Organic Waste Recycling

Algal production 233

Ca .h.z

HRT = (5.3)

1000 F.Io.Tc

or

Lt .h.z

HRT = (5.4)

1000 F.Io.Tc p

Where:

HRT = days

h = unit heat of combustion of the algae, kcal/g of algae (for sewage-

grown algae = 6 kcal/g)

F = efficiency of light energy conversion to chemical energy,

usually = 0.1

Io = the amount of visible solar energy penetrating a smooth water

surface, varying from 0 to 800 gcal/(cm^2 -day)

Tc = temperature coefficient, values given in Table 5.2

Lt = ultimate BOD or COD of influent wastewater, mg/L

p = ratio of weight of O 2 produced and weight of algal cells synthesized,

calculated from Equation 2.6 to be 1.58

The term Ca and z are as defined previously. Equation 5.4 indicates that HRT
varies inversely with Io. Therefore a strong Io may be associated with a reduced
HRT. Oswald et al. (1953) indicated that while other factors being constant, the
most favorable environment for algal growth in continuous cultures is obtained
at relatively short HRT. Under these conditions, algal growth is maintained in
logarithmic phase and cells are large and fat, rich in chlorophyll, low in
carbohydrate and produce protoplasm rapidly. For biological reasons the HRT
should be larger than 1.8 days which is a minimum time span for generation of
the algae in HRAP (Oron and Shelef 1982). Oswald et al. (1953) reported that
the algal fraction of the SS in effluent increases with increasing HRT, and at
HRT greater than 4 days, practically all of the VSS consist of algal cells.
Moraine et al. (1979) reported that algae constituted about 30-90% of the VSS
in an HRAP in Israel, and 65% being typical. They also found longer HRT to
favor higher algal fraction in the HRAP water.
The maximum value for HRT of HRAP should not exceed 8 days, since
under these conditions the pond will be under-loaded (lack of nutrients), causing
a decrease in algae concentration. Therefore, for the maximization of the net
production in HRAP, the choice of the optimum value of HRT/z (subject to
environmental and biological conditions) is of utmost importance. From a