Environmental Engineering Reference
In-Depth Information
how VOC phytoremediation systems can be designed as feasible installations for
general IAQ improvement in real buildings. It is thus hoped that future research
funding for IAQ improvement is directed towards disentangling this problem.
It has been proposed that the capacity-limiting step for biological air cleaning
is the rate at which gaseous air contaminants can infiltrate the substrate upon
which the biological component, whether plants or microorganisms, are growing
(Guieyesse et al.
2008
). Active or dynamic air cleaning systems operate on the
principle of using either pressure to increase the volume of air to which a biofil-
tration system is exposed, or a mobile aqueous phase into which contaminants are
accumulated and thus transferred to a bioreactor. Both methods thus potentially
increase the rate at which contaminants are removed. Passive systems rely on the
diffusion of contaminants to a static bioremediation system, such as the rhizo-
sphere of a potted plant or green wall, which could be expected to be slow for low
concentrations of pollutants in spaces with poor air circulation. Some green wall
systems, with constant trickling nutrient solutions but no pressure assistance to
increase airflow (e.g. Darlington et al.
2000
), might thus be considered as hybrid
passive-active systems.
Chen et al. (
2005
) found that whilst a plant-based air cleaning system was
effective in removing VOCs, its applicability was restricted by its inability to
process high rates of airflow, a disadvantage not suffered by most sorption fil-
tration systems. Darlington et al. (
2001
) assessed the effect of airflow rate on the
capacity of his active botanical biofiltration system to remove airborne toluene,
ethylbenzene and o-xylene. They found that, whilst the single pass efficiency of the
biofilter was greater when the airflow was very low (0.025 ms
-1
), the maximum
removal capacity of the system occurred at the highest airflow tested (0.2 ms
-1
).
The authors proposed that it was the diffusion of VOCs into the aqueous phase of
the system that was rate-limiting to biodegradation, suggesting that systems that
increase the airflow rate even further may have further increased efficiency.
However converse to these observations, the compost-based biofilter tested by
Delhoménie and Heitz (
2003
) had a gross toluene removal efficiency that was
proportional not to the airflow rate, but the residence time within the biofilter
column. This system was tested at relatively high VOC concentrations only.
To maximise the capacity of biofiltration systems, there is thus a need to
process as large a volume of contaminated air as possible, whilst exposing the air
to the biological material for the critical time period over which sufficient pollutant
removal will occur to allow air contaminants to be reduced to habitable levels.
These two variables: airflow versus residence time are clearly the key attributes to
maximise the efficiency of any system. It would thus be useful if, whilst devel-
oping and testing new biological air cleaning systems, these data could be included
in published results, as they allow a simple and direct comparison of new systems
for their efficacy.
Darlington et al. (
2000
) investigated a system installed in a 160 m
2
room with a
very low outdoor air change rate (0.2 air changes per hour). Air from the test room
was circulated through an 'ecologically complex' biofilter consisting of a 'biosc-
rubber', 30 m
2
of hydroponic plants and a 3,500 L aquarium containing aquatic and
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