Filtration is carried out commonly on membranes of modified cellulose, with the aid of a suction pump. The greatest advantage of this method as a concentrating device is that it is able to collect microalgae or cells of very low density. However, concentration by filtration is limited to small volumes and leads to the eventual clogging of the filter by the packed cells when vacuum is applied.
Several methods have been devised which avoid these problems. One involves the use of a reverse-flow vacuum in which the pressure operates from above, making the process more gentle and avoiding the packing of cells. 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 3 hours). A second process uses a direct vacuum but involves a stirring blade in the flask above the filter which prevents the particles from settling at all during the concentration process.
Sometimes they may be collected very well with the microstrainer. When a microstrainer is used to collect algae, the original suspension may be faintly green, which could be further concentrated.
See the following pages on Algae Harvesting: