Biological removal of siloxanes from landfill and digester gases: opportunities and challenges.

The presence of volatile methyl siloxanes (VMSs) presents challenges for using landfill and digester gases as energy fuels because of the formation of silicon dioxide deposits during combustion. This study looks at the feasibility of using biological treatment to control VMSs. Biotrickling filters removing octamethylcyclotetrasiloxane (D4), selected as a model VMS, from aerobic and anaerobic waste gas streams were setup. The efficacy of both aerobic and anaerobic biotrickling filters was low. The removal of D4 in the aerobic biotrickling filter followed a linear trend, reaching 43% at a gas empty bed residence time of 19.5 min. Aerobic biodegradation of D4 in shake flasks was found to be extremely slow, with trace concentrations requiring 3-4 months for complete degradation. Gas-liquid partition tests revealed that D4 partitions poorly into aqueous phases and that interphase mass transfer is slow. Using the mass transfer data, we estimated the maximum possible mass transfer rate of D4 in the biotrickling filter to be in the range of 30-100 mg m(-3) h(-1). These values are low and suggest that mass transfer limitations play an important role in the low treatment performance that was observed. The possibility of enhancing D4 mass transfer by using oleyl alcohol as a second nonmiscible liquid phase was unsuccessful. Overall, the results demonstrate that biological treatment of D4 vapors is possible but poses significant challenges.