Siloxane removal and sludge disintegration using thermo-alkaline treatments with air stripping prior to anaerobic sludge digestion

• Siloxanes hamper the energy-use of anaerobic digestion biogas.
• D5 siloxane was considered as target compound in this study.
• The treatment removed 80% of D5 in sewage sludge at 55 °C and 135 g-NaOH kg−1-VTS.
• D5 removal and the disintegration of VSS in the sludge were correlated.
• At the optimal conditions, the costs of anaerobic digestion were notably diminished.

A thermo-alkaline treatment with air stripping was applied before anaerobic sludge digestion for both siloxane removal and sludge disintegration. The treatment was expected to increase the amount of biogas produced and to reduce the amount of siloxane in the gas. Adding sodium hydroxide (NaOH) to the sludge improved the removal of siloxane from the sludge, with approximately 90% of the siloxane removed to the gas phase using a thermo-alkaline treatment. Over 80% of decamethylcyclopentasiloxane (D5) could be removed under the following conditions: 55 °C treatment temperature, 135 g-NaOH kg−1 volatile total solids (VTS), and 0.5 L min−1 air-stripping rate. The disintegration ratio of volatile suspended solids (VSS) in the sludge was correlated with the D5 removal ratio. Because most of the siloxane was adsorbed to, or was contained in the VSS, the siloxane removal ratio increased with VSS disintegration. Finally, the energy consumption and operational costs of this system were evaluated for several scenarios. Thermo-alkaline treatment at the indicated operational conditions had the lowest operating costs for a 400 m3 day−1 anaerobic sludge digestion system.