Abstract

Anaerobic digestion (AD) reduces greenhouse gas emissions by converting sludge into renewable energy such as biogas and hydrogen. However, the compact structure of raw sludge limits biodegradability and biogas yield. Pretreatment can enhance hydrolysis and improve energy recovery. This study investigates electrochemical (EC), thermal-pressure (TP), and combined TP-EC pretreatments and their effects on sludge properties and biogas production. The research was conducted in three phases. In Phase 1, EC pretreatment was tested using three reactor configurations. The reactor with 213 mm electrode spacing outperformed the 50 mm reactor, achieving 89% phase separation after 300 minutes at 1.3 V/cm. Scaling up with an enhanced 3.4 V/cm voltage gradient for 60 minutes significantly increased sludge temperature, solids content, and organic compound solubilization. The SCOD/TCOD ratio in EC-pretreated sludge was 11 times higher than in raw sludge, demonstrating scalability. Phase 2 examined TP pretreatment (90–180 °C, 0.6–1.3 MPa) and TP-EC combinations. TP pretreatment enhanced solubilization but consumed more energy than EC. For example, TP pretreatment at 165 °C and 1.1 MPa for 30 minutes yielded an SCOD/TCOD ratio 7 times higher than raw sludge but 1.5 times lower than EC. TP-EC sequences improved solubilization but required even higher energy input. Phase 3 compared all methods through physicochemical analyses, BMP test, LC-MS, and SEM. EC pretreatment achieved the highest biogas yield: 83% more than raw sludge, 42% more than TP-pretreated sludge, and 11% more than TP-EC sludge. These findings demonstrate the potential of EC pretreatment to enhance biogas production and support its integration into existing and new anaerobic digesters, including those in small and medium-sized municipalities.