Water Well Journal
Issue link: http://read.dmtmag.com/i/681918
The barrier method of solids removal is usually recom- mended when the amount of contaminants in the water is light to moderate and the contaminants consist of settleable and non-settleable solids of adequate physical size. A critical fac- tor to consider for a screening filter is that the screen element will require periodic cleaning and maintenance. The typical operation and backwash of a screen is shown in Figure 6a and Figure 6b. Water is normally flowing from right to left with the water passing through an inline screening ele- ment. Undesirable particles are trapped and impinged onto the surface and in the holes of the screen. As more and more par- ticles become trapped on and in the screen, a filter cake, simi- lar to that in a granular media filter, begins to develop. As the area of the screen becomes more clogged, a pressure decrease and reduction in flow is realized across the screen. The loss in pressure across the screen is also referred to as the pressure drop. Eventually, as the reduction in area across the screen increases, the pressure drop also increases proportional to the square of the velocity through the screen. With a pressure screen, backwash or removal/cleaning of the screen often occurs when the pressure drop is between 5- 10 PSI. Since this mathematical relationship applies to most fixed screening elements, this also means that doubling the open area of a screen increases the time interval between backwash cycles at the same pressure drop by a factor of four (4). For example, a screen with 20 square inches of open area will typically function four times longer between backwash or cleaning cycles than the same type of perforation design with 10 square inches of open area (both set at a 5 PSI pressure drop to initiate backwash). This is a primary reason why a screen's open area is so critical to the proper selection of a barrier-method device and why a larger screen is sometimes desirable for a given flow rate. Filters Depth filters rely on a torturous path to capture particles within the matrix or "depth" of the filter. Basically, particles are caught within the depth pores of the filter as they come into contact with obstructions within the filter element. There is rarely a uniform or defined pore structure in a depth filter and in many configurations, such as fibrous filters, there are no pores at all. Even though there may not be defined pores, depth filters can still be performance rated based on challenge testing. In these tests, the filter is challenged with a pre-set quantity of a defined size of particles or organisms. When finished, this type of testing renders the filter unusable and is referred to as destructive testing. Manufacturers perform these tests on a representative sample of each filter batch. Since every filter cannot be realistically tested and verified individually, a "nominal" rating is generally associated with depth filters ENGINEERING continues on page 48 Figure 6a. The typical operation of a filtering screen with the water passing through an inline screening element. Figure 7. This sediment filter is an example of a fiber filter. Figure 6b. The typical operation of a filtering screen with the water backwashing through an inline screening element. WWJ June 2016 47 Twitter @WaterWellJournl Sediment Filter in a Clear Housing. Figure 8. A sand separator using centrifugal force with gravity to separate and remove sand.