A variation of dissolved-air flotation is dispersed-air flotation, whereby air is directly in­troduced to the flotation tank by various means. Large bubbles of about 1 mm are generated by agitation combined with air injection (froth flotation) or by bubbling air through porous media (foam flotation). In froth flotation, the cultivator aerates the water into a froth, then skims the algae from the top. A highly efficient froth-flotation procedure was developed for harvesting algae from dilute suspensions (Levin et al., 1962). The method did not depend on the addition of surfactants. Harvesting was carried out in a long column containing the feed solution, which was aerated from below. A stable column of foam was produced and harvested from a side arm near the top of the column. The cell concentration of the harvest was a function of pH, aeration rate, aerator porosity, feed concentration, and height of foam in the harvesting column. The authors speculated that economic aspects of this process seemed favorable for mass harvesting of algae for food or other purposes.

The removal of algae and attached water using a froth-flotation method as a function of the collector type, aeration rates, the pH of die algal suspension, and temperature of operation was described by Phoochinda et al. (2005). Dispersed-air flotation was used in this study to remove Scenedesmus quadricauda. The addition of surfactants such as cetyltrimethy — lammonium bromide (CTAB) and sodium dodecyls ulfate (SIDS) increased the aeration rates and reduced the size of air bubbles. Only CTAB gave high algal removal (90%), whereas SIDS gave poor algal removal (16%). However, by decreasing the pH values of the algal suspen­sion, it was possible to increase the algal removal efficiency up to 80%. Low-temperature operation had an important effect on reducing the rate of algal removal, but when the temperature was 20°C or higher, there was little change with further temperature rises.

In a subsequent study, the removal efficiencies of both live and dead algae using the froth — flotation method as a function of the introduction of two types of surfactant, aeration rates, pH, and temperature of operation were compared (Phoochinda et al., 2005). CTAB, a cationic surfactant species, gave comparatively good algal removal efficiency, whereas SIDS, an an­ionic surfactant species, gave, in comparison, a relatively poor removal efficiency. By decreas­ing the ambient pH values of the algal suspensions, SIDS gave an increasingly better extent of separation. As the aeration rates were increased, the removal efficiencies of both the live and the dead algae were increased slightly, whereas when the temperature increased from 20-40°C, the removal rates were, more or less, unchanged. In most cases, the removal of the dead algae was greater than that of the live algae. The surface tension of the dead algal suspensions with CTAB was slightly lower than that of the live algal suspensions with CTAB at comparable concentrations, which may facilitate the removal of the dead algae.

Selectivity for air-bubble attachment is based on the relative degree of wetting (wettabil­ity), specifying the ability of the algal surface to be wetted when in contact with the liquid. Only particles having a specific affinity for air bubbles would rise to the surface (Svarovsky, 1979). Wettability and frothing are controlled by the following three classes of flotation reagents (Shelef et al., 1984):

1. Frothers, which provide stable froth

2. Collectors (promoters), which are surface-active agents that control the particle surface

wettability by varying the contact angle and the particles’ electrokinetic properties

3. Modifiers, which are pH regulators

Golueke and Oswald (1965) reported that only 2 out of 18 tested reagents gave satisfactory concentration of algae harvested, with poor algae removal efficiency. In another study, it was reported that algae harvest was primarily controlled by culture pH in the dispersed-air flotation system operated (Levin et al., 1962). Critical pH level was recorded at 4.0, which was attributed to the changes in the algae surface characteristics.