Microscreening removals

Table 1 Microscreens Removal from Secondary Effluents ...

This is mechanical separation process for removal of suspended sohds by passing the waste-water through a microscreen. A removal efficiency for TSS is 50-80% depending on the pore size of the microscreen to be used. 

In the integrated membrane system fed with seawater (Figure 12.4A) the pretreatment consists of two rotating microscreens of 150 pm used to remove the larger suspended particles from the water. Successively 8MF units (polypropylene (PP) hollow membranes placed in a vertical position and operated according to the deadend principle) are used to remove the suspended solids completely, algae and to disinfect the water. The MF section is designed to filter 700-750 m3/h [15]. 

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. 

Hot water. Where oil and grease are present, hot water and/or steam treatment can be used to remove these materials from the microscreens. Plastic screens with grease problems are cleaned monthly with hot water at 1200 F to prevent damage to the screen material. Downtime for cleaning may be up to 8 hours. 

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. 

With respect to microscreening of scleroglucan broths, tests should be carried out on broth neutralized by lime. Because some of the acid produced in the fermentation is oxalic, its removal is probably necessary before Injection, to avoid precipitation as calcium oxalate in the formation. Lime precipitation should effect this. It is probable that filtration with calcium oxalate in the feed will proceed somewhat differently than without, but it is difficult to predict whether performance will be better or worse. 

Deep bed or granular batch, for 0.01 to 50 J,m (see filter. Section 16.11.5.13). Fixed bar screen batch, grizzly (the filter cloth is made of rods and bars) removal of very coarse material of diameter >2 to 5 cm low concentration of solids <15 mg/L bars at 30 to 60° to the horizontal that can be cleaned manually or automatically. Microscreen (rotary drum or disk). Batch, removal of participles of diameter >20 jm 20 mg/L solids feed concentration. 

When the required particulate removal eflBciency has been determined for each size fraction in the distribution, the technical feasibility of available solids/liquid separation processes must be evaluated. Such a screening procedure will provide the framework for determining the extent of analytical modeling or pilot plant studies needed to develop process design criteria. The solids/liquid separation processes used in water and wastewater treatment exploit the physical-chemical properties of the particulates to achieve rapid and therefore economical separation from the treated water. Microscreening devices remove most particulates with at least one dimension larger than the minimum opening in the... 

For particulate distributions of relatively low mass concentration ( < 50 mg/L) with Znv > 30 /xm, microscreening devices may be a feasible separation process, provided the particulates have a rigid structure. Higher mass concentrations rapidly clog the screens, causing ineflBcient operation. More concentrated suspensions of large particulates are handled economically by sedimentation, which is eflBcient for removing particulate suspensions with Zxv > 100 /xm (33). 

Pd-Cu alloy coated plastic samples were first cut into discs 75 mm in diameter, sandwiched together with a porous metal disc (Monel) between two microscreens and then clamped (refer to schematic in Fig. 11.2). To remove the plastic backing material, the samples are then lowered horizontally with the coated polymer discs up. Monel mesh down, into the appropriate solvent hot water (60-80°C) for Solublon and chloroform (room temperature) for polystyrene. Polymer dissolution (removal rate) was evaluated as a function of temperature and time. Nominal times were 30 s for the Solublon and 600 s for the polystyrene. Upon dissolution of the polymer backing material, samples were removed from the solvent, carefully disassembled and then dried. 

Microscreen (rotary drum or disk) batch, removal of participles of diameter... 

Figure 6.15 Microscreens with continual removal of solids...

This issue is very significant for pretreatment systems for seawater desalination plants with open-ocean intakes. Often the source seawater contains small sharp objects (such as shell particles), which can easily puncture the pretreatment membranes and result in a very quick loss of their integrity, unless the damaging particles are removed upstream of the membrane pretreatment system. As discussed previously, to remove sharp seawater particles that can damage the membranes from the source water, the SWRO plant intake system has to incorporate a microscreening system that can remove particles larger than 120 p,m. 

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