by environmental modification may cause micro-algal damage and death and could be unreliable and
uneconomic on a commercial scale (Benemann and Oswald 1996; Lee et al. 2009)SK,§ M jfi. ,1 M Bio-flocculant can be produced by bacteria and bacteria can cause the flocculation of micro-algae (Shelef et al. 1984a). Bio-flocculants produced from bacteria have been shown to be effective in the flocculation of Chlorella (Molina Grima et al. 2003).
Bacteria have also been found to flocculate Pleurochrysis carterae, but a relatively high organic carbon content in the growth medium (0.01 %) is required to grow the bacteria to flocculate the microalgae; approximately 20 % of the carbon content in the growth media from the micro-algae (0.05 %) (Lee et al. 2009).
Bacteria can make up to 30 % of the biomass in the photic Zone of open waters (Sournia 1978), and a large
proportion of the mixed micro-algal biomass grown in wastewater. Many micro-algal species dominant in
wastewater treatment HRAPs often form large colonies (Park et al. 201 l). Effective separation of algae by
sedimentation due to their incorporation into biomass flocs has been demonstrated in symbiotic algal-bacterial wastewater treatment (Medina and Neis 2()07). The use of bacteria grown on waste or wastewater could hold the possibility of a low fossil fuel input method of separating micro-algae, especially if energy within the bacterial biomass could be recovered with that of the micro-algae. Microalgal bacterial floc from the secondary treatment of sewage supplemented by flue gas
from a coal power plant has recently been shown to settle readily, removing 97.5 % of the biomass from
the growth medium within 30 min and producing a sediment of 2 % bacterial-micro-algal dry biomass
(Van den Hende et al. 2011).Electro-coagulation-flocculation using sacrificial aluminium or iron anodes has been shown to be effective at a 1 l bench scale in the flocculation of Chlorella and Phaedactylum, with aluminium anodes
being superior to iron anodes (Vandamme et al. 201 l).Power consumption was favourable in comparison to
centrifugation, at between 0.3 and 2 kWh kg—' with
the lowest energy consumption in salt water, suggesting that electro-coagulation may be a “particularly
attractive method for harvesting of marine micro-algae” (Vandamme et al. 201 l). Aluminium concentration in the micro-algal biomass and growth medium
for recycling was lower than with the use of alum.Although electro-coagulation- flocculation may be a
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promising technology there are concerns about increased power consumption in scale-up as the
distance between electrodes greatly influences power consumption (Vandamme et al. 2011).
In electrolytic flocculation non-sacrificial anodes are used and negatively charged algae move towards
the anode where the negative charge is lost enabling flocs to be formed (Poelman et al. 1997). This has the
advantage that flocculants are not always required, but the electrodes are prone to fouling (Uduman et al.
2010). Electrolytic flocculation has been shown to be effective at a bench scale removing 95 % of the
original micro-algae in suspension with an energy consumption of 0.3 kWh m_3 (Poelman et al. l997).
Ultrasound has also been found to flocculate algae but concentration factors are lower than for other methods
with a maximum increase in concentration of twenty times the feed concentration (Bosma et al. 2003).
Electro-coagulation-flocculation, electrolytic flocculation and ultrasonic flocculation have been shown to
flocculate micro-algae, but there are disadvantages
with each method and none has yet appears to have been demonstrated on a commercial scale.
A wide range of flocculants are available, but there is currently no single flocculant or flocculation method
suitable for all types of micro-algae, and the flocculation of marine micro-algae on an industrial scale has yet to be
satisfactorily resolved. Sedimentation and flocculation appeared to offer potentially the lowest energy input. A
very high energy usage has been reported, however, of 14.8 kWh m'3 for a suspension of Tetraselmis, a
marine micro-alga, using a synthetic cationic polyelectrolyte polymer: a greater energy use per cubic metre
than centrifugation or filtration (Danquah et al. 2009).Another method that could be considered for the
increasing of the particle size to be harvested is by the use of micro-algal predators. Larger predators could
consume the micro-algae and be more easily harvested than the micro-algae. The conversion of plant biomass
to animal biomass is inefficient, however, due to
energy losses from respiration and other metabolic
processes and it appears unlikely that this method will
be a viable commercial option for micro-algal biofuel.
4 Flotation
Flotation can be relatively fast compared to sedimentation for a number of micro-algal species (Edzwald