158 Environmental Biotechnology
spp., for example, can increase their biomass by 10 g/m^2 /day under optimum
conditions, which represents an enormous demand for nitrogen and carbon from
their environment. The direct uptake of nitrogen from water by these floating
plants gives them an effective removal potential which approaches 6000 kg per
hectare per 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 bioengineered 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 phytotreatment 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 phytotreatment, 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 biosystem 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 vegeta-
tion. For the effective final removal of assimilated effluent components, accessibly
harvestable material is essential, and above water, standing biomass is ideal.
The link between the general desire for biodiversity conservation and the
acceptability of created wetlands was mentioned earlier. One of the most impor-
tant advantages of these systems is their potential to create habitats not just
for ‘popular’ species, like waterfowl, but also for many less well-known organ-
isms, which can be instrumental in bolstering the ecological integrity of the
area. This may be of particular relevance in industrial or urban districts. At the
same time, they can be ascetically pleasing, enhancing the landscape while per-
forming their function. These systems can have relatively low capital costs, but
inevitably every one must be heavily site specific, which means many aspects of
the establishment financing are variable. However, the running costs are gener-
ally significantly lower than for comparable conventional treatment operations of
similar capacity and efficacy. In part the reason for this is that once properly set
up, a well-designed and constructed facility is almost entirely self-maintaining.
However, the major contribution to low operational overheads comes from the
system’s low energy requirements, since gravity drives the water flow and all the