Biomedical Engineering Reference
In-Depth Information
are submerged, floating or emergent species. Both biotic and abiotic methods
are involved. The main biological mechanisms are direct uptake and accumula-
tion, performed in much the same manner as terrestrial plants. The remainder
of the effect is brought about by chemical and physical reactions, principally
at the interfaces of the water and sediment, the sediment and the root or the
plant body and the water. In general, it is possible to characterise the primary
processes within the MaTS as the uptake and transformation of contaminants by
microorganisms and plants and their subsequent biodegradation and biotransfor-
mation; the absorption, adsorption and ion exchange on the surfaces of plants and
the sediment; the filtration and chemical precipitation of pollutants via sediment
contact; the settlement of suspended solids; the chemical transformation of con-
taminants. It has been suggested that although settlement inevitably causes the
accrual of metals, in particular, within the sediment, the plants themselves do not
tend to accumulate them within their tissues. While this appears to be borne out,
particularly by original studies of natural wetlands used for the discharge of mine
washings (Hutchinson, 1975), this does not form any basis on which to disregard
the contribution the plants make to water treatment. For one thing, planting den-
sities in engineered systems are typically high and the species involved tend to be
included solely for their desired phytoremediation properties, both circumstances
seldom repeated in nature. Moreover, much of the biological pollutant abatement
potential of the system exists through the synergistic activity of the entire com-
munity and, in purely direct terms, this largely means the indigenous microbes.
Functionally, there are strong parallels between this and the processes of enhanced
rhizospheric biodegradation described for terrestrial applications. While exactly
the same mechanisms are available within the root zone in an aquatic setting,
in addition, and particularly in the case of submerged vegetation, the surface of
the plants themselves becomes a large extra substrate for the attached growth of
closely associated bacteria and other microbial species. The combined rhizo- and
circum-phyllo- spheres support a large total microbial biomass, with a distinctly
different compositional character, which exhibits a high level of bio-activity,
relative to other microbial communities. As with rhizodegradation on dry land,
part of the reason is the increased localised oxygenation in their vicinity and
the corresponding presence of significant quantities of plant metabolic exudates,
which, as was mentioned in the relevant earlier section, represents a major pro-
portion of the yearly photosynthetic output. In this way, the main role of the
macrophytes themselves clearly is more of an indirect one, bringing about local
environmental enhancement and optimisation for remediative microbes, rather
than being directly implicated in activities of primary biodegradation. In addi-
tion, physico-chemical mechanisms are also at work. The iron plaques which
form on the plant roots trap certain metals, notably arsenic (Otte, Kearns and
Doyle, 1995), while direct adsorption and chemical/biochemical reactions play a
role in the removal of metals from the wastewater and their subsequent retention
in sediments.
The ability of emergent macrophytes to transfer oxygen to their submerged
portions is a well appreciated phenomenon, which in nature enables them to
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