Froth Flotation for
Harvesting Algae and Its Possible
Application to Sewage Treatment
GILBERT B. LEVIN, Director of Special Research
JOHN M. BARNES, Plant Pathologist
Bioengineering Department
Hazleton Laboratories, Inc.
Falls Church, Virginia
INTRODUCTION
The modified froth flotation process previously reported (1) as promising economic harvesting of algae has been further developed. In addition to bringing a
practicable algal harvesting system nearer, the new work indicates that the
method may also be useful in the treatment of sewage. The fact that laboratory-
grown cultures of algae frothed without the addition of flotants indicates that a
frothing agent was produced by the cultures themselves. A high-temperature strain
of Chlorella pyrenoidosa was initially selected for use in the studies because its
adaptability to the conditions of laboratory culture and its relatively high reproductive rate make it desirable for mass culturing as a food source or for gas exchange in closed systems. Following acid pH adjustment of the feed culture, it
was placed in a glass cylinder equipped with a porous diffusion plate at the
bottom. Air was forced through the plate, creating fine bubbles in the liquid
above. A stable foam, highly concentrated in algae, formed at the liquid surface
and rose up the column; the foam was collected at a discharge orifice. The feed
solution was virtually clarified by this process which harvested almost all of the
suspended cells (Figure 1).
Hydrochloric acid was used to lower the pH of the harvest feed cultures and
sodium hydroxide for subsequent neutralization. It was found that aeration rate
and pH level influenced the degree of concentration obtained.
Subsequent work has considered the desirability of harvesting only a portion of
the algae and recycling the remaining culture in a continuous growth system.
Successful operation of such a system requires that the algae remain viable despite
exposure to the low pH condition in the harvester. Investigations were directed
toward determination of viability of cells repeatedly exposed to harvesting conditions; and their subsequent growth in fresh media, supplemented, unsupple-
mented, and diluted harvest liquor.
EXPERIMENTAL PROCEDURES AND RESULTS
Viability of Cells
In order to determine the viability of cells which were continuously recycled,
and resuspended in fresh medium, the following determinations were made.
Seven hundred ml of urea medium (Composition in mg/1 deionized-distilled
water: MgS04-7H2O, 1, 000; KH2PO4, 250; Urea, 1,000; iron sequestrine, 35;
manganese sequestrine,   3; and sequestrines of copper, zinc and cobalt, 1.) were
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