The main step after the bulk cultivation of algae is its harvesting, which performs a very vital part in shaping the process budget of algal biofuels. Despite the excessive presence of algal biomass, the harvesting of macro-algal biomass is considered as simpler and less costly as compared to the harvesting of algal biomass. Due to the diluted nature of algal culture cells and small size, the operating expenses of dewa­tering and harvesting of algal biomass is high. The typical size of single-celled eukaryotic algae is measured around 3-30 pm (Grima et al. 2003), and the range of cyanobacteria is 0.2-3 pm (Chorus and Bartram 1999) . The improvement and wide-scale application of different technologies for energy generation is currently a great challenge and of a significance to the scientists and the machinists of active systems. It is generally believed that numerous roots, properties, and active transfor­mation of biomass are the main bases of renewable energy (McKendry 2002; Goyal et al. 2008). A number of procedures including chemical as well as mechanical can be performed, which includes centrifugation, flotation, flocculation, filtration, screening and gravity sedimentation, and electrophoresis for harvesting of algal biomass (Uduman et al. 2010). There are critical parameters to consider for the selection process of algae for harvesting. Such parameters include density, size, and value of the desired products. Two-step processes are usually used for the har­vesting of algae:

1. Bulk harvesting: This step is performed for the separation of algal biomass from the bulk suspension. The techniques that can be used to complete this process are flocculation, flotation, or gravity sedimentation.

2. Thickening: the second step required for harvesting of algae is thickening which is performed to thicken the slurry by filtration or centrifugation (Brennan and Owende 2010).

The most important and the most operative method used for the separation of algal biomass is by centrifugation technique in algae harvesting, but it is only done on high-valued products due to high operational and functional cost (Grima et al. 2003).

13.5.1 Flocculation

This is the process in which circulated algae cells are combined together to form bulky biomass collection for settling. The precipitate of carbonates with algal cells at high pH, due to CO2 ingestion by the algae, results in auto-flocculation (Sukenik and Shelef 1984).