Organic Waste Recycling

Fish, chitin, and chitosan production 287

without effluent overflow. All the loaded organic wastes stay in the ponds and
exert an oxygen demand which keeps on increasing due to the accumulation of
the residual organic compounds and solubilization of some organics from the
sediment layer (see Figures 6.8 a and b). After some period of pond operation,
the DOd level seemed to reach a plateau in which a further decline of DOd was
not observed much, an indication of a somewhat steady-steady condition with
respect to DOd dynamics in the waste-fed ponds.
For the flow-through ponds fed with wastewater, there is effluent flow that is
more or less equal to that of the influent flow. In this case there is less sediment
accumulation at the pond bottom and consequently less variation of the DOd
concentration with time of pond operation.

Tilapia growth model
An empirical model of Tilapia growth in septage-fed fish ponds was developed
by Bhattarai (1985) using the data of Edwards et al. (1984). The model
considered the effects of: a) fish stocking density (SD), as evidenced from
Figure 6.11; b) nitrogen loading, as nutrient source for algal growth (or Tilapia
food); and c) time of fish culture. The effect of initial fish weight was not
included because all the fish stocked had approximately the same initial weight
of about 2.7 g. The developed model, assuming a constant temperature of 30 °C
and neglecting spawning, is

Wt = 12.032 (t.N/SD)0.707 (6.6)

Where:

Wt = mean fish weight at time t, g

t = time of fish rearing, months

N = total Kjeldahl nitrogen (organic N + NH 3 -N + NH 4 -N) loading, kg/(ha-day)

SD = fish stocking density, no./m^2

Equation 6.6 is valid when N, t and SD are greater than zero, and the term
(t.N/SD) can be interpreted as the applied weight of nitrogen per fish. Figure
6.14 shows a typical relationship between the observed and predicted fish
weight (using Equation 6.6) in which a good correlation is observed.
It should be pointed out that the development of Equations 6.5 and 6.6 and
their validations were undertaken by using different sets of experimental data.
However, it is apparent that, to ensure their applicability, these two equations
should be validated further with other field-scale data and with data of fish
ponds having different modes of operation, fish species and types of organic
waste input.