Influence of NaOH and thermal pretreatment on dewatered activated sludge solubilisation and subsequent anaerobic digestion: Focused on high-solid state

• Comparative effects of NaOH and thermal pretreatment on dewatered sludge.
• A combination of high-solid pretreatment and high-solid digestion.
• Cumulative methane yield was improved by 71% after 120 °C pretreatment.

In this study, the influence of NaOH and thermal pretreatment of dewatered activated sludge (DAS) on the high-solid solubilisation and anaerobic digestion was separately investigated by monitoring common parameters. The results indicated that COD, proteins and carbohydrates were efficiently solubilised in both NaOH and thermal pretreated DAS samples. For NaOH pretreatment, the concentrations of volatile fatty acids (VFAs) and total ammonium nitrogen (TAN) firstly increased followed by decreasing with NaOH dose increasing. However, they decreased with the severity of thermal pretreatment. During the batch digestion experiments (at 37 °C), for 80 mg NaOH g−1 total solid (TS) DAS pretreatment it resulted in a 6.99% decrease in cumulative methane yield (CMY) compared to untreated DAS. While for 80, 100, 120 °C and 20 mg NaOH pretreatment, CMY increased by 15%, 42%, 71% and 35%, respectively, in comparison to untreated DAS.