Environmental Engineering Reference
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toluene removal from an undescribed, 'botanical' air cleaning system, although
the constituents and construction details of the system were not supplied, so it
cannot be speculated on what, if any, microbial community was present in the
apparatus used. Surprisingly, Ondarts et al. ( 2012 ) did not detect trichloroethylene
removal by a compost-based biofilter, unless activated carbon was added to the
substrate.
An alternative use for microbial biofilters that bypasses problems associated
with the low background concentrations of indoor airborne VOCs is to use the
systems as an initial clean-up process to remove high VOC levels in new buildings
(Lu et al. 2010 ). This can be achieved by using an initial bake-out process,
whereby a newly constructed room is heated to accelerate VOC emissions which
can then be effectively removed in the biofilter (Lu et al. 2012a ). However, such
technology would appear to have little application in the ongoing maintenance of
building air quality.
Another putative concern raised with respect to microbial air cleaners is that
they would lose their capacity to remove trace VOCs over time if not constantly
exposed to them (Guieyesse et al. 2008 ). Whilst empirical research has shown that
the rate of VOC biodegradation increases markedly on repeated exposure, and that
the rate of removal is proportional to the concentration of VOC present, there is
also evidence that the basic capacity to biodegrade some VOCs is innate and not
lost in the absence of the gases (Orwell et al. 2004 , 2006 ).
The capacity of microbial biofilters to remove VOCs in high concentrations is
inarguable. Delhoménie et al. (2002) developed a novel vegetable and sewage
sludge compost-based biofiltration system and tested its capacity to remove high
concentrations of toluene. Contaminated air was exposed to the biofilter at
0.4-1 m 3 h -1 . The compost column was fed with a nutrient solution to sustain the
microbial population. Whilst the system removed toluene with high efficiency, the
system also produced significant quantities of CO 2 , and the release of microor-
ganisms from the system may also have been possible (see Sect. 8.12 ) but was not
tested. Lu et al. ( 2010 ) detected very high rates of biodegradation of formaldehyde,
benzene, toluene and xylene from a biotrickling filter dominated by four common
bacterial genera. Ondarts et al. ( 2012 ) used a compost-based system, which had
very high removal efficiency for many VOCs, including the highly hydrophobic
undecane and limonene, leading to the hypothesis that an abiotic sorption process
was important in the overall biodegradation efficiency of the system, possibly due
to the hydrophobic components of the organic matter.
Logically, the rate of removal of any air pollutant will be limited by the rate at
which it is exposed to any air filtration apparatus, whether biological or physical.
This has been seen as a major constraint on the development of biological air
cleaning systems (e.g. Guieyesse et al. 2008 ). The large magnitude of the biofilter:
room volume ratio for the effective air cleaning system developed by Darlington
et al. ( 2001 ) reinforces the difficulties in gaining effective air cleaning performance
with a reasonably compact system. Future research into increasing the effective air
exposure of biofilters, whether by examining airflow rates over the filters or by
increasing their effective surface area, would be highly valuable.
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