Pollution and Pollution Control - Environmental Biotechnology: Theory and Application - page 73

the resultant solution, thereby bringing about the desired effect. The medium
itself provides physical support for microbial growth, with a large surface area to
volume ratio, high in internal void spaces and rich in nutrients to stimulate and
sustain bacterial activity. Biofilters need to be watered sufficiently to maintain
optimum internal conditions, but waterlogging is to be avoided as this leads to
compaction, and hence, reduced efficiency. Properly maintained, biofilters can
reduce odour release by 95% or more.
Biotrickling filters
As shown in Figure 4.3, in many respects these represent an intermediate
technology between biofilters and bioscrubbers, sharing certain features of each.
Once again, an engineered vessel holds a quantity of filter medium, but in
this case, it is an inert material, often clinker or slag. Being highly resistant to
compaction, this also provides a large number of void spaces between particles
and a high surface area relative to the overall volume of the filter. The microbes
form an attached growth biofilm on the surfaces of the medium. The odourous
air is again forced through the filter, while water simultaneously recirculates
through it, trickling down from the top, hence the name. Thus a counter-current
flow is established between the rising gas and the falling water, as shown
in the diagram, which improves the efficiency of dissolution. The biofilm
communities feed on substances in the solution passing over them, biodegrading
the constituents of the smell.
Process monitoring can be achieved relatively simply by directly sampling the
water recirculating within the filter vessel. Process control is similarly straight-
forward, since appropriate additions to the circulating liquid can be made, as
Figure4.3 Biotricklingfilter