Membrane filtration is one of the algae harvesting methods and is usually aided by a vacuum pump. Membrane filtration provides well-defined pore openings to separate algal cells from the culture. An advantage of the mem­brane filtration harvesting method is that it is capable of collecting and con­centrating microalgae or cells of very low initial density (concentration). However, concentration by membrane filtration is somewhat limited to small volumes and leads to the clogging and fouling of the filter (membranes) by the packed cells when vacuum is applied. Fouling and clogging of the mem­brane surface due to increased concentration of algal cells results in sharp declines in flux and requires maintenance.

A modified filtration method involves the use of a reverse-flow vacuum in which the pressure operates from above rather than below, making the process gentler and avoiding or alleviating the packing of cells on the mem­brane. This method itself has been modified to allow a relatively large vol­ume of water to be concentrated in a short period of time (20 liters to 300 ml in three hours or a concentration of nearly 70 times in three hours) [21].

Cross-flow filtration is a purification separation technique, typically employed for submicron-sized materials, where the majority of the feed (algae-water suspension) flow travels tangentially across the surface of the filter rather than perpendicularly into the filter. It is advantageous over standard filtration, because the filter cake is being constantly washed away during the filtration process, thereby increasing the service time that the fil­tration device can be used without maintenance stoppage.

Cross-flow microfiltration (MF) was investigated by Hung and Liu [22] for separation of green algae, Chlorella sp., from freshwater under several dif­ferent transmembrane pressures (TMP) and also with both laminar and turbulent flows by varying the cross-flow velocity. The study examined the hydrodynamic conditions and interfacial phenomena of microfiltration of green algae and revealed the interrelations among the cross-flow velocity, MF flux decline, and TMP [22].

Forward osmosis (FO) is an emerging membrane separation process, and it has recently been explored for microalgae separation. It is claimed that for­ward osmosis membranes use relatively small amounts of external energy compared to the conventional methods of algae harvesting. The driving force through a semi-permeable membrane for forward osmosis separa­tion is an osmotic pressure gradient, such that a "draw" solution of high concentration (relative to that of the feed solution of dilute algae suspen­sion) is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feedwater from its solutes (microalgae). Zou et al. [23] studied the FO algae separation by comparing two different draw solutions of NaCl and MgCl2 and also examining the efficacy as well as their membrane fouling characteristics.

Sometimes concentrated algae may be collected with a microstrainer. When a microstrainer is used to collect algae, the processed algae-water suspension may look faintly green, indicating that it could be further con­centrated. However, due to its eventual clogging, a microscreen alone is usually insufficient for long-term continuous or large-scale operation; sub­stantial energy and labor input are required to remove the clogging and reopen the flow channels.

A novel process for harvesting, dewatering, and drying (HDD) of algae from an algae-water suspension has been developed by Algaeventure Systems, LLC. In this process, a superabsorbent polymer (SAP) fabric belt is put in contact with the bottom of the screen (water meniscus), thereby enabling the movement of a vast amount of water without moving the algae and achieving dewatering. This is based on the fact that water-water hydro­gen bonding is stronger than water-to-algae’s weak intermolecular forces. As such, reduced surface tension, enhanced capillary effect, and modified adhe­sion effect can be built in and the system can be designed to be continuous. In a prototype testing, an exceptional rate of HDD was achieved with a very low power input. A schematic of Algaeventure Systems harvester is shown in Figure 2.2 [24].

Superabsorbant polymers (SAPs) can absorb and retain very large amounts of a liquid (such as an aqueous solution) in comparison to their own mass and have been widely used in baby diapers, personal hygiene and care products, and water soluble or hydrophilic polymer applications.