Environmental Engineering Reference
In-Depth Information
and samples were taken one year apart to test the long-term effect of the system.
Total fungal, yeast, mould and bacterial numbers declined in the second year's
samples, suggesting that this active biofiltration system went through a stabilisa-
tion period over which microbial release declined, probably due to a reduction in
the quantity of compostable organic matter. Whilst the biofilter increased fungal
spore numbers relative to adjoining rooms and buildings, the spore density
detected (mean 115 CFU/m
-3
) was considered within the range expected from
normal buildings by Darlington et al. (
2000
), and is certainly surprisingly low
given the large quantity of biological material used in the system. The community
present in the biofiltered room was dominated by Penicillium spp., suggesting the
indoor sources predominated over outdoor sourced genera, which are normally
dominated by Cladosporium spp. (Cabral
2010
). In contrast, Wang and Zhang
(
2011
) performed a pilot trial for their previously described biofiltration systems.
Whilst they could not culture any microbial growth in the outlet air of their system,
the authors used a single culture medium only, and recognised the need for further
research into whether long-term use may lead to the release of microorganisms
from their system. The presence of non-viable bioparticles was also not assessed
by either study. Microbial by-products, such as ergosterol and lipopolysaccharide,
as well as non-living cellular material are well known to lead to disease states such
as allergy, headache, coughing and dermatitis (Mendell et al.
2011
).
A study into the effect of passive botanical air cleaning systems on the indoor
airborne fungal community was performed by Torpy et al. (
2013b
). The air of 54
offices both with and without potted plants was tested with an impaction air
sampler and fungal-specific growth media for viable fungal spores. The authors
found that whilst potted plants were associated with an increase in total spore
numbers, the maximal level reached with two large indoor plants was approxi-
mately an order of magnitude lower than matched outdoor air samples. The
absence of any major fungal pathogens (e.g. Aspergillus fumigatus) in the air
samples provided further evidence that the passive system tested was generally
safe for most building occupants with regard to viable fungi.
As potted plants have become more popular as installations within buildings, some
concerns have been raised as to whether the plants and their growth substrates could
be a reservoir for Legionella spp, the bacteria responsible for Legionnaires disease.
The conditions for Legionella proliferation within man-made environments include
stagnant water, temperatures around 38 C, and some form of carbohydrate nutri-
tional source (Grimes
1991
). Once bacterial growth has taken place, the water then
needs to be aerosolised or come into immediate human contact for infection to occur
(Verissimo et al.
1990
). As most buildings maintain temperatures well below optimal
for Legionella growth, and current indoor plant husbandry practices utilise little or no
excess exposed water, the potential for the proliferation and transmission of
Legionella is limited (Burchett et al.
2007
). However, with active green wall and
biofiltration technologies, there is potential for increased temperatures due to both
mechanical water circulation pumps or integrated lighting systems, coupled with
increased quantities of biological material and the potential requirement for larger
watering and draining basins. These factors may thus lead to significant concern for
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