The driving force for vacuum filtration results from the application of suction on the filtrate side of the medium. Although the theoretical pressure drop for vacuum filtration is 100 kPa, it is normally limited to 70 or 80 kPa in actual operation (Shelef et al., 1984). Vacuum filtration can yield algal harvests with moisture contents comparable to those of pressure filtration at lower operating cost if the content of large algal cells in the feed is high.

Five different vacuum filters—vacuum drum filter (not precoated), vacuum drum filter precoated with potato starch, suction filter, belt filter, and filter thickener—have been tested for the harvesting of Coelastrum (Mohn, 1980). Suspended-solids content of the harvested al­gae was in the range of 5-37%. Based on energy consideration, reliability, and dewatering capability, the precoated vacuum drum filter, the suction filter, and the belt filter were recommended. The precoated filter can also be used to harvest tiny microalgae such as Scenedesmus (Shelef et al., 1984). The nonprecoated vacuum drum filter was ineffective and not reliable due to clogging problems. The filter thickeners were not recommended because of low solids content (3-7%) of the algal cake, low filtration velocity, high energy demand, and poor reliability.

Dodd and Anderson (1977) were the first to harvest microalgae by a belt filter precoated with eucalyptus and pine-crafts fibers. The use of a precoat was found to cause undesirable operational complexity and increased costs. In another study, fine-weave cloth rather than the precoated filter was investigated (Dodd, 1980). This method required a relatively low energy input and no chemicals were added. It was found to be efficient in harvesting larger species of algae such as Micractinium, but it had problems with fouling in smaller algal species such as Chlorella. Its capital costs are higher than dissolved-air floatation, but the operating expendi­tures are the lowest among all harvesting methods with the exception of natural settling (Dodd, 1980).