Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 159

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 organisms,
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 performing 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 generally significantly
lower than for comparable conventional treatment operations of similar capac-
ity 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 reme-
diating organisms are ultimately solar powered, either directly or indirectly, via
the photosynthetic action of the resident autotrophe community.
Aside from cost and amenity grounds, one major positive feature is that
the effluent treatment itself is as good or better than that from conventional
systems. When correctly designed, constructed, maintained and managed, plant-
based treatment is a very efficient method of ameliorating wastewaters from a
wide range of sources and in addition, is very tolerant of variance in organic
loadings and effluent quality, which can cause problems for some of the alter-
native options. In addition, phyto-systems can often be very effective at odour
reduction, which is often a major concern for the producers and processors of
effluents rich in biodegradable substances.
Invariably, the better designed, the easier the treatment facility is to manage
and in most cases, 'better' means simpler in practice, since this helps to keep the
maintenance requirement to a minimum and makes maximum use of the existing
topography and resources. Provision should also be made for climatic factors and
most especially, for the possibility of flooding or drought. It is imperative that
adequate consideration is given to the total water budget at the project planning
stage. Although an obvious point, it is important to bear in mind that one of the
major constraints on the use of aquatic systems is an adequate supply of water
throughout the year. While ensuring this is seldom a problem for temperate lands,
for some regions of the world it is a significant concern. Water budgeting is an
attempt to model the total requirement, accounting for the net overall in- and out-
puts, together with the average steady-state volume resident within the system in
operation. Thus, effluent inflow, supplementary 'clean' water and rainfall need
to be balanced against off-take, evaporative and transpirational losses and the
demands of the intended retention time required to treat the particular contami-
nant profile of a given wastewater. One apparent consideration in this process is