Biochemical assays of intensified methane content in biogas from low-temperature processing of waste activated sludge

Anaerobic digestion (AD) is implemented as an important ‘waste to energy’ approach for converting organic-rich byproducts such as sewage sludge into biogas and nutrient-rich digestate. Sewage sludge consists of primary sludge and waste activated sludge (WAS), and low biodegradability of WAS limits methane production. This study presents the influence of the low-temperature pretreatment (LT-PT) of WAS on the efficiency of AD. Different conditions of LT-PT process were tested in terms of temperature (45 °C/50 °C/55 °C/60 °C) and exposure time (24h/48h). LT-PT caused the increase of biomethane potential, which was even 43% higher if compared with the untreated WAS; 90% of all methane production was reached between 6 and 12 day of the LT-PT process. The highest improvements were observed in samples pretreated at 55 °C and 60 °C for 48 h (198.8 ± 5.5 NmL CH4/g VS and 194.3 ± 4.2 NmL CH4/g VS, respectively). Changes in the microbial community during LT-PT of WAS showed an oxygen-driven decrease among the predominant phyla except for Firmicutes. Taxonomic diversity and numerous rare taxa with the potential for functional compensation were observed. It was also proved that AD of pretreated WAS, combined with the heat recovery, achieved a positive net energy balance (improved by over 40%) and better energy performance, than AD without LT-PT.