Organic Waste Recycling

324 Organic waste recycling: technology and management

can be manufactured locally). Estimates of aquatic weed pressing costs range
from US$5.0 to US$10.0/ton of dry matter (depending on machine cost,
machine use, production rate, amortization time and labour costs) (NAS 1976;
Bagnall 1979; Bagnall et al. 1974).
In some arid climates solar drying may be feasible, either by exposing a thin
layer of the plants to the sun or by more sophisticated solar collector-heat
transfer systems. In the U.S., a solar drying system for aquatic weeds was
constructed at the National Space Technology Laboratories. Depending on the
weather, it is capable of drying 15-17 tons of newly harvested water hyacinth in
36-120 hours (Figure 7.7). Drying of the aquatic weeds with conventional fuel-
heated air is economically impractical because of the high moisture content.

Figure 7.7 Solar energy being harnessed to dry aquatic weeds at the National Space
Technology Laboratories (NSTL) near Bay St. Louis, Mississippi (NAS 1976). Note: this
system is connected to a lagoon full of water hyacinth growing in sewage effluent.

Water hyancinth-filled

sewage treatment lagoon

NSTL vascular aquatic

plant drying system

Greenhouse

Solar heat

collectors