The algal biomass production process requires one or more solid-liquid separation steps. Generally, first stage involves a separation of biomass from the bulk sus­pension (including flocculation, flotation, or gravity sedimentation). The second stage (thickening) raises the concentration of the slurry through techniques such as centrifugation, filtration, and ultrasonic aggregation; hence, it is generally a more energy intensive step than bulk harvesting (Brennan and Owende 2010).

The flocculation is the first (preparatory) stage that is intended to aggregate the microalgae cells in order to increase the effective “particle” size. Unlike floccula­tion, flotation methods are based on the trapping of algae cells, using dispersed microair bubbles. Gravity and centrifugation sedimentation methods are based on characteristics of suspended solids and are determined by density and radius of algae cells and sedimentation velocity. It is the most common harvesting technique for algae biomass in wastewater treatment because of the large volumes treated and the low value of the biomass generated. The filtration process is better suited for harvesting relatively large (>70 mm) microalgae such as Coelastrum and Spirulina. The membrane microfiltration and ultrafiltration (hydrostatic pressure) are viable alternatives to recovery of biomass from smaller algae cells (<30 mm), such as Dunaliella and Chlorella (Brennan and Owende 2010). Some species are much easier to harvest, considering algae densities and size. The strain character­istics, cost, and energy efficiency are the main factors to select harvesting technol­ogy (Brennan and Owende 2010).