Harvesting of Microalgal Biomass - Biotechnological Applications of Microalgae - page 83

by dissolution of the reactive anode, destabilization of colloidal suspensions, and
aggregation of destabilized suspensions, resulting in the formation of algal flocs.
The EC process is a more efficient chemical flocculation technique compared to
conventional processes of direct interaction of the aluminum sulfate with algal
suspensions (Aragón et al., 1992). The flocculated biomass is removed from water,
either by sedimentation or flotation and skimming. For the latter to be effective,
inactive metal cathodes are used to generate micro-gas (mainly hydrogen) bubbles
that get entrapped in algae flocs and float them to the surface. Complete biomass
removal from algal cultures having cell densities of 0.55 × 10 9 to 1.55 × 10 9 cells
mL −1 has been reported using this process (Gao et al., 2010). However, the process
may not be very effective for very dilute algal solutions because at low concentra-
tions of total suspended solids (representing algal cells), the amount of colloids pres-
ent in the culture solution may not be sufficient for significant amounts of settleable
solids (Azarian et al., 2007).

6.9 ENERGY EFFICIENCIES OF HARVESTING PROCESSES
In terms of energy inputs, harvesting of algal biomass is the most energy-intensive
process in biomass production. To date, there has been no specific commercial-
scale algal harvesting technique that has been developed, and the approach has been
to adapt separation technologies already in use in wastewater treatment and food
processing industries. Therefore, the energy consumption and energy efficiency
information available from those industries are discussed in this chapter to compare
the energy efficiency of different algal harvesting techniques. The highest possible
solids recovery (as %(w/v) total suspended solids (TSS)) and energy requirements for
each of the harvesting processes are given in Table 6.1.

TABLE 6.1
Summary of Energy Usage and Highest Possible Solids (%w/v) Yields of
Different Algae Harvesting Techniques

Highest Yield
(% solids)

Energy Usage
(kW-hm −3 )

Harvesting Process Ref.

Centrifugation 22.0 8.00 Girma et al., 2003

Gravity sedimentation 1.5 0.1 Shelef et al., 1984

Filtration (natural) 6.0 0.4 Semerjian et al., 2003

Filtration (pressurized) 27.0 0.88 Semerjian et al., 2003

Tangential flow filtration 8.9 2.06 Danquah et al., 2009

Vacuum filtration 18 5.9 Girma et al., 2003

Polymer flocculation 15.0 14.81 Danquah et al., 2009

Electro-coagulation NA 1.5

Bektaş et al., 2004

Electro-flotation 5.0 5.0 Shelef et al., 1984;

Azarian et al., 2007

Electro-flocculation NA 0.331 Edzwald, 1995