Organic Waste Recycling

Fish, chitin, and chitosan production 285

However, these fish yield data were reported to be at least comparable to or
better than those ponds fed with conventional fish feed.

DO-at-dawn (DOd) model
Since the critical period of DO depletion in fish ponds occurs during night time
or the early morning period, a model to predict this critical DO (or DOd) would
be useful for fish pond operators. Boyd (1979) proposed a mass-balance
equation to estimate the amount of DO remaining at dawn.

DOd = DOdusk ± DOdf – DOf –DOin – DOp (6.4)

Where,

DOd = DO concentration at dawn, mg/L

DOdusk = DO concentration at dusk, mg/L

DOdf = gain or loss of oxygen due to diffusion, mg/L

DOf = DO used by fish, mg/L

DOm = DO consumed by sediment, mg/L

DOp = DO used by planktonic community, mg/L

To determine DOd from Equation 6.4, values of the parameters on the right
hand side have to be determined, either experimentally or obtained from
literature. Computer simulations programs have been developed by Boyd
(1979), which could model the dynamics or fluctuation of DOd in channel
catfish ponds satisfactorily.
From a practical point of view, DOd model should be a simple one to enable
fish pond operators to easily determine the occurrence of critical DO, so that
appropriate measures (such as mechanical aeration or temporary discontinuation
of organic waste feeding) can be undertaken. Bhattarai (1985) proposed an
empirical DOd model for waste-fed fish ponds as follows:

DOd = 10.745 exp {-(0.017 t + 0.002 Lc )} (6.5)

Where,

exp = exponential

t = time of septage loading, days

Lc = cumulative organic loading to fish ponds up to time t, kg COD (per

200 m^3 pond volume).

Equation 6.5 was developed and validated with experimental data of Edwards
et al. (1984), as shown in Figure 6.13 for the experiments with a septage loading
of 100 kg COD/(ha-day). These experiments were conducted in Thailand with