Thermal hydrolysis on the edge of thermophilic anaerobic digestion: a pilot-scale operation experience

Abstract

This study investigated the integration of the thermal hydrolysis process (THP) as a pretreatment with thermophilic anaerobic digestion (TAD) at a pilot scale using sludge from a full-scale wastewater treatment plant. This is the first pilot-scale evaluation of THP–TAD employing thermophilic inoculum adapted to hydrolysed sludge, offering critical insights into the potential of THP (155 °C, 30 minutes) to enhance TAD (55 °C) performance and contribute to sustainable sludge management. This study assessed the effects of THP on process stability at reduced hydraulic retention times (HRTs), biogas production, sludge dewaterability, and antibiotic resistance gene (ARG) reduction. The THP achieved a sludge disintegration degree of 26.8%, enabling a 50% reduction in HRT without compromising the reactor stability or process efficiency. At an HRT of 12 days, the specific biogas production averaged 0.28 Nm³ kg⁻¹ VS_in. Additionally, compared with traditional processes with longer HRTs, THP significantly enhanced ARG reduction, achieving a maximum reduction of 3.5 log units, while improving sludge hygienization and maintaining volatile solids reduction (VSR). Despite performance improvements, THP–TAD requires higher energy input, underscoring the need for optimization strategies. This study demonstrated that THP–TAD is a robust and effective approach for intensifying anaerobic digestion, offering notable reductions in capital costs (digester volume) while addressing critical environmental challenges such as ARG mitigation. Further investigations into sludge thickening and energy efficiency optimization are necessary to fully realize the potential of this technology as a cornerstone of sustainable wastewater management.