Organic Waste Recycling

318 Organic waste recycling: technology and management

Floating weeds are also equally or more productive than emergent plants as
they can be moved with winds and currents and their productivity far exceed the
biomass yields of many subtropical terrestrial, salt water, and freshwater plants
(Wolverton and McDonald 1978).
The growth rate for aquatic weeds can also be expressed as the specific
growth rate independent of biomass or the units in which biomass is measured
(dry weight). The values can be measured as fractional increase per day or
percent increase per day. In 1969, Bock calculated the daily incremental factor
using the following formula (Mitchell 1974):

t

Nt=N 0 .x (7.1)

Where:

No= initial number of plants

Nt= final number of plants

t = time interval in days

x = daily incremental factor

However, the specific growth rates are generally high at low plant density
and decrease as the plant density increases.
Growth of aquatic weeds can also be reported in terms of doubling time, the
time taken for the material present to double itself (or Nt /No = 2). In 1974,
Gaudet while working with Salvinia minima and S. molesta under standardized
culture conditions calculated the doubling time using the following formula
(Mitchell 1974):

Doubling time = ln 2 (7.2)

(ln Nt – ln N 0 )/t

The notations are the same as used in Equation 7.1.
Under favorable conditions, the area doubling time for water hyacinth ranges
between 11-18 days, depending on the weather. Mitchell (1974) obtained
doubling times for S. molesta of 4.6-8.9 days in culture solutions in the
laboratory, compared to 8.6 days on Lake Kariba, Africa. Bagnall et al. (1974)
and Cornwell et al. (1977) both reported doubling times of 6.2 days for water
hyacinth grown on a stabilization pond receiving secondarily treated effluent.
The variation of doubling times reported in the literature is due to the effects of
weather, nutrients, growing season and plant density.
Knowledge of area doubling time is useful to determine the frequency of
plant harvesting and the amount of harvested plant biomass to be further