Biogas purification through membrane bioreactors: Experimental study on siloxane separation and biodegradation

Abstract

Sewage biogas valorization to different energy applications is hampered by the presence of volatile methyl siloxanes. Despite the high operating costs, adsorption onto activated carbon is the most implemented technology for siloxane removal from biogas. In order to purify biogas sustainably, the current work explores the diffusion of siloxanes (octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane) together with other biogas impurities (limonene, toluene and hexane) through polydimethylsiloxane membranes. Abiotic tests revealed transport efficiencies above 75% towards a clean air stream for most compounds, although the transport of the most hydrophobic pollutants was challenged when water was circulated through the shell side of the membrane. Moreover, the performance of a hollow-fiber membrane bioreactor, inoculated with anerobic active sludge, was evaluated towards biogas purification in anoxic conditions. Toluene and limonene were successfully degraded, hexane's removal efficiency was positively correlated with the residence time, and siloxanes removal was achieved up to 21%. CO2 was detected in the outlet gas as the mineralization product as well as some byproducts from the degradation of limonene and siloxanes. The presence of 1% of O2 in the gas, as a strategy to substitute NO3−, efficiently supported high removal for volatile organic compounds and moderate for siloxanes, which would ultimately reduce the operating costs of the technology