234 Organic waste recycling: technology and management
practical point of view, HRT/z at a range of 6 to 12 day/m seems to be
appropriate for all operational purposes (Oron and Shelef 1982).
BOD loading
BOD loading of HRAP should have an influence on algal yield because a too-
high BOD loading can result in anaerobiosis and interference with the algal-
bacterial symbiosis. Hsu (1970) did an experiment on HRAP in Thailand using
diluted-fresh Bangkok night soil as raw material to find out the optimum design
parameters of HRAP. He concluded that, for tropical region like Bangkok, the
optimum pond depth, HRT and BOD 5 loading should be 0.35 m, 1.5 days, and
300 lbs/(acre-day) (336 kg/(ha-day)), respectively. Under these conditions,
BOD 5 removal efficiency was about 95%, the effluent BOD 5 was below 10
mg/L, and the algal yield was around 350 lbs/(acre-day) (390 kg/(ha-day)). An
experiment on a 200-m^2 HRAP by Edwards and Sinchumpasak (1981) using
weak sewage (mean BOD 5 = 45 mg/L) resulted in the mean algal concentration
of 94 mg/L or the mean algal yield of 157 kg/(ha-day); the organic load and
HRT applied to this pond were 75 kg BOD 5 /(ha-day) and 3 days, respectively.
Table 5.5 Temperature coefficients for chlorella grown in pilot-plant (Oswald and
Gotaas 1955)
Mean temperature, in degrees
Centigrade Fahrenheit Photosynthetic temperature coefficient, Tc
0 32 -
5 41 0.26
10 50 0.49
15 59 0.87
20 68 1.00
25 77 0.91
30 86 0.82
35 95 0.69
40 104 -Mixing and recirculation
Mixing of HRAP content is essential to prevent algal sedimentation and provide
interactions between the benthic deposits and free oxygen containing
supernatant. Mixing keeps the nutrients in active contact with the algal cell
surface, leading to a stimulation of metabolic activities and a more effective
utilization of incident light (Persoone et al. 1980). In large-scale HRAP, mixing
can prevent thermal stratification and the development of anaerobic condition at
the pond bottom, and avoid photo-inhibition through a decrease of the duration