Organic Waste Recycling

286 Organic waste recycling: technology and management

earthern ponds, each with working dimensions of 20x10x1 m3: length x width x
depth. These ponds were daily fed with septage at organic loadings of 50, 100,
150, 200, 250 and 300 kg COD/(ha-day), and Tilapia stocking densities were
varied at 1, 3, 5, 10 and 20 fish/m^2. The ponds were operated as “non-flow-
through” in which there was no effluent overflow, but canal water was
periodically added to make up for water losses due to evaporation and seepage.
It is apparent that Equation 6.5 is applicable to only the above experimental
conditions. Other types of fish pond conditions and operation will have different
coefficient values for Equation 6.5, but the effects of t and Lc on DOd might be
similar.

Figure 6.13 Cumulative load versus DOd for 100 kg COD/(ha-day) loaded septage ponds

The trend of DOd decline with time as shown in Figures 6.8 and 6.13 is
usually expected in fish ponds fed with organic wastes such as septage, sludge
or manure. This phenomenon is due to the pond hydraulics that are static and

Equation 6.5

DO

(mg/L) d

Time of pond operation, days

Cumulative Load, Lc(kg COD per 200m^3 pond volume)

Predicted data from Equation 6.5

Observed date from Edwards et al. (1984)