Automatic backwash filtration

A DAFF clarifier s top portion is similar to a DAF clarifier, but its bottom portion is an automatic backwash filtration unit. The readers are referred to elsewhere for a detailed description of a DAFF clarifier.41 43... 

An innovative potable filtration plant with a design capacity of 1.2 MGD has been reliably serving 10,000 residents and tourists in the town of Lenox, Massachusetts, USA, since July 1982. Its process system consists of chemical flocculation, dissolved air flotation, and automatic backwash and sand filtration. It substantially improves upon the conventional flocculation, sedimentation, and filtration system in performance, capability, operation, maintenance, and energy use (19 2). 

Fig. 19 Automatic backwash sand filter (Centra-flo ) 1, Overflow 2, Alter influent 3, coarse media 4, fine media 5, filtrate nozzles 6, filtrate chamber 7, level controller 8, filter reject 9, washbox 10, counter-current washer 11, airlift 12, central feed chamber. (View this art in color at www. dekker.com.)...

Hollow Fiber The hoUow-fiber configuration is the most common configuration for MF and UF membranes. The hollow fibers arc 0.5-1.5 mm (less than 5 mm) in diameter, and several thousands of hollow fibers are packed into a modtrle (Fig. 6.4). The most important merit of hollow-fiber membranes in water treatment applications is that extensive pretreatment is not needed as the membrane can be backwashed (back-flushed) automatically. Backwashing is carried out by changing the direction of flow of the permeate stream so that it flushes out the deposited particles that formed a cake layer on the membrane surface during the filtration cycle. 

Several plate and frame MF units have been installed in Japanese nuclear power plants. The units operate with automatic periodic backwash using accumulated filtrate. Typically, the units backwash every 3 hours at a back pressure twice that of the final forward pressure. The backwashed crud is removed from the system as concentrated RAD wastes. In this sense, these back-wash systems are not dissimilar to cross-flow units using MF or UF-membranes. In either case, a reject stream containing concentrated crud must be disposed of. 

Figure 10.69. These are automatic filters and are normally used for the first filtration to remove objectionable particles in such a way that the backwash is recycled, thereby reducing waste and extending the life of second stage and other filtration steps used to pohsh the viscose. Because these filters are automatic, there is a significant reduction in the labor required to change and dress filter presses. The filtered viscose is next pumped to a ripening tank.

Although continuous operation can be achieved using conventional sand filters in parallel, truly continuous depth filters exist where filtration proceeds with virtually no head loss. These units, which are typified by the Tenten inclined bed and Dynasand devices, operate in the normal manner except a portion of the filter media is intermittently removed (see Figure 1.44). This portion is washed, and then automatically returned to the top of the filter bed in a cleaned condition. While being more expensive than conventional sand filters, continuous depth filters find use when frequent backwashing would otherwise be required. 

---