Separate treatment of liquid and solid fractions derived from thermally alkaline-treated sewage sludge: impact on energy and material balance in sludge processing

Thermal hydrolysis process (THP) can break down sludge structures and enhance organic solubilization into the liquid fraction. Separately treating liquid and solid fractions could benefit energy and material balances. In this study, five primary sludges (PSs) and seven waste activated sludges (WASs) were treated with thermal alkaline process (TAP) (160 °C, 30 min, pH 9-12), followed by centrifugation for liquid and solid separation. The liquid fractions of PSs after TAP yielded 348 ± 66 L/kg COD_added of biogas. They produced up to 29 ± 7% of the biogas from hydrolysates. In contrast, the liquid fraction of WASs after TAP yielded 395 ± 34 L/kg COD_added of biogas and produced up to 77 ± 4% of the biogas from hydrolysates. Additionally, the dewaterability of WAS increased by about 54%, while the dewaterability of PS decreased by roughly 48% after thermal treatment. With digestion of the liquid fraction and disposal of the solid fraction in WAS treatment with TAP at pH 11, biogas production increased by 4%, and the discharged sludge volume decreased by 10%. Although net electricity generation decreased by 22%, the digester volume could be reduced by 90%, allowing for more capacity for co-substrate digestion in WWTPs.