Acidic thermal post-treatment for enhancing anaerobic digestion of sewage sludge

Acidic thermal post-treatment (ATPT) was examined for treatment conditions in a batch study, and was demonstrated to enhance anaerobic digestion of sewage sludge in a continuous study. In the batch study where anaerobically digested sludge was the substrate in view of a post-treatment mode, higher ATPT temperatures between 25 and 180 °C improved volatile suspended solids (VSS) destruction and methane production, but generated color significantly at 180 °C. Lower ATPT pH between 2 and 6 enhanced sludge dewaterability (as capillary suction time), and slightly suppressed color generation. In the continuous study, two single-stage anaerobic digestion processes were operated at 35 °C and 20 days hydraulic retention time. For one of the processes, ATPT at 170 °C and pH 5–6 for 1 h was incorporated in the recycle line. Approximately 75% of VSS destruction was achieved in the ATPT process, which was 2–2.5 times more than that in the control process, 30–37%. The ATPT process also showed 14–21% more methane production and 22–23% better dewaterability, but formed around three times more color, compared to the control process. Sulfuric acid as the acidifying agent caused more release of phosphate from the digested sludge, which enables efficient phosphorus recovery.