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

remediating organisms are ultimately solar powered, either directly or indirectly,
via the photosynthetic action of the resident autotrophe community.
Aside of cost and amenity grounds, one major positive feature is that the efflu-
ent 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 alternative 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
the capacity of the facility. Determining the 'required' size for a treatment wet-
land is often complicated by uncertainty regarding the full range of wastewater
volumes and component character likely to be encountered over the lifetime of
the operation. The traditional response to this is to err on the side of caution and
oversize, which, of course, has inevitable cost implications, but in addition, also
affects the overall water budget. If the effluent character is known, or a sample
can be obtained, its BOD can be found and it is then a relatively simple pro-
cedure to use this to calculate the necessary system size. However, this should
only ever be taken as indicative. For one thing, bioengineered treatment systems
typically have a lifespan of 15-20 years and the character of the effluent being
treated may well change radically over this time, particularly in response to shifts
in local industrial practice or profile. In addition, though BOD assessment is a
useful point of reference, it is not a uniform indicator of the treatment require-
ments of all effluent components. For the bioamelioration process to proceed
efficiently, a fairly constant water level is necessary. Although the importance
of this in a drought scenario is self-evident, an unwanted influx of water can