Damage mechanisms of sludge flocs and cell structures by different pretreatment methods

Pretreatment is an important way to break sludge cells and to release intracellular organic substances in the recycling utilization of excess sludge. Thermal-alkaline pretreatment released the highest DNA, SPro, SCarb and SCOD (26.2, 144.2, 41.3 and 380.8 g/kgTS, respectively) by solubilizing 76.5% of sludge protein and 13.0% of TS. The reason was because that the heat directly acted on the sludge cell walls since 4.0 times of N-acetylglucosamine increase was observed, and the alkaline had tremendous effect on cell membranes once the cell walls were damaged. Ultrasonic pretreatment achieved the highest sludge particle size reduction of 49.7% but only 1.5 times of N-acetylglucosamine increase, thus the hydro-mechanical forces of ultrasound only caused serious disaggregation of sludge flocs without the disruption of sludge cells. It was the Reaggregation effect of sludge flocs led to the insignificant reduction of sludge particle size in thermal and alkaline associated pretreatments, though they solubilized more EPS-Pro and EPS-Carb than ultrasound. The lowest sludge size reduction (2.0%) and N-acetylglucosamine increase (0.1 times) obtained in enzymatic pretreatment showed it had little effect neither on the sludge flocs disaggregation nor on the cell structures disruption. Besides, there was no significant difference in the types of released organic matters among those pretreatments.