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.

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