Alkaline treatment of high-solids sludge and its application to anaerobic digestion

High-solids anaerobic digestion is a promising new process for sludge reduction and bioenergy recovery, requiring smaller digestion tanks and less energy for heating, but a longer digestion time, than traditional low-solids anaerobic digestion. To accelerate this process, alkaline sludge disintegration was tested as a pretreatment method for anaerobic digestion of high-solids sludge. The results showed that alkaline treatment effectively disintegrated both low-solids sludge and high-solids sludge, and treatment duration of 30 min was the most efficient. The relation between sludge disintegration degree and NaOH dose can be described by a transmutative power function model. At NaOH dose lower than 0.2 mol/L, sludge disintegration degree remained virtually unchanged when sludge total solids (TS) content increased from 2.0 to 11.0%, and decreased only slightly when sludge TS increased to 14.2%. Although high-solids sludge required a slightly higher molarity of NaOH to reach the same disintegration level of low-solids sludge, the required mass of NaOH actually decreased due to sludge thickening. From the view of NaOH consumption, sludge TS of 8–12% and a NaOH dose of 0.05 mol/L were optimum conditions for alkaline pretreatment, which resulted in a slight increase in accumulative biogas yield, but a decrease by 24–29% in digestion time during the subsequent anaerobic digestion.