Organic Waste Recycling

306 Organic waste recycling: technology and management

6.5

a) A small community produces septic tank sludge (septage) with the

following characteristics:

Quantity = 10 m^3 /day

COD = 20000 mg/L

Total Kjeldahl N = 1800 mg/L

This septage is to be used to feed to fish ponds to produce Tilapia with a mean
weight of about 30 g for use as animal feed. Three fish harvests are planned
annually, with 100 days for fish culture and 3 weeks for pond maintenance.
Suppose Tilapia can tolerate DO concentration at dawn of no lower than 1.5
mg/L. What should be the highest COD loading to be applied to the septage-fed
fish pond(s)? Also determine the total land area required, stocking density, the
number of fingerlings needed and the expected fish output for each cycle (120
days).

b) Suppose that for efficient fish harvesting, the pond water is to be drained

out, what should be done with this pond water and the pond sediment that

needs to be removed?

6.6 What are the fish species that the people in your country like to eat? Would
the people mind if the fish sold in the market are reared in waste-fed ponds?
Suggest methods to make the waste-grown fish more acceptable to the
people.

6.7 An agro-industrial factory produces wastewater with the following
characteristics
Flow rate = 10 m^3 /day
COD = 5,000 g/m^3
NH 3 + NH 4 + = 10 g/m^3

The factory plans to apply this wastewater to a fish pond to raise tilapia for use
as animal feed (mean individual fish weight = 30 g)

a) If, to maintain satisfactory D.O. levels at dawn, the organic loading rate

to be operated in the tilapia pond is 100 kg COD/(ha-day), determine

the required dimension (length × width × depth) of the tilapia pond,

appropriate fish stocking density and fish productivity in kg/year (if

this tilapia is to be harvested 2 times per year). Data on fish

productivity is given in Figure 6.11.