Wood Chips as an Air Filtration Media

One of the more unusual uses for woody biomass is in bio-filtration.  In this example the wood chips are used to form a bed and malodorous air is fed through to remove the offensive smell.  Facilities that need this kind of filtration include sewage treatment plants and waste processing facilities.  There are a number of examples in California including the facility pictured that processes municipal solid waste. 

How does it work?
In the filter shown the material is 8 ft deep and arranged in 24 cells.  Each cell holds approximately 2,000 cubic yards of wood chip.  The filter reduces volatile organic content (VOC) emissions more effectively than inorganic filtration media.  Bacteria grow on the moist wood particles and "eat" the odors.  Effectiveness of the filter increases as the bacteria settle in.  The filters may last for 5 years before requiring replacement.  The filter will settle and reduce in volume over time.  In this instance the old filter bed is 6 foot deep.  The old filter bed is composted when it is replaced.  More information on how the process works is included in a short literature review on our website here. 

Replacement wood chip bed

What is the chip specification and price?
For this type of filtration, material from a grinder (followed by screening) is best as the stringy nature of the particles allows for air flow through the bed.  The chip specification is important to avoid restriction of air flow or forced air channeling in any part of the filtration bed.  Particles in the 3-6 inch range are optimal and no 1 inch minus material is acceptable.  The material typically comes from forestry sources (pine and fir) or orchard removals (citrus and nut).  No recycled wood is allowed and species such as redwood and eucalyptus are not appropriate as they would inhibit the growth of the bacteria.  Facilities may pay $40 per cubic yard delivered.  For a 50,000 cubic yard filter this represents an investment of $2m.  So although they last for 5 years and have quite a tight specification it could represent a potential source of income for a business that had other markets for materials from a grinding operation.