A cross-flow ultrafiltration system was adopted for treatment of algae pond effluents to produce thickened algae for animal feed. Up to 20 times of the concentration of the algae had been collected with very high-quality filtered effluent. The main disadvantage of this system is the high energy requirements, which rendered this process uneconomical.

5.3.3.1 Magnetic Filtration

Magnetic filtration was initially used in wastewater treatment for removal of suspended solids and heavy metals (Bitton et al., 1974; Okuda et al., 1975). Magnetic separation using suspended magnetic particles (such as Fe3O4 magnetite) was subsequently used in algae re­moval (Yadidia et al., 1977). Algal cells and the magnetic particles were coagulated, and the fluid was passed through a filter screen encompassed by magnetic field to retain the magnetic precipitates. Algae removal efficiency of between 55% and 94% by a commercial magnetic filter dosed with alum coagulant was reported (Bitton et al., 1974). Higher algae removal (>90%) was achieved using 5-13 mg/L Iron (III) Chloride as primary coagulant and 500-1,200 mg/L magnetite as magnetic particles for pond algal harvesting (Yadidia et al., 1977).