Volatile Methyl Siloxanes as Key Biogas Pollutants: Occurrence, Impacts and Treatment Technologies

Volatile methyl siloxanes (VMS) are biogas pollutants generated from the metabolism of polydimethylsiloxanes (PDMS) in anaerobic digestion processes. In the past, VMS were considered an issue only for biogas produced in landfills. However, the widespread presence of VMS in different types of biogas has been demonstrated in recent years as a consequence of the intensive use of PDMS in the formulation of personal care products, industrial lubricants, glues, paints, and detergents. Burning biogas (or biomethane) laden with VMS leads the formation of silicate (SiO2) deposits, resulting in abrasion and severe lubrication issues, which finally produce irreversible damage to energy production devices. The present work provides a comprehensive review of the VMS concentrations recently reported in several types of biogas, as well as the physical-chemical technologies available on a commercial scale for the removal of VMS. Alternative biological processes for VMS removal are also described, including the most recent advances in microbial degradation mechanisms, bioreactor configurations, and operation modes. Critical research niches and challenges towards the consolidation of biotechnologies as efficient and cost-effective VMS treatment systems are identified and critically discussed.