162 Environmental Biotechnology
mains sewerage. At its heart is the ability of reeds, often established as mono-
cultures of individual species, or sometimes as oligocultures of a few, closely
related forms, to force oxygen down into the rhizosphere, as has been previously
discussed. Many examples featurePhragmitesorTyphaspecies, which appear to
be particularly good exponents of the oxygen pump, while simultaneously able
to support a healthy rhizospheric microfloral complement and provide a stable
root zone lattice for associated bacterial growth and physico-chemical process-
ing of rhizo-contiguous contaminants. Isolated from the surrounding ground by
an impermeable clay or polymer layer, the reed bed is almost the archetypal
emergent macrophyte treatment system.
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.
Nevertheless, reed beds and root zone treatment techniques in general are not
immune from a range of characteristic potential operational problems, which can
act to limit the efficacy of the process. Thus, excessive waterlogging, surface run-
off, poor or irregular substrate penetration and the development of preferential
drainage channels across the beds may all contribute to a lessening of the system’s
performance, in varying degrees.
Nutrient Film Techniques (NFT)
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 phytotreatment 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 applica-
tion. One interesting aspect which did, however, emerge was that the cultivation
system could also be extended to most terrestrial plants, which may yet be of
possible relevance to the future development of land-based phytotreatments.