Advancing wastewater sludge valorisation: Evaluating hydrothermal treatment and anaerobic digestion integration pathways

Integrating hydrothermal treatment (HT) with anaerobic digestion (AD) offers an innovative approach to enhance energy and nutrient recovery from sewage sludge. While previous studies have primarily focused on downstream configurations employing digested sludge (DS) as HT feedstock, upstream coupling involving the use of primary sludge (PS) and thickened waste-activated sludge (TWAS) has received limited attention. This study systematically evaluates one downstream and two upstream HT-AD configurations to identify the most efficient valorisation route. The digestion of PS with the liquid phase from hydrothermally treated TWAS resulted in the highest biomethane yield of 184.1 mL/g VS and achieved over 90 % biodegradability. Despite an initial lag phase of 25 days due to high NH₄⁺ concentrations and microbial inhibition (11.4 %), microbial viability improved to 61.3 % post-digestion, with Methanosarcina emerging as the dominant genus. Enhanced nutrient content and reduced heavy metals in the digestate underscore the benefits of upstream HT-AD integration.