Aquatic weeds and their utilization 339pond will be dominated by one species of plant, but in some cases a variety of
plants will be combined for a particular treatment objective.
The presence of aquatic weeds in place of suspended algae is the major
physical difference between aquatic treatment systems and waste stabilization
ponds. A waste stabilization pond is highly effective if the algal population
(often occurring as blooms of extremely high density) can be removed prior to
final discharge of the effluent. However, algal biomass is usually carried away
with the pond effluent resulting in an increase in the BOD and the production of
inferior quality effluent. Thus, waste stabilization ponds which sustain good
algal growth are often less effective in producing effluents having acceptable
levels of suspended solids or BOD. Some of these problems associated with
waste stabilization ponds can be overcome by the use of aquatic weeds.
Although the latter may have slower growth rates, they can be more readily
harvested, especially if they are free-floating, and therefore amenable to a
simple mechanized system of continuous harvesting. Moreover, the weed
growth tends to shade out the algae and prevents the erratic fluctuations in algal
population densities that are a frequent characteristic of algal ponds or waste
stabilization ponds.
Contrary to common opinion, the aquatic plants themselves bring about very
little actual treatment of the wastewater. Their function is to provide
components of the aquatic environment that improve the wastewater treatment
capability and/or reliability of that environment. Reported values of oxygen
transfer from leaves to the root surface of aquatic plants are 5-45 g O 2 /(cm^2 -
day), with an average being 20 g O 2 /(cm^2 -day) (WEF 2001). Some specific
functions of aquatic plants in aquatic treatment systems are presented in Table
7.5. These functions may be more clearly envisioned and understood by
considering them in light of the morphologies of aquatic plants, some of which
are shown in Figures 7.1, 7.2, 7.3 and 7.4.
Table 7.5 Functions of aquatic plants in aquatic treatment systems (Stowell et al. 1980)
Plant parts Function
Roots and / or stem Surfaces on which biofilm bacteria grow in the
water column.
Media for filtration and adsorption of solids
Stem and/or leaves at or above
the water surface- Attenuate sunlight and thus can prevent the
growth of suspended algae - Reduce the effect of wind on water
- Reduce the transfer of gases between the
atmosphere and water - Transfer of oxygen from leaves to the root
surfaces