Role of free nitrous acid in the pretreatment of waste activated sludge: Extracellular polymeric substances disruption or cells lysis?

Free nitrous acid (FNA) has positive effect on the solubilization of waste activated sludge (WAS), but the mechanism is uncertain. In this study, five different pretreatments were compared to elucidate the enhanced mechanism through the variation of sludge physicochemical properties, including capillary suction time (CST), extracellular polymeric substances (EPS) and bound water. The CST values showed a significant positive correlation with soluble protein (PN) and polysaccharide (PS) under different FNA concentration and contact time. After being pretreated, the CST values increased from 156.5 (raw sludge) to 335.4 (ultrasonic), 237.5 (FNA), 258.7 (ultrasonic/FNA) and 214.4 (ultrasonic (washed)/FNA), respectively. The soluble extracellular polymeric substances (S-EPS) and loosely bound EPS (LB-EPS) increased for all pretreatment, but the tightly bound EPS (TB-EPS) decreased. The sludge filterability was in connection with the existence of humic acid in LB-EPS. Contrary to other pretreatments, bound water decreased from 82% to 79% after FNA treatment. This study showed that FNA was very effective in disintegrating cell membrane, which was indicated by increasing percentage of DNA in solution, however, its role in disintegration of EPS fractions was limited.