Removal of volatile methyl siloxanes in an anoxic two-phase partitioning bioreactor operated with hydrophobic biomass

Abstract

Volatile methyl siloxanes (VMS) generated during the anaerobic digestion of organic matter laden with polydimethylsiloxane are key biogas pollutants responsible of irreversible damage to energy-production equipment. This study presents a next-generation two-phase partitioning bioreactor operated with hydrophobic biomass under anoxic conditions for the removal of octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) as model VMS commonly found in biogas. When supplied in mixture, D4 and D5 average removal efficiencies (RE) of 26.7 ± 6.4 and 24.1 ± 5.9 % were achieved in the control operating phase using only aqueous mineral salt medium. Addition of 15 % (v/v) of silicone oil boosted the reactor performance, resulting in RE of up to 69.4 ± 9.7 and 85.1 ± 1.6 % for D4 and D5, respectively. D5 removal was enhanced when no D4 was fed, reaching a D5 RE of 91.8 ± 4.0 % and revealing a competitive inhibition between both VMS. The VMS removal in presence of silicone oil was concomitant with nitrate consumption rates of 412 ± 267–1846 ± 1605 mg m−3h−1. Microbial attachment on the water/silicone oil interface was confirmed by observations in the microscope at a 100 × magnification as well as by changes in the silicone oil color and texture. 16S rRNA high-throughput sequencing showed that while Mycobacterium was by far the predominant genera attached to silicone oil with a relative abundance of 92.2 %, Mycobacterium, Romboutsia, Clostridium, Intestinibacter, and Terrisporobacter were the predominant bacterial genera in aqueous phase after 127 days of operation with relative abundances of 25.3, 25.2, 12.3, 7.7, and 5.9 %, respectively.