Microscreening backwashing

The biomass concentration in the microscreen backwash is low. We have established however that centrifugation at 2800 g appears to allow concentration up to 2 or 3% dry solids, or by about a factor of ten. The pasty cake obtained could be used directly as an animal-feed additive if there were a market nearby, or could be dried for shipment. Centrifugation of microscreen backwash is a relatively low cost operation we estimate that it would add 10,000 to the capital cost and 2 hp to the power requirement of the estimates in Table 1. 

Introduction Microscreening Process Design Criteria Backwashing Design oe Microscreens Energy and Costs Design Example Nomenclature Reeerences Appendix... 

Microscreening is a method of filtration that uses fabric as the filtering medium. Microscreens (microstrainers) usually consist of a special metallic or plastic fabric mounted on the periphery of a revolving drum. The untreated water flows into the drum and radiates outward through the microfabric, leaving behind the suspended solids removed by the cloth. The solids retained on the inside of the rotating screen are carried upward to a row of backwash jets that flush them into a hopper, which is mounted on a hollow axle of the drum, for return to the treatment plant. 

The annual energy consumption by microscreens is shown in Fig. 2 (9). Electrical energy requirements include backwash water pumping and screen drive (10). 

Microscreens. In the course of this research, we came to the opinion that a preliminary screening to remove most of the biomass was desirable. We shall report evaluations of microscreens, or mlcrostralners, for this purpose. These devices have been available for several decades for waste water treatment (11). They are low-hydraulic-head filters, comprised of a screen mounted on a rotating drum. Apertures of available screens range down to micron sizes. Feed is introduced into the drum. Filter-cake control is by backwash with air or a portion of the filtrate, once each rotation, the backwash being caught in a tray. A schematic is shown in Figure 6. 

Figure 16. Concentrations at various points in the microscreen system equipped with 21-fim screens a, volatile suspended solids b, polymer. , jeed tank O, influent , effluent +, backwash.

Moderate decreases in viscosity in polishing filtration do not necessarily imply blopolymer loss. So long as polymer is not Incorporated In filtercake as it would be in dlatomaceous earth filtration (or if it can be backwashed off, even if it is), it should be possible to operate with a feed of higher viscosity than necessary for injection, and to recycle feed through the filtration apparatus, perhaps after dilution. Another possibility would be to recycle polishing filtration blowdown into the microscreening system. 

Rotating microscreen batch, fluid loading 3 to 6 L/s m of submerged area usually 66% area submerged solids loading 0.05 to 0.1 g/s m headless 7 to 14 cm to max. of 45 cm. Clean by backwash at 2 to 5 % volumetric throughput capacity. Screen pack downstream of extruders screen pack area directly proportional to rate of extrusion. Cycle time for the screen to blind > time between shutdowns. For example, screen pack area 0.17 m s/kg and screen pack diameter 20 cm s/kg but must be custom selected for the polymer. 

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