Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 158

cope with effective waterlogging and functional anoxia. As much as 60% of the
oxygen transported to these parts of the plant can pass out into the rhizosphere,
creating aerobic conditions for the thriving microbial community associated with
the root zone, the leaf surfaces and the surrounding substrate. This accounts for
a significant increase in the dissolved oxygen levels within the water generally
and, most particularly, immediately adjacent to the macrophytes themselves.
The aerobic breakdown of carbon sources is facilitated by this oxygen
transfer, for obvious reasons, and consequently it can be seen to have a major
bearing on the rate of organic carbon biodegradation within the treatment
system, since its adequate removal requires a minimum oxygen flux of one and
a half times the input BOD loading. Importantly, this also makes possible the
direct oxidation of hydrogen sulphide (H 2 S) within the root zone and, in some
cases, iron and manganese.
While from the earlier investigations mentioned on plant/metal interactions
(Hutchinson, 1975) their direct contribution to metal removal is small, fast grow-
ing macrophytes have a high potential uptake rate of some commonly encountered
effluent components. Some kinds of water hyacinth, Eichhornia spp ., for example
can increase their biomass by 10 g/m 2 /day under optimum conditions, which rep-
resents an enormous demand for nitrogen and carbon from their environment. The
direct uptake of nitrogen from water by these floating plants gives them an effec-
tive removal potential which approaches 6000 kg/ha/year and this, coupled with
their effectiveness in degrading phenols and in reducing copper, lead, mercury,
nickel and zinc levels in effluents, explains their use in bio-engineered treatment
systems in warm climates.
Emergent macrophytes are also particularly efficient at removing and storing
nitrogen in their roots, and some can do the same for phosphorus. However,
the position of this latter contaminant in respect of phyto-treatment in general
is less straightforward. In a number of constructed wetland systems, though the
overall efficacy in the reduction of BOD, and the removal of nitrogenous com-
pounds and suspended solids has been high, the allied phosphorus components
have been dealt with much less effectively. This may be of particular concern
if phosphorus-rich effluents are to be routinely treated and there is a consequent
risk of eutrophication resulting. It has been suggested that, while the reasons for
this poor performance are not entirely understood, nor is it a universal finding for
all applications of phyto-treatment, it may be linked to low root zone oxygena-
tion in slow-moving waters (Heathcote, 2000). If this is indeed the case, then the
preceding discussion on the oxygen pump effect of many emergent macrophytes
has clear implications for bio-system design.
As has been established earlier, associated bacteria play a major part in aquatic
plant treatment systems and microbial nitrification and denitrification processes
are the major nitrogen-affecting mechanisms, with anaerobic denitrification,
which typically takes place in the sediment, causing loss to atmosphere, while
aerobic nitrification promotes and facilitates nitrogenous incorporation within the
vegetation. For the effective final removal of assimilated effluent components,