Biogas coming from uncontrolled systems (E.g. from landfill or marshland) or from controlled systems (E.g. municipal wastewater treatment plants and anaerobic digestion plants) contains siloxanes.
A siloxane is any chemical compound composed of units in the form of R2SiO (R = H or HC group).
Siloxanes are found in significant quantities in a wide and varied range of household products, such as detergents, shampoos, deodorants, toothpastes and cosmetics.
Most of them quickly evaporate into the atmosphere, and over time they break down into carbon dioxide, silica and water. However, some end up in wastewater and in municipal solid waste, and inevitably accumulate in landfills and treatment plants, where they are considered one of the most difficult pollutants to control.