Organic Waste Recycling

Algal production 249

coagulant materials and other operating conditions for the coagulation-
flocculation process.

Figure 5.12 Schematic of a dissolved air flotation unit

Dissolved-air flotation (DAF)

Theoretically, there is a tendency for algal cells to float to the water surface due
to the release of supersaturated oxygen gas generated during the photosynthetic
period. To make the algal cells float more effectively DAF is normally
employed. In DAF systems, air dissolved in the water under a pressure of
several atmospheres, and the pressure is later released to the atmospheric level
in the flotation tank. For small-scale operation, part or all of the influent flow is
pressurized and is added with coagulant materials such as alum (Figure 5.12).
For large-scale operation and to improve the system performance, the clarified
effluent is normally recycled to mix with the influent feed prior to being
released into the flotation tank. In this case, the recycled effluent is pressurized
and semi-saturated with air, and is mixed with the unpressurized influent water.
DAF efficiency depends on various factors such as the air/solids ratio, dissolved
air pressure, flotation time, types and dose of coagulant aids, and pH of the
water. There are various types of DAF units produced by some manufacturers in
Europe and U.S.A. for full-scale application.
McGarry and Durrani (1970) conducted DAF experiments on HRAP water
and found that at the alum dosages of 125-145 mg/L, pH = 5-7, air pressure =
35-50 psi gage (2.4-3.4 atmospheric pressures), and flotation time = 6-10 min,
an algal concentration of 8% was obtained in the overflow effluent. Bare et al.

Pressurized

influent

wastewater

Algal concentrate to

be removed manually

or mechanically

Chemical

coagulant

Clarified

effluent

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