Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 161

Engineered reed beds are probably the most familiar of all MaTS, with several
high profile installations in various parts of the globe having made the technology
very widely accessible and well appreciated. This approach has been successfully
applied to a wide variety of industrial effluents, in many different climatic condi-
tions and has currently been enjoying considerable interest as a 'green' alternative
to septic tanks for houses not joined up to mains sewerage. At its heart is the
ability of reeds, often established as monocultures of individual species, or some-
times as oligocultures of a few, closely related forms, to force oxygen down into
the rhizosphere, as has been previously discussed. Many examples feature Phrag-
mites or Typha species, which appear to be particularly good exponents of the
oxygen pump, while simultaneously able to support a healthy rhizospheric micro-
floral complement and provide a stable root zone lattice for associated bacterial
growth and physico-chemical processing of rhizo-contiguous contaminants. Iso-
lated from the surrounding ground by an impermeable clay or polymer layer, the
reed bed is almost the archetypal emergent MaTS.
The mechanisms of action are shown in Figure 7.3 and may be categorised
as surface entrapment of any solids or relatively large particulates on the grow-
ing medium or upper root surface. The hydraulic flow draws the effluent down
through the rhizosphere, where the biodegradable components come into direct
contact with the root zone's indigenous micro-organisms, which are stimulated
to enhanced metabolic activity by the elevated aeration and greater nutrient
availability. There is a net movement of oxygen down through the plant and
a corresponding take up by the reeds of nitrates and minerals made accessible
by the action of nitrifying and other bacteria.
These systems are very efficient at contamination removal, typically achieving
95% or better remediation of a wide variety of pollutant substances, as demon-
strated in Table 7.1, which shows illustrative data on the amelioration of landfill
leachates by this system.
Figure7.3 Diagrammaticrootzoneactivity