The upsurge of strict environmental regulations to maintain good air, soil and water quality requires the use of proper pollution control and pollution prevention equipment. Traditional pollution control technologies such as incineration, carbon adsorption and wet-scrubbing may be used to control pollution. However, these conventional techniques are becoming more expensive due to more stringent environmental regulations [1]. In the field of pollution control, biological treatment such as biofiltration is continuing to gain attention as an alternative to the conventional techniques.

Biofiltration is a pollution control technique that uses living microorganisms to capture and degrade pollutants from air. It is a process that combines basic mechanisms of adsorption, biodegradation and desorption of gas phase pollutants [2]. A biofilter is simply a packed bed that utilizes a packing material with microorganisms such as bacteria immobilized as biofilm on the surface and the pore structures of the packing material. The packing material may be particles such as peat, compost, peat/perlite mixture or organic or inorganic commercial media materials. The flow of contaminated air through the biofilter results in the degradation of the pollutants by the immobilized microorganisms. Biofilter performance is affected by a number of factors such as the composition and relative humidity of the waste stream, airflow velocity, temperature and pH of the biofilter bed, the pore size distribution, and other structural characteristics [1]. The surface of the porous material in the biofilter is covered with biofilms which are made of microorganisms. Treatment begins with the transfer of the contaminants from the air stream to the biofilm phase. Then, the dissolved contaminant is moved by diffusion and by advection in the air. Biotransformation finally converts the contaminant to biomass, metabolic by-products, carbon dioxide and water.

Several types of biofilters have been developed, the most typical of which include bioscrubbers, trickling bed biofilter and packed bed biofilter [3]. In all these technologies, pollutants in the gas stream are transferred to the biofilm and are degraded by the microorganisms. The bioscubber consists of a scrubber and a bioreactor. In the scrubber, water is sprayed counter-current to the polluted gas flow resulting in absorption of the pollutant into the water. This water is then directed to a bioreactor containing activated sludge where microorganisms degrade the pollutants. The trickling bed biofilter, on the other hand, relies on the inert packing media to support bacterial growth. The packed bed biofilter does not use a large continuous flow of water. The media used in the packed bed biofilter acts as a water reservoir as well as a support structure for the bacteria.

Figure 1 shows a simplified schematic of a biofilter [3].