Start-up of a lab-scale biotrickling filter for octamethylcyclotetrasiloxane (D4) removal from biogas

Abstract

The biogas generated during anaerobic digestion in wastewater treatment plants and landfills is considered as a resource of renewable energy. Whereas its use not only contributes to reducing greenhouse gas emissions but also obtaining heat and electricity, biogas contains trace undesired compounds such as H2S, volatile organic compounds and siloxanes among others. Volatile methyl siloxanes present in biogas cause the formation of silicate deposits during combustion affecting the efficiency of energy recovery systems. The most widespread technology commercially available for siloxanes removal is adsorption onto porous materials, though their regeneration or replacement once exhausted is energy-consuming and expensive. On the contrary, biodegradation technologies may (i) reduce investment and operating costs, (ii) increment treatment capacities and (iii) require low energy and chemicals. It needs to be highlighted that siloxanes challenge biotechnologies due to their low solubility into water, which hinders their gas-liquid mass transfer and subsequently the liquid-biofilm transfer. Within the present work, an aerobic biotrickling filter was operated for 56 days for the treatment of an air stream contaminated with octamethylcyclotetrasiloxane. At the beginning, D4 stripping from the laboratory-grown inoculum was observed and confirmed by cutting the siloxane entrance of D4. Even at high gas flows, D4 concentration of around 10 mg m-3 were observed. After inoculating with activated sludge from a wastewater treatment plant, the performance of the reactor showed 10% removal efficiencies at an empty bed contact time of 10 min. Moreover, decreasing the medium recirculation flow down to 15 mL min-1, higher REs up to 21% were observed. Although being a pretty high gas residence time, low removal efficiencies were obtained due to the low solubility of D4 which leads to a low mass transfer to the liquid phase and consequently to the biomass