and less thixotropy. Both MW heating temperature and pH played important roles in improving sludge dewaterability. Higher temperature was beneficial for sludge disintegration, but the released polymers resulted in highly negative zeta potential and deteriorated sludge dewaterability. The acidification was capable of reducing the negative zeta potential, increasing flocs size and finally improving sludge dewaterability. According to the analysis of molecule weight distribution and 3D-EEM, the fractions of polymers especially protein-like substances at molecule weight of 104–105 Da were the key organics related to sludge dewaterability, but not the humic acid-like and fulvic acid-like substances. [Display omitted] •Microwave-acid treatment is efficient in enhancing sludge dewatering.•Microwave and acidification has synergetic effect on sludge dewatering.•Protein-like polymers of 104–105 Da are the keys related to sludge dewatering.