Ultrasound-assisted valorisation of anaerobically digested sludge: Enhancing biodegradability and PAHs reduction

Among the issues related to anthropogenic pollution, the recovery of nutrient-rich biosolids from sewage sludge residues represents an emerging challenge due to the presence of trace organic contaminants (TrOCs). This study proposes the ultrasound (US) treatment of anaerobically digested sludge (ADS) as a strategy to improve biosolids quality, while improving energy recovery and TrOCs transfer and bioavailability. Polycyclic aromatic hydrocarbons (PAHs) were selected as target substances and the US treatment effects on their mobilisation and degradation were studied at environmentally relevant concentrations (50–500 ng/g DW ). The sonolysis of the more recalcitrant organic constituents of sludge demonstrated an increase in soluble carbohydrates and proteins in the supernatant up to 15 and 45 times the initial value, as well as up to 400 % increase in volatile fatty acids. These solubilised compounds served as readily available substrates during anaerobic digestion, enhancing microbial activity and improving the overall biodegradability of ADS. Subsequent re-digestion of the US-treated ADS revealed an increase in specific methane production up to 51 % compared to the untreated ADS. Additionally, a 34 % and 29 % higher reduction in acenaphthylene and benzo[ghi]perylene, respectively, likely resulted from their redistribution into more bioavailable organic fractions. Microbial community analysis showed the taxonomic effects induced by the US disintegration. This study provides a comprehensive evaluation, aiming to open new perspectives on the US technology for sludge treatment. • Low-frequency ultrasound (US) was tested as a strategy for sludge upcycling. • US increased soluble organics in anaerobically digested sludge (ADS). • Polycyclic aromatic hydrocarbons (PAHs) were tested at relevant concentrations. • Methane yield increased up to 51 % after US and re-digestion. • US enhanced PAHs removal through desorption.