Enhanced anaerobic digestion and sludge dewaterability by alkaline pretreatment and its mechanism

To investigate the influences of alkaline pretreatment on anaerobic digestion (AD) and sludge dewaterability after AD, waste activated sludge was adjusted to different pH values (8, 9, 10, 11, 12) and placed at ambient temperature for 24 hr. The samples were then adjusted to the initial pH and subjected to 25 days of AD. The results showed that, when compared with the control (pH 6.8), total suspended solids (TSS) and volatile suspended solids (VSS) reduction following pretreatment at pH 9–11 increased by 10.7%–13.1% and 6.5%–12.8%, respectively, while biogas production improved by 7.2%–15.4%. Additionally, significant enhancement of sludge dewaterability after AD occurred when pretreatment at pH 8–9 was conducted. The proteins and carbohydrates transferred from the pellet and tightly bound extracellular polymeric substances (TB-EPS) fractions to the slime and loosely bound EPS (LB-EPS) fractions after pretreatment and during the AD process, and the concentrations of proteins and carbohydrates in the slime fraction had a good linear relationship with the normalized capillary suction time (CST). During the AD process, the normalized CST was positively correlated with the organic materials in the loosely bound fraction of the sludge matrix (R2 ⩾ 0.700, p < 0.01), while it was negatively correlated with the organic materials in the tightly bound fraction (R2 ⩾ 0.702, p < 0.01). These results suggest that alkaline pretreatment could break the EPS matrix and release inner organic materials, thus influencing the efficiency of the AD process and dewaterability after AD.