Microstrainers consist of a rotary drum covered by a straining fabric, stainless steel or poly­ester. The partially submerged drum rotates slowly in a trough of suspended algal particles. The screen is fine mesh that captures only fairly large particles such as algae. As the mesh moves to the top, water spray dislodges the drained particles. When a microstrainer is used to harvest algae, the concentration of harvested algae is still low. Smaller algae can still pass through the screen and are thus not harvested.

Unit costs of microstraining range between $5 and $15 per 106 liters, depending on algae size and scale of operation (Benemann et al., 1980). For larger algae, even lower costs may be achieved. Favorable features of microstraining include simple function and construction, simple operation, low investment, neg1igable wear and tear due to absence of fast-moving mechanical parts, low energy consumption, and high filtration ratios.

Problems encountered with microstrainers include low harvesting efficiency and difficulty in handling particles fluctuations. These problems may be overcome in part by varying the drum rotation speed (Reynolds et al., 1975). Another problem associated with microstraining is the buildup of bacterial and algae biofilm slime on the fabric or mesh. Ultraviolet irradia­tion, in addition to periodic fabric or mesh cleaning, may help inhibit this biomass growth.

Microstrainers have been widely used in the removal of particles from sewage effluents and in removal of algae from the water supply (Berry, 1961). Successful removal of Micractinium from algae ponds has been reported under a condition that growth of unicellular strains of Scenedesmus and Chlorella does not overcompete the algae to cause deterioration of algae removal (van Vuuren and van Duuren, 1965). Thickening of Coelastrum proboscideum to about 1.5% suspended solids by microstrainers was reported when operating at a cost of about DM 0.02/m3 and power consumption of 0.2 kWh/m3 (Mohn, 1980). Some success in clarifying high rate pond effluent with continuous backwashing in microstrainers was achieved (Koopman et al., 1978; Shelef et al., 1980). However, the success was confined to ef­fluent dominated by algae species such as Micractinium and Scenedesmus, since the smallest mesh available at that time was of 23 pm openings. Greater success has been reported in clar­ifying stabilization lagoon effluent in reducing suspended solids from up to 80 mg/L to 20 mg/L or less by rotary microstrainers mounted with screens as fine as 1 pm (Wettman and Cravens, 1980).

In a study using microstrainers fitted with 6 pm and 1 pm meshes in clarifying algae pond effluents, the Francea Micractinium algae were completely retained by the 6 pm screen, whereas the Chlorella algae passed through the 1 pm screen (Shelef et al., 1980). The distinction in algae retention on the screens was evidently due to the difference in size of the algae in each pond. It was noted that although the size of the Chlorella algae were larger than 1 pm, they were not retained by the microstrainers. A possible reason could be due to the poor

quality control of mesh size. Continuous operation may overcome part of the problem by building up and maintaining an algal biofilm base layer that serves as a biological fine screen.