Aquatic weeds and their utilization 323algae for space and food have to be stocked at higher initial stocking density to
prevent algal blooms.
After the weeds have been harvested, their handling is another problem.
Because of their enormous water content (which results in very low bulk
density), the weeds are exceedingly difficult to handle and their transportation is
very expensive. Thus, choppers are often incorporated into harvesting
machinery designed for aquatic weeds. Chopping makes the plants much easier
to handle and reduces their bulk to less than a fourth of the original volume,
greatly simplifying transportation, processing, and storage.
The energy cost of combined harvesting / chopping amounts to about 0.34
kWh/ton and the economic cost to about US$0.14/ton of fresh plants (Bagnall
1979).
7.5.2 Dewatering
Because harvested or chopped aquatic weeds contain 80-95% water, they should
be dewatered prior to being used as animal feed or for other purposes. Although
sun drying can be employed, this must occur so rapidly that mold and decay do
not ruin it, and this practice is not possible all year round.
About half the moisture is on the surface and some is loosely contained in the
vascular system. This water can be removed relatively easily by lightly pressing
the plant. The squeezed out water contains only about 2% of the plant solids and
often can be returned directly to the waterway without causing pollution. Even
with half the water removed, aquatic vegetation is still much wetter than
terrestrial grass. In order to reduce the moisture further, heavier pressing is
required. This process can remove about 70% of the water content and yielding
a product that is comparable to terrestrial forage grasses in moisture content.
Depending on plant species, process design and operating conditions, the
water removed can carry with it 10-30 percent of the plant's solid matter, 15-
35% of the protein, and up to 50% of the ash (for the most part, silt and mineral
encrustations caught on the plants). Roller, belt, cone and screw presses are the
different types commercially available to dewater weeds, but they are usually in
heavy, durable designs that are unnecessarily complex and cumbersome for
pressing aquatic vegetation. Lightweight experimental screw presses (suitable
for developing countries) have been designed. For example, a small screw press
of a simple design has been constructed at the University of Florida, USA (NAS
1976). With its 23 cm bore, this press weighs between 200-250 kg. Complete
with a power plant, it can be carried by a truck, trailer or barge to remote
locations, and can press 4 tons of chopped water hyacinth per hour. Such presses
are compatible with a program of manual harvest and with the small-scale needs
for animal feed in rural areas in developing countries (in addition, these presses