Conventional processes used to harvest microalgae include concentration through centrifugation (Haesman et al. 2000), foam fractionation (Csordas and Wang 2004), flocculation (Poelman et al. 1997; Knuckey et al. 2006), membrane filtration (Ros — signol et al. 2000), and ultrasonic separation (Bosma et al. 2003). Harvesting costs may contribute 20 to 30% to the total cost of algal biomass (Molina Grima et al. 2003). Microalgae are typically small with a diameter of 3 to 30 pm, and the culture broths may be quite dilute at less than 0.5 g/L. Thus, large volumes must be handled. The harvesting method depends on the species, on the cell density, and often also on the culture conditions (Carlsson et al. 2007).

Algae pressing is very similar to the techniques used to press flowers and is used widely by scientists as a means of preserving algal specimens and observing their features.

Algae can be harvested by centrifugation, flocculation, or froth flotation. Alum and ferric chloride are chemical flocculants used to harvest algae. Water that is brackish or salty requires additional chemical flocculants to induce flocculation. Harvesting by chemical is a method that is often too expensive for large operations. However, interrupting the carbon dioxide supply to an algal system can cause algae to flocculate on its own, which is called “autoflocculation.” In froth flotation, the water and algae are aerated into froth and algae and then removed from the water.

The typical cell density achieved in the industrial application is between 0.3 and 0.5 g dry cell/L or 5 g dry cell/L at best, which makes harvest difficult and expensive (Wang et al. 2008). Two processes are involved in harvesting, bulk harvesting and thickening. Bulk harvesting is a large-scale operation separating biomass from bulk culture. It has a concentration factor of 100 to 800 times, depending on the culture and harvesting method. Bulk harvesting can be categorized into flocculation and floatation. Flocculation reduces/neutralizes the negative surface charge of microal­gal cells, allowing them to aggregate into larger lumps with an efficiency of >80. The thickening process consists of either centrifugation or filtration. Centrifugation, a semicontinuous or continuous process, utilizes centrifugal force generated by the spinning of a suspension to separate and harvest algal cells.

For mass cultivation of algae, optimization of algae harvesting and processing is needed. Algae properties such as algae size, cell wall sensitivity to shear force, ease of flocculation, and oil content need to be taken into consideration in the design process.