Rev Environ Sci Biotechnol (2013) l2:l65—l78
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1993; Oswald 1988; Singh et al. 201 1). Although some
micro-algae naturally float to the surface, flotation can
be promoted by addition of air bubbles (Singh et al.
2011). [As with micro-algal sedimentation, the addition of flocculants is required in most cases for
flotation to be effective (Edzwald 1993; Mohn 1988).'Flocculation flotation was found to be superior to
sedimentation for the separation of a marine micro-alga, Isochrysis galbana, but only when large strong
flocs were formed by the addition of a combination of organic and inorganic polymers (Shelef et al. 1984b).
The reduced density of micro-al/gal flocs compared to micro-algal cells could favour flotation over sedimen-
tation as a method of separating flocculated micro-algae. I he concentration of micro-algae in the separated suspension from flotation separation (7 %) is generally higher than micro-algal suspension from
sedimentation (Mohn 1988; Oswald 1988).Flotation processes are classified according to the
method of bubble production: dissolved air flotation,
electrolytic flotation and dispersed air flotation (Shelef et al. 1984a). Flocculation and froth flotation has been
found to be effective in the removal of micro-algae from wastewater using fine air bubbles (no dimensions
given) generated by a sparger with gas pressure of 3 atmospheres (Moraine et al. 1979). Dissolved air flotation (DAF) is a process where small bubbles are generated, with a mean size of 40 pm and ranging from 10 to 100 um (Edzwald
1993). Most wastewater treatment lagoons in the USA do not harvest algae, but at plants that do, chemical
coagulation followed by dissolved air flotation (DAF)
is the most common method; the micro-algae removal is for purification of effluent, however, rather than for
micro-algal biomass production (Christenson and Sims 2011). DAF has been found to be effective for
harvesting of micro-algae grown on pig slurry, but a high dosage of alum (0.3 g l_1) was required (Goh
1984). Unfortunately DAF although an efficient flotation option, is energy intensive due to the high
pressure required (Hanotu et al. 2012). Electro-flotation has been found to be effective at a
bench scale on a range of micro-algae, but as with DAF it is energy intensive (Shelef et al. 1984a) and not
the ‘best choice for micro-algal recovery’ (Uduman et al. 2010). Oswald (1988) suggested that it could be
more useful in salt rather than fresh water. Origin Oil developed a process called Quantum FracturingTM, in
which pulsed electromagnetic fields and pH modification fracture the micro-algal cells with lipid
floating to the surface and the remaining micro-algal biomass settling out (Gouveia 2011), but there appears
to be little independent published information on energy consumption.
Micro-bubble generation by fluidic oscillation is a
method for generating small bubbles using less energy
than traditional methods, developed at the University
of Sheffield (Zimmerman et al. 2009). Micro-bubbles
generated by fluidic oscillation have recently been
shown to be effective in the recovery of algal biomass
from growth medium (Hanotu et al. 2012). Considerably more research is required, however, to establish
whether an energy-efficient large-scale fluidic oscillation micro-bubble method for micro-algae harvesting is practicable.
Flotation can have high investment and operational costs and high energy usage (Mohn 1988) especially if small bubbles are required. It has been suggested that the cost of flotation can be as great or greater than centrifugation when the cost of flocculants are included (Mohn 1988) and a recent review has concluded that there is little evidence of the technical
or economic feasibility of flotation (Brennan and Owende 2010).