Algal production 245openings are normally in the range of 23 to 60 ȝm, but microscreens with 1 ȝm
mesh size made of polyester fabric has been developed (Harrelson and Cravens
1982). Wastewater enters the open end of the drum and flows outward through
the rotating fabric. The collected solids are continuously removed by high-
pressure jets (located outside at the top of the drum) into a trough within the
drum (Middlebrooks et al. 1974 and Metcalf and Eddy Inc. 2003).
Microstrainers are normally operated at hydraulic loading rates of 5-15 m^3 /(m^2 -
day), drum submergence 70-75% of height or 60-70% of filter area, and drum
diameters varying from 2.5-5 m depending on the design of screen. The full-
scale microstrainers with a 1 ȝm mesh size installed at the Camden waste
stabilization ponds in South Carolina, U.S.A. (flow rate = 7200 m^3 /day) were
operated at hydraulic loadings between 60-120 m^3 /(m^2 -day).
Typical SS and algal removal achieved with microstrainers is about 50
percent, the range being 10 to 80 percent. Reed et al. (1988) suggested that
microstrainers with 1 ȝm polyester screening media are capable of producing an
effluent with BOD 5 and SS concentrations lower than 30 mg/L. However, the
service life of the screen was found to be about 1.5 years, which is considerably
less than the manufacturer's prediction of 5 years: this was probably due to
operational and maintenance problems associated with this type of screen. A
simple mesh screen with pore opening of 50 μm (Figure 5.10) can be used to
separate algal cells from HRAP effluent. Although low cost in investment, algal
cell removal with this method is about 50 %.
Figure 5.10 Mesh screen for separation of algal cells