Composting 1393.8.2 Utilization as feed for fish
In waste recycling through aquaculture, herbivorous fish species (e.g. Tilapia)
feeding on phytoplankton are commonly used. The addition of compost to fish
ponds could increase the growth of phytoplankton, and will increase the fish
growth and yield.
A preliminary experiment was carried out at AIT on Tilapia ponds fed with
the composts whose characteristics are given in Table 3.11. The fish ponds were
earthen, each having a working dimension of 20 x 10 x 1 m^3 , length x width x
depth. Three experimental runs, each lasting 6 months were carried out on 4 fish
ponds. In experiment 1, ponds 2 and 4 served as control without compost
feeding. Although the compost feeding rate for ponds 1 and 3 was 50 kg
COD/(ha-day), the compost materials fed were obtained from the compost piles
where initial C/N ratios were 30/1 and 20/1, respectively. Consequently,
because of the higher nutrients content of the compost pile with the initial C/N
ratio of 20/1 (Table 3.11), the fish yield in pond 3 was almost double of that in
pond 1, while ponds 2 and 4 without compost feeding had fish yields several
times lower than those of the other two ponds (Table 3.12). Based on these
results, the compost materials used in experiments 2 and 3 were obtained from
the compost piles with the initial C/N ratio of 20/1.
The data from Table 3.12 showed that the compost of nightsoil mixed with
water hyacinth and vegetable leaves added to fish ponds increase the fish yield
considerably and the amount of fish yields were almost proportional to the rate
of compost feeding. Also these experimental results strongly indicated the
technical feasibility of using composted nightsoil as feed for Tilapia growth.
More details of the waste fish ponds are given in chapter 6.
The public health hazard resulting from this practice is considered to be
much lower than the direct feeding of septic tank sludge or nightsoil to fish
ponds. This is because most of the enteric microorganisms had been inactivated
by heat during composting, and the remaining in the compost, when applying to
fish ponds, would be diluted and, eventually, subject to natural die-off.
However, in areas where certain helminthic diseases are endemic, care should
be taken to control the transmission of these helminths, whose life cycle include
pond fauna such as snails and/or fish as their intermediate hosts.