Harvesting of Microalgal Biomass - Biotechnological Applications of Microalgae - page 78

multi-stage harvesting techniques for algae farms. In such systems, a variety of har-
vesting technologies are arranged in a sequence based on culture chemistry, specific
characteristics of each technique, and its energy requirements to dewater pond water
to either 5% or 10-20% solids.
Techniques and processes of microalgae cultivation, harvesting, and dewatering
have been reviewed extensively in the literature (Lee et al., 1998; Spolaore et al., 2006;
Khan et al., 2009; Harun et al., 2010; Uduman et al., 2010). Due to the very dilute
culture (<1.0 g of solids L −1 ) and typically small size of microalgae with a diameter
of 3 to 30 μm, large volumes must be handled to harvest algal biomass, which is an
energy-intensive process. Therefore, harvesting microalgal biomass is considered a
challenging issue for commercial-scale production of algal biofuels. Conventional
processes used to harvest microalgae include concentration through centrifugation,
foam fractionation, chemical flocculation, electro-flocculation, membrane filtra-
tion, and ultrasonic separation. The resulting high cost of biofuel production is a
major bottleneck to its commercial application. The cost of harvesting may itself
contribute to approximately 20% to 30% of the total cost of algal biomass, and the
above methods would be viable only if the biomass harvested is used for extract-
ing high-value products such as nutraceuticals (Girma et al., 2003). Harvesting, in
general, can be defined as a series of processes for removing water from the algal
growth culture and increasing the solids content from <1.0% to a consistency of up to
20% solids, depending on the downstream processing requirements for conversion to
fuel. Thermal drying is generally discouraged (except when sufficient waste heat is
available), because the amount of thermal energy needed to dry the algae would be a
major fraction, if not all, of the energy content of the algal biomass.
Industrial production of algal biofuel is still in its infancy and therefore uncer-
tainty in all stages of production and unpredictability of economy has been highly
debated. Some very optimistic estimates on algae biofuels propose that the cost of
algal oil production must be reduced by 5 to 6 times, in addition to the tax and envi-
ronmental subsidies, to make them competitive with petroleum fuels (Chisti, 2007).
For economical production of algal biomass, the selection of harvesting technol-
ogies is so crucial. Several factors, such as algal strain, ionic strength of culture
media, recycling of filtrate, and final fate of harvested biomass, must be consid-
ered when selecting the harvesting technique. For example, the filamentous alga
Cladophora with very long thread-like filaments (several centimeters long) lends
itself to relatively cost-effective harvesting using membrane filtration. In contrast,
chemical flocculation is not recommended if the harvested biomass must be pro-
cessed for nutraceutical and pharmaceutical products because of the residual con-
tamination caused by the flocculants. In general, it is quite difficult to recommend
a single technique as the best for harvesting and recovery without consideration
of specific process conditions and downstream product use. Scientists all over the
world have developed several techniques for harvesting and recovery processes that
rely on facts to simplify this overall process. Judicious exploitation of the different
harvesting technologies is therefore necessary to reduce the harvesting cost and
energy requirements by the desired factor of 2 if algal biomass production is tar-
geted for very low-cost products such as biofuel. Advances in different methods
of algal harvesting and dewatering to resolve our energy crisis, along with energy