Aquatic weeds and their utilization 351Use of plants and animals in aquatic treatment
The fundamental purpose of plants, animals and the management of these
organisms in aquatic systems is to provide components to the aquatic
environments that improve the rate and/or reliability of one or more of the
contaminant removal mechanisms to be operative by design in the aquatic
treatment system. Plants play a more dominant role than animals in aquatic
systems because plants have a greater effect on the aquatic environment and are
more adaptable to harsh and/or fluctuating environment.
Aquatic animals are used, primarily, as biological controls of insect vectors
and as bioassay organisms for monitoring the treatment performance of an APU
or aquatic system. In certain situations animals can also play a role in plant
harvesting and suspended solids removal. In general, however, the direct uptake
and/or removal of wastewater contaminants by plants and animals are not
significant treatment mechanisms. An observation of interest is that some
molecular oxygen produced by the photosynthetic tissue of plants is translocated
to the roots and may keep root zone microorganisms metabolizing aerobically,
though the surrounding water is anoxic (Tchobanoglous et al. 1979).
7.7.3 Process design parameters
The different parameters currently being used in the design of aquatic systems
include organic loading rate, hydraulic loading rate, hydraulic application rate,
hydraulic detention time, and nitrogen loading rate. Depending on the treatment
objective the major design parameter will be different, for example, organic
loading rate is important for organic matter removal. Root depth of the plant is
important as it provides the bacterial support and the media for interaction
between wastewater and the bacterial mass. Even though there are many
potential aquatic plants that can be used for waste treatment they have not yet
been investigated to develop rational design criteria. Design criteria for water
hyacinth treatment systems are given below, while some data from the existing
systems are given in section 7.7.4.
Hydraulic retention time and process kinetics
Hydraulic retention time, typically expressed in days, is the most frequent
parameter used to design aquatic systems. The greatest benefit derived from the
use of hydraulic retention time is that a majority of performance data reported in
the literature are correlated to retention time. The use of hydraulic retention
time, however, has some serious drawbacks. Depending on system geometry,
systems with similar hydraulic retention times may be quite different
hydraulically. Also, accurate determination of hydraulic retention time is