Assessing microbial acclimation and mild hydrothermal treatment to improve methane production from paper sludge

The paper manufacturing and recycling industry generates large volumes of paper sludge, a lignocellulosic by-product that poses environmental and economic challenges due to its high moisture content and structural recalcitrance. Anaerobic digestion offers a sustainable route for energy recovery; however, cellulose hydrolysis remains a major bottleneck due to poor enzymatic accessibility under standard conditions. This study addresses this critical knowledge gap by comparing two enhancement strategies for improving sludge digestibility and methane production: mild hydrothermal pretreatment at 250 °C and 5 MPa, and microbial acclimation. A two-way ANOVA analysis was employed to quantify individual and interactive effects during 28-day batch digestion. Results showed that acclimation was the dominant factor, while pretreatment improved methane yield only in non-acclimated systems. Untreated sludge produced 147.8 mL CH4/gVS, whereas hydrothermally treated sludge achieved 15–22% higher yields. Despite this improvement, energy balance analysis revealed a negative net gain for pretreatment due to high thermal input. Conversely, acclimatised inoculum delivered 206.9 mL CH4/gVS without external energy input, confirming microbial adaptation as a more effective and energy-efficient strategy. Microbial community profiling indicated enrichment of hydrolytic and fermentative bacteria in acclimated systems, supporting enhanced process stability. Semi-continuous validation over 90 days further demonstrated that extended acclimation beyond 60 days stabilised reactor performance and achieved methane yields exceeding 350 mL CH4/gVS, eliminating any long-term advantage of pretreatment. Overall, this work provides an integrated assessment of biological and thermal approaches for sludge valorisation and underscores the potential of inoculum adaptation as a low-energy, practical solution for industrial-scale biogas production.