Reducing Indoor Air Pollutants Through Biotechnology - Biotechnologies and Biomimetics for Civil Engineering - page 182

Sorption filtration is the most commonly used air cleaning process (Chen et al.
2005 ). It involves the removal of pollutants (typically VOCs) by adsorption on
solid media. Common adsorbent substrates include activated carbon, zeolite and
activated alumina. Adsorbents generally suffer from VOC saturation after long-
term use, which can be as little as 3-6 months (Chen et al. 2005 ; Wang and Zhang
2011 ). Furthermore, re-emission of the adsorbed VOCs is possible from some
systems (Chen et al. 2005 ), leading to an increased necessity for maintenance.
Ultraviolet-photocatalytic oxidation (UV-PCO) removes gaseous contaminants
via chemical reactions on a semiconductor catalyst surface under UV irradiation.
Whilst the system has been shown to be effective in reducing a range of VOCs
(Zhao and Yang 2003 ; Chen et al. 2005 ;Moetal. 2009 ), this system has not yet
been widely commercialised, although it shows considerable potential (Guieyesse
et al., 2008 ; Mo et al. 2009 ). Photocatalysts may also be added to interior paint
(Auvinen and Wirtanen 2008 ), although such systems are yet to be developed in an
effective form.
Ozone-based air cleaners work on the principle of VOC oxidation. However,
ozone is itself hazardous and the oxidation process may in some cases lead to the
generation of harmful by-products (Chen et al. 2005 ; Kwong et al. 2008 ).
Air ionizers also work on the principle of the oxidation of VOCs, however,
through the action of radicals rather than ozone. This process suffers from the same
disadvantage as ozone air cleaners, and may, in fact, generate ozone (Chen et al.
2005 ).
All physiochemical air cleaners may employ a high efficiency particulate air
(HEPA) filter and various types of pre-filters within the systems for reduction of
particulate matter.
Chen et al. ( 2005 ) compared 15 different air cleaning technologies for their
capacity to reduce airborne concentrations of 16 VOCs. The technologies evalu-
ated included sorption filtration, UV-PCO, ozone oxidation and air ionization
(plasma decomposition), which were compared to an undescribed proprietary
botanical process. Twelve of the air cleaners were portable and three were resi-
dential scale in-duct systems. The study found that sorption-based air cleaners
were the most effective for both VOCs and ketones (formaldehyde and acetalde-
hyde), although some UV-PCO methods were also quite effective. The botanical
filter had little effect on chamber concentrations of VOCs other than n-hexanal and
ketones, although these were removed at a substantially lower rate than the
sorption processes except those using only activated carbon, which did not reduce
ketone concentrations. Ozone generating methods were ineffective, and are not
recommended by the authors. Saturation effects and ozone generation were
observed for several systems, reducing their practical application for air cleaning.
Whilst it is not known how effective the proprietary botanical process in Chen
et al.'s ( 2005 ) study was relative to other biological systems, it would thus appear
that for a simple, short-term comparison, physiochemical methods are more
effective for general removal of VOCs, and certainly particulates, from indoor air.
However when the long-term effectiveness of the systems are taken into account,
along with the simultaneous capacity to remove CO 2 , reduced maintenance and