Organic Waste Recycling

Fish, chitin, and chitosan production 265

6.1.2 Upgrading effluent from waste stabilization ponds

Algae in suspension increase the suspended solids content of most waste
stabilization pond effluents. (Waste stabilization ponds are a waste treatment
process encompassing reactions of the algal-bacteria symbiosis as described in
Chapters 2 and 5) Effluent discharge without the removal of algae might cause
algal blooms or increasing organic and nutrient loads to the receiving waters.
Introducing fish ponds in series with waste stabilization ponds or its rearing in
the waste stabilization ponds was found to reduce the algal and bacterial
concentrations to a considerable extent when herbivorous fish species (Tilapia
and silver carp) were used (Schroeder 1975). The layouts in Figure 6.1 could be
used for the above purpose. In this way, the death of fish in the fish ponds may
also indicate the presence of some toxic materials and/or low dissolved oxygen
(DO) levels in the effluent discharging into the receiving water.

Figure 6.1 Layouts to upgrade waste stabilization pond (WSP) effluents using fish

6.1.3 Better food conversion ratio

Fish are cold-blooded animals and, unlike other farm animals such as cattle or
poultry, do not have to spend a lot of energy for movement because friction is
less in water. Thus fish have a better food conversion ration than any other farm
animals. Table 6.2 compares the relative growth and feed utilization of farm
animals and fish during a period of rapid growth. Food conversion ratio (FCR)
is defined as:

Wet weightofanimalgained(kg)

Dry weightoffeedgiven(kg)

FCR=

therefore, the lower the FCR, the better the food conversion efficiency.

Waste stabilization ponds Fish pond

Influent Effluent Upgraded effluent

Waste stabilization ponds + Fish

Influent Upgraded effluent