Environmental Engineering Reference
In-Depth Information
VOCs are degraded in plant-rhizosphere microbial community systems is gener-
ally poorly described, it is probable that it resembles the processes whereby liquid
or solid phase hydrocarbon water and soil contaminants are biodegraded, although
in many cases the details of the biochemical pathways involved are also still
unknown (e.g. Haritash and Kaushik
2009
; McGenity
2014
).
Research suggests that all potted plant /substrate combinations can remove
VOCs at similar rates for a given volume of growth substrate (Orwell et al.
2004
),
with slightly reduced rates for plants grown in hydroculture, probably due to the
lower microbial density in the substrate (Irga et al.
2013
). Two recent studies have
reported significant variation in VOC removal rates between plant species (Yang
et al.
2009
; Liu et al.
2007
), however, in each study rates were computed on the
basis of removal per unit leaf area, rather than on pot volume, which reveals
removal rates of the rhizosphere. A large number of potted plant species and
varieties have been tested to date (approx. 120: Soreanu et al.
2013
), and all have
shown VOC removal capacity. However, there is still a clear need for further
research into how plants influence the biodegradative capacity of their rhizosphere
microflora.
The identification of bacteria as the primary agents in the biodegradation of
VOCs leads to the potential for improvement of the system's capacity. Zhang et al.
(
2013
) identified the rhizospheric bacteria directly associated with toluene
removal, and postulated that the specific bioaugmentation with these species may
increase the rate of VOC removal. Torpy et al. (
2013a
) demonstrated the microbial
community-level activity specific to biodegradation, and that the benzene removal
efficiency of the bacterial community can be manipulated with the addition of
specific bacterial nutrients, leading to a simple means of increasing the capacity
of the system. Once the concept of using bacterial communities for the biore-
mediation of indoor air pollutants has become widely accepted, it is clear that the
potential for developing very large capacities for VOC removal will be the next
target in the development of these systems for air cleaning.
The great majority of the botanical VOC removal literature describes manip-
ulative laboratory experiments, where potted plants are placed in small, sealed
chambers, into which one or more VOCs are introduced and the rate of decay
determined by analysis of the chamber atmosphere (Fig.
8.1
). Most studies correct
for background VOC losses with no plant or substrate in the chambers, which are
substantial in most studies (e.g. Yoo et al.
2006
). It is obvious that chamber studies
cannot realistically be extrapolated directly to real environments in buildings (e.g.
Llewellyn and Dixon
2011
; Irga et al.
2013
; Soreanu et al.
2013
), primarily
because the plant density per unit volume of chamber atmosphere is far higher than
would be possible in buildings. Future research thus needs to test objectively the
relationship between the chamber performance of potted plants and their perfor-
mance in buildings.
Recent research has attempted to estimate the efficacy of biological air cleaning
systems by determining their outdoor air ventilation rate equivalence. An active
botanical air cleaning system (Wang and Zhang
2011
) was found to be equivalent
to a ventilation rate with 20 % outdoor air, measured on the basis of VOC
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