Biomedical Engineering Reference
In-Depth Information
Nutrient Film Techniques (NFTs)
An alternative approach to the use of aquatic macrophytes, which was tried exper-
imentally, involved growing plants on an impermeable containment layer, in a
thin film of water. In this system, the wastewater flowed directly over the root
mass, thereby avoiding some of the mass transfer problems sometimes encoun-
tered by other aquatic phyto-treatment regimes. Though the early work indicated
that it had considerable potential for use in the biological treatment of sewage and
other nutrient-rich effluents, it does not appear to have been developed further
and little is known as to the conditions which govern its successful practical
application. One interesting aspect which did, however, emerge at the time was
that the cultivation system could also be extended to most terrestrial plants. Sub-
sequently, the potential for developing Nutrient Film Techniques (NFTs) into
a relatively simple and inexpensive means to treat municipal wastewaters suf-
ficiently to meet discharge standards has indeed been shown for a number of
such species, including
Datura innoxia
(Vaillant
et al
., 2003)
Chrysanthemum
cinerariaefolium
(Vaillant
et al
., 2002), a variety of commercial roses (Monnet
et al
., 2002) and both
Digitalis lanata
and
purpurea
(Vaillant
et al
., 2004). This
may ultimately pave the way for the further development of such land-based
phyto-treatments in the future.
Algal Treatment Systems (ATS)
Algae have principally been employed to remove nitrogen and phosphorus
from wastewaters, though some organic chemicals can also be treated and
there has been renewed interest
in making use of
their efficient carbon
sequestration potential.
Effluent treatment
Algal effluent treatment systems work on the basis of functional eutrophication
and rely on a dynamic equilibrium between the autotrophic algae themselves and
the resident heterotrophic bacteria, which establishes a two stage biodegrada-
tion/assimilation process, as shown in Figure 7.4. In effect this is an ecological
microcosm in which organic contaminants present in the wastewater are biolog-
ically decomposed by the aerobic bacteria, which make use of oxygen provided
by algal photosynthesis, while the algae grow using the nutrients produced by
this bacterial breakdown, and photosynthesise producing more oxygen.
Though the process is self-sustaining, it is also self-limiting and left to pro-
ceed unchecked, will result in the well-appreciated characteristic eutrophic stages
leading to the eventual death of all component organisms, since true climactic
balance is never achieved in the presence of continuously high additional nutrient
inputs. The removal of excess algal and bacterial biomass is, therefore, an essen-
tial feature, vital to maintaining the system's efficiency.
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