In deep-bed filtration, algae particles are harvested in a depth filter. Smaller than the medium openings, algal particles flow into the medium and are retained within the filter bed. Deep-bed filtration is most often operated as a batch process. When the pressure drop reaches the maximum available, the filter must be taken out of service for backwashing.

Harris et al. (1978) and Reinolds et al. (1974) reported successful separation of algal cells from pond effluent with average solids concentration of 30 mg/L by intermediate sand filtration. The filtration systems, however, rapidly experienced a severe clogging problem and filtration flux dropped drastically.

Intermittent sand filtration was also investigated in a wastewater treatment plant upgrading (Middlebrooks and Marshall, 1974; Marshall and Middlebooks, 1973). The inves­tigation revealed that only large algal particles can be harvested by separating the dried cake from the surface of the bed. Fine algal particles infiltrated and trapped within the bed could not be efficiently harvested.