Harvesting of algal biomass refers to the separation of algae from water for subsequent biofuel production. The process consists of two distinctive steps: (1) bulk harvesting, to separate algae from bulk suspension via gravity sedimentation, flocculation, and flotation, and (2) thickening, to concentrate the algal slurry after bulk harvesting using techniques such as centrifugation and filtration (Brennan and Owende, 2010; Chen et al., 2011). Harvesting of algal biomass is extremely challenging because of algae’s small cell size (gen­erally 1-20 pm) and suspension in water (Lam and Lee, 2012; Suali and Sarbatly, 2012). The mass ratio of algal biomass to water is considered very low, even if the algae are cultivated in a closed photobioreactor (Chen et al., 2011). For example, the mass ratio of algal biomass to wa­ter lies in the range of 0.00035-0.027 for algae cultivated in a closed photobioreactor, assuming a biomass productivity of 0.05-3.8 g/L/day and cultivated for seven days. When the algal cultivation system (typically a closed photobioreactor) is scaled up for mass production of algal biomass, an average of 73 tonnes of water need to be processed when harvesting 1 tonne of algal biomass. This amount of water is quite substantial; thus, developing effective algal harvesting methods is exceptionally important to strengthen the possibility of commercia­lizing algal biofuel production. Table 12.4 summarizes the current available algal biomass harvesting technologies.

A recent LCA study revealed that current technologies for harvesting and drying algal biomass consumed a significant amount of energy input to produce algal biodiesel (Sander and Murthy, 2010). The study assessed two types of algal thickening methods (without prior bulk harvesting), namely, filter press and centrifugation, and reported that each method contributed 88.6% and 92.7%, respectively, to the total energy input for the LCA. Thus,

harvesting algal biomass using solely centrifugation or filtration is still far from commercial application because of the high energy consumption and high operating cost.

On the other hand, bulk harvesting methods such as flocculation offer an alternative approach to harvesting algal biomass with lower energy input and at a reasonable cost. Conventional flocculants, such as ferric chloride (FeCl3), aluminium sulphate (Al2(SO4)3), and ferric sulphate (Fe2(SO4)3) (Brennan and Owende, 2010), which are widely used in waste­water treatment plants, can be used to agglomerate algal cells to become dense flocs (slurry) and subsequently settle out of the cultivation medium (de Godos et al., 2011). After the flocculation process, water that is still retained in the algal slurry can be concentrated further through centrifugation or filtration (Suali and Sarbatly, 2012).

Nevertheless, conventional flocculants that are always referred to as multivalent salts could contaminate the algal biomass and may affect the quality of the final product. Although no scientific work or assessment has been carried out to justify this claim, flocculant toxicity should not be ignored, especially if health-related products are to be extracted from algal biomass before the algal biomass is diverted to biofuel production. Other organic polymeric flocculants that are biodegradable and less toxic offer an alternative and environmentally friendly way to harvest algal biomass, but these organic polymeric flocculants require further development prior to application on the commercial scale.

After concentrating the algal slurry to 5-15% dry solid content through centrifugation or filtration, further dehydration or drying of the slurry is necessary to facilitate subsequent biofuel production (Brennan and Owende, 2010; Lam and Lee, 2012). The presence of water could severely inhibit the biofuel processing and conversion, including lipid extraction using

chemical solvents and biodiesel production through transesterification (Ehimen et al., 2010). The water would cause some difficulty in recovery of chemical solvents as well as biodiesel refining, requiring even higher energy input for subsequent water separation.

Several dehydration methods are currently applicable to drying the algal slurry, including solar drying, spray drying, freeze drying, and fluidized bed drying (Brennan and Owende, 2010; Desmorieux and Decaen, 2005; Orset et al., 1999; Prakash et al., 1997). Solar drying is apparently the most inexpensive dehydration method because it is free, but a large drying surface is required, and it is time-consuming (Prakash et al., 1997). Nevertheless, solar drying is not feasible in temperate countries where sunlight is not always available throughout the year (Lam and Lee, 2012). Thus, the use of heat generated from fossil fuels cannot be avoided to ensure that the algal slurry is continuously dried for each cultivation cycle. Some LCA stud­ies have emphasized that a large amount of energy is consumed in drying the algal slurry, making commercial algal biofuel production even more challenging (Cooney et al., 2011; Lardon et al., 2009; Lohrey and Kochergin, 2012; Sander and Murthy, 2010; Xu et al., 2011). For example, Sander and Murthy (2010) revealed that using natural gas as the fuel to dry the algal slurry consumed nearly 69% of the overall energy input and consequently resulted in a negative energy balance for producing algal biofuels. Heavy dependence on fossil fuels to dry the algal slurry could reduce the market potential and feasibility for producing algal biofuels; thus, new development of an efficient drying method is required to ensure that the energy input in this step is minimized (Lohrey and Kochergin, 2012).