Biofiltration is an evolving technology for treating gas-phase volatile organic compounds (VOCs), Hazardous Air Pollutants (HAPs), and odorous contaminants by utilizing naturally-occurring bacteria, immobilized on surface biofilms. Biofilms exist in naturally bioactive materials, such as soil, peat, and compost, and can also be formed on synthetic materials, such as plastic and metal.
Traditionally, biofilters used naturally bioactive media, such as compost and wood chips. These biofilters had a large footprint and could treat odorous contaminants, such as hydrogen sulfide, and easy biodegradable contaminants. Natural media is depleted of nutrients over time and clogs due to biomass growth, eventually requiring replacement. Later, high surface area plastic and ceramic media were developed, which offered the significant benefit of not requiring a replacement over the lifetime of the equipment. Figure 2 shows the various types of biofilters that have been used. Although the term “biofilter” has been used to designate all these different type, recently, a Dual-BioPhase™ biofilter has been developed (Figure 3). This configuration simultaneously biodegrades the contaminants both in the gas and liquid phases. Treated water from the sump is recycled back to the biofilter section to keep the biomedia wet and periodically slough-off any excessive biomass growth on the biofilter biomedia.
Biofilters differ in the following aspects: (1) the biomedia used to support the bacteria and manage their growth; (2) the ability to handle the treatment of the water since water soluble compounds will partition from the gas phase into the aqueous phase; (3) the ability to dissipate heat if the inlet gas is above the ambient temperature, as in the case of exhaust gases from wood dryers; and (4) the ability to handle particulate matter and condensable compounds in the inlet gas.
Figure 2. Types of Biofilters
Figure 3. Dual-BioPhase Biofilter, developed by PRD Tech, Inc. and licensed to Process Combustion Corporation, Inc. (Pittsburgh, PA).
Dual-BioPhase Biofilters have been installed to treat odors and combinations of odors and volatile organic compounds (VOCs)/Hazardous Air Pollutants (HAPs).
The major disadvantages of biofiltration are:
(a) Significant investment cost of the process;
(b) Large diameter biofilter vessel, which generally operates at a superficial gas velocity of 100 ft/min or less;
(c) Requires a liquid-phase bioreactor in addition to the gas-phase biofilter;
(d) Exhibits poor removals for water-insoluble compounds;
(e) Requires several weeks to establish active biofilms in the biofilter vessel; and
(f) Exhibits poor response to sudden increases in inlet gas concentration of the contaminants.