Harvesting of Microalgal Biomass - Biotechnological Applications of Microalgae - page 79

utilization by various techniques with respective constraints and drawbacks of each
method, are discussed in this chapter. In addition, this chapter discusses the pros and
cons of different algae harvesting techniques along with their energy requirements.
The potential advantages of multi-stage hybrid harvesting systems involving more
than one technique deployed in a specific sequence for efficient and energy-effective
biomass recovery are discussed.
6.2 HARVESTING PROCESSES
The term harvesting refers to the concentration of dilute microalgal culture sus-
pension to slurry or paste containing 5% to 25%, or more, total suspended solids
(TSS). This slurry can be obtained in either a one-step or two-step harvesting pro-
cess. Subsequent processing of the algal paste depends on the concentration of the
algal paste. Increased product concentration decreases the cost of extraction and
purification, as well as the effective unit cost of biomass. Concentration of algal
paste significantly influences downstream processes, including drying. Microalgae
are particles that have a colloidal character in suspension. Electric repulsive inter-
action between algal cells and cell interaction with the surrounding water provide
stability to the algal suspension. Algal cells are usually characterized as negatively
charged surfaces where the intensity of charge is a function of the species, ionic
strength, and pH of the cultivation media (Taylor et al., 1998). These surface charges
are helpful in the growth culture because they assist in keeping the cells in the water
column so that they do not settle to the bottom of the pond, particularly in regions of
the pond where the water velocity is low. However, the charges pose a challenge to
the dewatering process because they eliminate the option of using a simple settling
tank (or pond) for harvesting.
Harvesting and dewatering processes can be divided into two categories, namely
(1) those in which the dewatering is performed directly on the algae culture, and
(2) those involving agglomeration of the algae into macroscopic masses to facilitate
the dewatering process. The former, which include centrifuges and membrane filters,
avoid the complications and costs associated with the addition of coagulation and
flocculation chemicals. Processes like flocculation, flotation and gravity sedimenta-
tion have acceptable energy requirements but have a fairly wide range of costs asso-
ciated with motors and controls.
6.3 GRAVITY SEDIMENTATION
The sedimentation rates of algae are influenced by the settling velocity of micro-
algae, which can be increased by increasing cell dimensions (i.e., by aggregation
of cells into large bodies) (Schenk et al., 2008). This principle is being applied to
algal harvesting, wherein chemicals are added to enhance flocculation, causing the
large algal flocs to settle more readily to the bottom of the container. The floccula-
tion of algal biomass is generally followed by gravity sedimentation for settling
of algal flocs, thus enhancing the efficiency of this process. Gravity sedimentation
preceded by flocculation is one of the most commonly used techniques for first-
stage (1% to 5% solids) algae biomass harvesting (Girma et al., 2003; Pittman