Effects of municipal sludge composition on hydrothermal liquefaction products: Aqueous phase characterization and biodegradability assessment

Hydrothermal liquefaction (HTL) aqueous phases derived from mixed sludge and digested sludge of two wastewater treatment plants (WWTP) were characterized considering variations in primary-secondary sludge ratios, an aspect previously overlooked in the literature. Mixed sludge was obtained by mixing primary and secondary sludge to simulate high primary sludge, average, and high secondary sludge cases. Aerobic and mesophilic/thermophilic anaerobic biodegradability tests were conducted. Higher chemical oxygen demand, total ammonium-N, orthophosphate-P, fatty acids, and N-heterocycles in HTL aqueous samples were detected as the secondary sludge ratio increased in mixed sludge. A similar trend was observed in the biodegradability tests. Characteristics of HTL aqueous derived from mixed sludge of WWTP 1 showed much higher variation, whereas WWTP 2 mixed sludge was not affected significantly by primary-secondary sludge ratios. Finally, the biodegradability levels of HTL aqueous samples were determined to be 69–78% under aerobic, 58–70% under mesophilic anaerobic, and 42–56% under thermophilic anaerobic conditions.