Enhancing Anaerobic Digestion of Waste Activated Sludge by Solid–Liquid Separation via Isoelectric Point Pretreatment

Abstract

Anaerobic bioconversion of waste activated sludge (WAS) into methane is often limited by the slow hydrolysis rate and/or poor methane potential. In this study, a novel pretreatment strategy based on the isoelectric point of sludge particulate is proposed to enhance methane production from WAS. Pretreatment of WAS for 4 h at room temperature substantially changed the physicochemical properties of sludge particulate, resulting in the disintegration of WAS. The experimental results showed that the apparent activation energy of sludge organic solubilization was decreased by 49.5% and the surface site density of sludge particulate was increased by 71.2%, indicating that by this pretreatment WAS solubilization was enhanced and the surface binding sites for enzymes were decreased. In addition, compared to the conventional pretreatment methods, the main metal contents of WAS were decreased by 41.4% after this pretreatment, suggesting the possibility of simultaneously enhancing methane conversion and removing heavy metals from WAS. The biochemical methane potential assays demonstrated that after pretreatment the hydrolysis and acidification of sludge were significantly improved, with the highest volatile fatty acid concentration (3364 ± 28 mg/L) being more than twice that of the untreated samples. Model-based analysis indicated that pretreatment improved methane potential by approximately 19% (from 197.5 ± 8.3 to 242.8 ± 6.2 mL CH4/g volatile solids (VS)), thereby changing methane production kinetics. Further analysis indicated that after pretreatment the methanogenic bacteria growth, rather than the hydrolysis reaction of sludge, became the rate-limiting step in the anaerobic digestion (AD) process. These findings, however, may provide a new direction for enhancing the AD efficiency of WAS.