Microalgal Biotechnology: Today’s (Green) Gold Rush - Biotechnological Applications of Microalgae - page 215

potassium, zinc, chitosan, extracellular polymeric substances, bioflocculants such as
Paenibacillus + aluminum sulfate, and organic cationic polymers. Co-bioflocculation
( Nannochloris + diatoms) is also used, but necessitates extra effort to grow another
alga. Pressure filtration (10 PSIG [pounds per square inch gage]) through four coni-
cal felt media bags (1 µm), ultrasonication and grinding, cross-flow microfiltration/
ultrafiltration, and continuous foam separation are some of the other methods used
for algal harvest. These techniques are labor intensive, expensive, and inefficient,
with yields in the range of 30% biomass. Coagulation in the presence of chemicals is
an alternative method for harvesting algae. Scenedesmus subspicatus, Selenastrum
capricornutum, and Nannochloropsis sp . exposed to 5 mg L −1 barium concentra-
tions bio-accumulate up to 88% to 99% of barium within 10 days (Theegala et al.,
2001). Further treatment with 200 mg L −1 ferric chloride facilitated harvest of the
metal-laden microalgae with an efficiency of nearly 99%.
Tetraselmis suecica can be concentrated up to 148 times using tangential flow
filtration (TFF) and up to 357 times with polymer flocculation (PF); TFF requires a
high initial capital investment and consumes 2.06 kWh m −3 while PF requires low
initial investment with energy consumption in the range of 14.81 kWh m −3 (Danquah
et al., 2009). The payback period, an important criterion for the investor, is 1.5 years
for TFF and 3 years for PF. Passive and active immobilization techniques hold
promise for harvesting high-value molecules such as storage products, antibiotics,
hydrocarbons, hydrogen, and glycerol, and should be explored (Lebeau and Robert,
2006). Algaeventure Systems (AVS) has been developing an AVS Harvester that
uses conveyor belts of capillaries to concentrate and dry Chlorella cultures. The esti-
mated processing cost is $1.92 per ton compared to $875 per ton by centrifugation.
One of the drawbacks is the required algal concentration of 3 g L −1 ; improvements
are being pursued to increase harvesting efficiency.
An aqueous and a biocompatible organic phase (dodecane) bioreactor exist to
extract β-carotene from Dunaliella salina cells. The organic phase continuously
removes β-carotene (“milked”) from the cells with greater than 55% efficiency, and
productivity is 2.45 mg m −2 d −1 , which is much higher than that of commercial plants
(Hejazi et al., 2004). Several other methods of harvesting microalgae from liquid
cultures are being developed, looking for a breakthrough to drastically reduce har-
vesting and dewatering costs. Although details have not been published, mention
should be made of the following:
1. Pretreatment of algae that involves application of 10 to 30 kV cm −1 electri-
cal pulses for 2 to 20 μs to an algal slurry to rupture the cell walls and
to release biodiesel compounds such as methyl hexadecanoate (Diversified
Technologies at the University of Galway, Ireland).
2. Usage of amphiphilic solvents, such as acetone, methanol, ethanol, isopro-
panol, butanone, dimethyl ether, or propionaldehyde, to separate out the
proteins and carbohydrates from the lipids (Aurora Algae).
3. Harvesting, dewatering, and drying system utilizing surface physics and
low-energy capillary action (Algaeventure Systems).
4. Single-step and live extraction of lipids (Origin Oil, James Cook University).
5. Hydrothermal liquefaction or thermal depolymerization (New Oil Resources).