Case StudyUsing mechanisms of hydrolysis and sorption to reduce siloxanes occurrence in biogas of anaerobic sludge digesters

- Hydrolysis and sorption constants for sludge (Kd) determined for cyclic siloxanes.
- Hydrolysis increased with temperature and decreased with size of cyclic siloxanes.
- Sorption increased with molecule size with LogKd values from 1.46 to 3.84.
- During single-stage anaerobic digestion, >91% of siloxanes end up in biogas.
- Cyclic siloxanes volatilization predicted to be mitigated using a pre-digester.

Hydrolysis of hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4), decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6) and dodecamethylcyclohexasilane (D6_silane) and their sorption to digested sludge was studied in batch experiments. Hydrolysis was affected by the type of the compound and the applied temperature, while the relevant half-life values ranged between 0.07 ± 0.01 d (D3, 55 °C) and 48.4 ± 17.1 d (D6_silane, 4 °C). D5 showed the greatest affinity for sorption to digested sludge (logKd: 3.84 ± 3.42), the lowest LogKd value was found for D3 (1.46 ± 0.95). Prediction of investigated compounds' fate in a single-stage anaerobic digestion system indicated that volatilization seems to be the major fate in both mesophilic and thermophilic conditions. The addition of a pre-digester with 3 d retention time would significantly decrease the expected concentrations of all siloxanes in biogas, enhancing their removal through hydrolysis and sorption to sludge.

104