Flotation

An alternative to gravity sedimentation is a process calledflotation, which is particularly effective for very thin algae suspension. Whereas gravitational separation works best with heavy algae suspension, flotation is used when suspended particles have a settling velocity so low that they are not able to settle in sedimentation tanks. Flotation is simply gravity thick- ening upside down. Instead of waiting for the sludge particles to settle to the bottom of the tank, liquid-solid separation is brought about by introducing fine air bubbles at the bottom of a flotation tank. The bubbles attach themselves to the particulate matter, and their combined buoyancy encourages the particles to rise to the surface. Once the particles have been floated to the surface, a layer of thickened slurry will be formed and can be collected by a skimming operation. The air-to-solids ratio is probably the most important factor affecting performance of the flotation thickener, which is defined as the weight ratio of air available for flotation to the solids to be floated in the feed stream.

Limited algae biomass is harvested by flotation processes unless coagulant in optimal dose is injected to the algae suspension (Bare et al., 1975). Different coagulants have been used in flotation systems. Chemicals such as aluminum and ferric salts as well as polymers are used to facilitate the flotation. The overall objective is to increase allowable solids loading, percent- age of floated solids, and clarity of the effluent.

The principal advantage of flotation over sedimentation is that very small or light algal particles that settle slowly can be harvested in a much shorter time. Flotation systems also offer higher solids concentrations and lower initial equipment cost. There are three basic variations of the flotation thickening system: dissolved-air flotation, electroflotation (also calledelectrolytic flotation), and dispersed-air flotation.

Based on observation of partial natural flotation of algae (van Vuuren and van Duuren, 1965), a full-scale flotation project was carried out (van Vuuren et al., 1965). Subsequently, work on the flocculation-flotation process for clarifying algae pond effluents was conducted (Bare et al., 1975; Moraine et al., 1980; Sandbank et al., 1974).

Other than algae, flotation of other microorganisms (bacteria) was suggested as a classi- fication and separation process.Gaudin et al. (1962)found thatE. colimay be floated success- fully with 4% sodium chloride. Quaternary ammonium salts were used as surface-active agents for effective bacterial flotation (Grieves and Wang, 1966).

5.3.5.1 Dissolved-Air Flotation

In the dissolved-air flotation system, a liquid stream saturated with pressurized air is added to the dissolved-air flotation unit, where it is mixed with the incoming feed. As the pressure returns to atmosphere, the dissolved air comes out of the liquid, forming fine bubbles that bring fine particles with them as they rise to the surface, where they are removed by a skimmer.

The production of fine air bubbles in the dissolved-air flotation process is based on the higher solubility of air in water as pressure increases. Saturation at pressures higher than at- mospheric and higher than flotation under atmospheric conditions was examined and used for algae separation (Sandbank, 1979). It was suggested that algae separation by dissolved-air flotation should be operated in conjunction with chemical flocculation (Bare et al., 1975;

McGarry and Durrani, 1970). The clarified effluent quality depends on operational parameters

98 5. ALGAL BIOMASS HARVESTING

such as recycling rate, air tank pressure, hydraulic retention time, and particle floating rate (Bare et al., 1975; Sandbank 1979), whereas slurry concentration depends on the skimmer speed and its overboard above-water surface (Moraine et al., 1980).

Algae pond effluent containing a wide range of algae species may successfully be clarified by dissolved-air flotation, achieving thickened slurry up to 6%. The solids concentration of harvested slurry could be further increased by a downstream second-stage flotation (Bare et al., 1975; Friedman et al., 1977; Moraine et al., 1980; Viviers and Briers, 1982). High reliabil- ity of dissolved-air flotation algae separation can be achieved after optimal operating param- eters have been ascertained. Autoflotation of algae by photosynthetically produced dissolved oxygen (DO) following flocculation with alum or C-31 polymer was examined (Koopman and Lincoln, 1983). Algae removal of 80–90%, along with skimmed algal concentrations averaging more than 6% solids, was achieved at liquid overflow rates of up to 2 m/hr. It was reported that the autoflotation was subject to dissolved oxygen concentration. No autoflotation was observed below 16 mg DO/L.

5.3.5.2 Electroflotation

In electroflotation or electrolytic flotation, fine gas bubbles are formed by electrolysis. The formed hydrogen gas attaches to fine algal particles, which float to the surface, where they are removed by a skimmer. Instead of a saturator, a costly rectifier supplying 5–20 DC volts at approximately 11 Amperes per square meter is required. The voltage required to maintain the necessary current density for bubble generation depends on the conductivity of the feed suspension. Further discussion of research on electroflotation is presented inSection 5.3.7.

5.3.5.3 Dispersed-Air Flotation

A variation of dissolved-air flotation is dispersed-air flotation, whereby air is directly in- troduced 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 suspen- sion, it was possible to increase the algal removal efficiency up to 80%. Low-temperature 99

5.3 METHODS OF ALGAE HARVESTING

operation had an important effect on reducing the rate of algal removal, but when the temperature was 20C 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 an- ionic surfactant species, gave, in comparison, a relatively poor removal efficiency. By decreas- ing 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 20–40C, 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 (wettabil- ity), 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.

5.3.5.4 Ozone Flotation

An injected air stream containing ozone gas was used in separating microalgae from high rate oxidation pond effluent by ozone flotation. Use of ozone-induced flotation for algae recovery and effluent treatment was studied (Betzer et al., 1980). The ozone gas promotes cell flotation by modification of algae cell wall surface and by releasing some surface active agents from algae cells. The ozone-flotation process has been studied in numerous applications (Jin et al., 2006; Benoufella et al., 1994).

Elimination of aMicrocystisstrain of cyanobacteria (blue-green algae) by the use of ozone flotation was investigated in a pilot study (Benoufella et al., 1994). The oxidizing properties of ozone and the physical aspects of flotation were exploited in the flotation process. A specific ozone utilization rate ofMicrocystiswas calculated, and ozone concentration and contact time curves were plotted versus algal removal. The study found that use of ozone in pretreatment leads to an inactivation of the algal cells. A prior coagulation stage was necessary for satis- factory cyanobacteria removal, and use of ferric chloride as a coagulant produced the best performance. Preozonation was also of influence on enhancement of the coagulation

100 5. ALGAL BIOMASS HARVESTING

efficiency. Coupling ozone flotation with filtration can improve water quality, with treated effluent indicating low turbidity and low organic content.