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bioactive or inert in origin. As will be described below, the choice will also be dictated
by the type/configuration of biofilter chosen.
All good biofilter support media share several common characteristics. These include
the ability to support microbial growth on the surface of the particles. Materials that have
rough surfaces, significant intraparticle porosity, and no inhibitory properties to bacteria
are generally good at supporting a microbial population. The ideal situation is that they
are resistant to breakdown and subsequent compaction. The breakdown and compaction
of the media leads to numerous operational problems and requires that the media be
replaced more often, thus adding cost. Often materials such as perlite are added as an aid
to stop bed compaction. The medium should possess adequate water-holding capacity:
usually between 40% and 70% for bioactive media and between 30% and 60% for some
inerts. Unless the biofilter is of unique design, the media should possess a pH of between
6 and 8 and, ideally, would have some buffering capacity. The cost of the media relative
to its lifetime should be acceptable to the operator. Each type of medium has a different
cost and lifetime associated with it. It is critical that this be considered in the design, as
media replacement can be a significant portion of the operating costs of a biofilter.
3.2.1. Bioactive Media
Some advantages of natural biofilter media are the relatively low cost and its ready
availability. Natural materials such as compost and wood chips are readily available.
However, they often vary significantly in their composition from one time/place to
another. Bark chips can be an effective medium, but the choice of wood species is very
important. For example, Douglas fir bark resists degradation more than pine bark and
would save the operator the cost of media replacement and operations via lower energy
costs. Although natural media have several advantages, they often encounter problems
with breakdown and compaction that lead to channeling and large pressure drops across
the systems. Once the media starts to break down, it can lead to significant increases in
operating costs as a result of increases in energy costs. Most importantly, the degrada-
tion of natural media can lead to poor performance of the system in terms of removal
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efficiency. Natural solid support media can range in price from $10/ft to more than $75
for such items as bagged bark.
3.2.2. Inert Media
Inert media has one obvious advantage: It does not break down, as natural material
will. The life is often much longer and there is little, if any, degradation of the media
because of microbial activity or chemical effects in the system. This allows for long-
term operation and very consistent operational parameters (i.e., flow rates, pressure
drops, etc.). Nevertheless, inert media may have several disadvantages. Inert material
can be much more expensive than the more readily available natural material (although
not always). Furthermore, many inert materials do not have much in the way of inher-
ent nutritional value (N and P) for supporting microbial populations and thus rely more
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on the addition of these materials. Synthetic materials can range in cost from $40/ft for
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lava rock to > $100/ft for ceramic or plastic supports.
When a biofilter is being designed for a particular application, it is critical to evalu-
ate the physical properties of the chemicals in that application to select the best solid
support. It is not uncommon for a biofilter vendor to sell a “proprietary” media with