Organic Waste Recycling

Algal production 231

Iz

= exp (Ca.Į.z) (5.1)

Ii

Where,

Ii = the measured light intensity at the pond surface, varying from 0 to

20,000 ft-candle

Iz = the measured light intensity at depth z, ft-candle

Į = the specific absorption coefficient, ranging from 1x10-3 to 2x10-3

Ca = the concentration of algae, mg/L

z = pond depth, cm

For practical design it should be assumed that all available light is absorbed;
therefore, at the pond bottom the transmitted light, Iz, should be relatively small.
If Iz is taken as equal to 1, Equation 5.1 can be written as:

z =1n Ii (5.2)

Ca.Į

Equation 5.1 defines the effective depth for photosynthetic oxygen
production in as much as there is no visible light and, hence, no algal growth
below depth z.
To determine z for an HRAP, the values of Ii, Ca and Į have to be selected. Ii
values can be obtained from meteorological data of that particular area which
will vary as seasons, climate and latitude, normally from a few hundred ft-
candles to more than 10,000 ft-candles. Ca concentrations in HRAP are between
200 and 300 mg/L. The value of Į depends on the algal species and their
pigmentation, which practically may be taken as 1.5x10-3.
It appears from Equation 5.2 that Ca is the only controllable parameter, which
determines optimum pond depth for an HRAP. Theoretically, the depth for
maximum algal growth should be in the range of from 4.5 to 5 in. (about 12.5
cm). Oswald (1963) carried out laboratory and pilot-scale experiments and
found the optimum depth to range from 8 to 10 in. (20 to 25 cm). However from
a practical point of view, the depth should be greater than 20 cm (i.e. 40-50 cm)
to allow for the sludge layer and for maintaining the needed HRT (Moraine et
al. 1979).