Non-Invasive Acidic Pretreatment Technology of Anaerobic Digestion of Waste-Activated Sludge (WAS) on Biogas Production: Unveiling the Role of Extracellular Polymeric Substances (EPSs) and Pharmaceutical Degradation

Non-invasive acidic pretreatments using acetic acid (1-5 mM) and citric acid (0.02-0.1 g g^-1 TS) were investigated to enhance anaerobic digestion (AD) of waste-activated sludge (WAS). Both pretreatments improved short-term process stability, with pH (6.5-7.1) and alkalinity (1000-5000 mg CaCO₃ L^-1) remaining within optimal ranges during 10-day digestion. Acetic acid markedly enhanced solubilization and acidification, increasing volatile fatty acids to ~2500 mg L^-1 (+67% vs. control), whereas citric acid achieved ~2000 mg L^-1 (+37%). EPS analysis revealed pronounced redistribution of polysaccharides and proteins, with acetic acid inducing stronger disruption of the EPS matrix (SB-EPS polysaccharides up to 34.1 mg eq Glc mL^-1). Specific methane yield increased from 28.5 mL CH₄ g^-1 VS (control) to 101.7 mL CH₄ g^-1 VS with acetic acid (3.6-fold) and to 73.8 mL CH₄ g^-1 VS with citric acid (2.5-fold). Gompertz modeling confirmed higher maximum methane potential, ~68% higher maximum methane production rates, and reduced lag phases for both pretreatments. In contrast, pharmaceutical concentrations (31 compounds) were largely unaffected by acid pretreatment, with significant reductions observed only for selected biodegradable molecules.