Algae can be dewatered and harvested by pressure filtration using either plate-and-frame filter presses or pressure vessels containing filter elements. In plate-and-frame filter press filtration, dewatering is achieved by forcing the fluid from the algal suspension under high pressure. The press consists of a series of rectangular plates with recesses on both sides, which are supported face to face in a vertical position on a frame with a fixed and movable head. A filter cloth is hung or fitted over each plate. The plates are held together with sufficient force to seal them so as to withstand the pressure applied during the filtration process.

In the operation, fluid containing algal suspension is pumped into the space between the plates, and pressure is applied and maintained for several hours, forcing the liquid through the filter cloth and plate outlet ports. The plates are then separated and the dewatered algal cake is harvested. The filtration cycle involves filling the press, maintain the press under pres­sure, opening the press, washing and discharging the cake, and closing the press. Chemical conditioners such as polyelectrolytes may be used to increase the solids content of the cake.

In filtration by pressure vessel containing filter elements, a number of designs have been devised, such as rotary-drum pressure filters, cylindrical-element filters, vertical tank vertical leaf filters, horizontal tank vertical leaf filters, and horizontal leaf filters. A comparison of the use of different pressure filters for Coelastrum harvesting has been investigated (Mohn, 1980). Five different pressure filters—chamber filter press, belt press, pressure-suction filter, cylin — dric sieve, and filter basket—were operated. Solids concentrations in the range of 5% to 27% were measured for the harvested algae. Chamber filter press, cylindric sieve, and filter basket were recommended for algae filtration with respect to energy consideration, reliability, and concentrating capability. A belt filter press was not recommended because of low-density al­gal cake if filtration was carried out without prior coagulants dosing to the feed. A pressure — suction filter was also not recommended because of low filtration ratio, high investment costs, and unclear operational expenses.