Siloxane removal from biosolids by peroxidation

The anaerobic digestion of WAS (commonly called biosolids) is widely used since it offers advantages of decomposing organic material, destroying pathogens, stabilising the sludge and producing a high calorific biogas which can be energetically valorised e.g. in a CHP unit. The presence of volatile siloxanes (SX) in the sludge can however largely hamper the energy use of the biogas since they are transformed into an abrasive microcrystalline silica during the biogas combustion: this silica coats and damages the gas beneficiation equipment.This paper studies the use of peroxidation to reduce the siloxane content of WAS. These methods both transform the siloxanes into lower molecular weight siloxanes and silica, and degrade the EPS to which the siloxanes are bound, thus enhancing their volatilisation before digestion. A reduction of approximately 50% is observed for all peroxidants studied, except in the DMDO peroxidation of D4, where a reduction of 85% is observed. Although the reduction percentages are significant, an additional biogas purification step might remain necessary to reduce the siloxane concentrations below the limits defined by the engine manufacturers.