310 Organic waste recycling: technology and management
However, interests in the beneficial uses of aquatic weeds are increasing
worldwide and the methods of aquatic weed utilization, as listed in sections 7.5
and 7.6, are being applied in several countries with positive results.
7.2 Major types and functions
Aquatic weeds may be divided into several life forms, a somewhat arbitrary
separation since there are plants, which may change their life form depending on
their stage of growth or on the depth of water. The major types are: submerged,
floating and emergent weeds, as shown in Figures 7.1, 7.2 and 7.3, respectively.
The most common plants and their scientific names are given in Table 7.1.
7.2.1 Submerged type
Weeds that grow below the water surface are called submerged. Often they form
a dense wall of vegetation from the bottom of the water surface. Submerged
species can only grow where there is sufficient light and may be adversely
affected by the onset of factors such as turbidity and excessive populations of
planktonic algae, which decreases the penetration of light into the water. Thus
these species are not effective for effluent polishing.
7.2.2 Floating type
According to Reimer (1984), there are two subtypes of floating weeds, i.e.
floating unattached and floating attached. The roots of floating unattached plants
hang in the water and are not attached to the soil. The leaves and stems are
above the water thus receiving sunlight directly. The submerged roots and stems
serve as habitat for the bacteria responsible for waste stabilization, the aquatic
weeds that cause some of the most widespread and serious problems fall into
this type. As these plants are unattached, populations of the plants are seldom
permanent and, if space permits, are liable to be moved by wind and water
currents or, if confined in one place, initiate the accumulation of organic matter
which decreases the depth of water until it is sufficiently shallow for the
establishment of emergent swamp vegetation. Thus they are essentially primary
colonizers in aquatic ecosystems.
Floating attached plants have their leaves floating on the water surface, but
their roots are anchored in the sediment. The leaves are connected to the bottom
of petioles (e.g. water lilies, Figure 7.2 f and g), or by a combination of petioles
and stems.