Water Well Journal
Issue link: http://read.dmtmag.com/i/681918
which when combined with a biological action on the upper surfaces of the media removes undesirable material. The straining action is performed at very low rates, usually between .05 and .25 GPM/ft 2 or 72-360 GPD/ft 2 —also known as the hydraulic loading rate, which can require a large land footprint. For example, up to 2000 square feet of filter area (5% of an acre) is often needed for just 100 GPM of flow. The biological action is performed at the very highest re- gions of the filter bed where a biological layer of fine material (a "schmutzdecke") forms from the reaction between a growth that occurs with an accumulation of material in the water and biological reactions. This is called ripening of the filter. Even though both of these methods are extensively used for potable water treatment, the rapid sand method, owing to the larger filtration rate of 2-10 GPM/ft 2 , is much more commonly used in practice. A pressure filter, on the other hand, uses much higher val- ues of hydraulic head to operate, typically measured in pounds per square inch (PSI) rather than feet of head. These systems, as shown in Figure 4, consist of a closed filter vessel, gener- ally round in shape, where the water flows through a single or series of layers of granular media under pressure. The flow rate per area (the hydraulic loading rate) is typi- cally much higher than with a gravity filter, averaging around 5-7 GPM/ft 2 but often up to 10-15 GPM/ft 2 , depending on the media and the desired removal rate of the harmful material. Although the depth of filter media can vary with the applica- tion and filter media, the depth of a filter media bed is gener- ally between 24 inches up to 48 inches for pressure filters and up to 72 inches for slow sand and rapid sand filters. As also seen in Figure 4, the filtering process generally consists of a downward flow through the media, with back- wash occurring in the opposite direction (upwards flow). Backwash of a granular media is needed to remove the en- trapped particles and adequately clean and fluidize the bed. Backwash through a rapid or pressure filter is generally much higher than the hydraulic loading rate, in values typically be- tween three up to ten times higher than the hydraulic loading rate, or 15 up to 30 GPM/ft 2 . Filter manufacturers use a statistical method for rating a filter's capability to remove particles or organisms, called a log removal rating or value (LRV). In order to determine the validity of a manufacturer's claim, the designer must under- stand how LRVs work and how they are reported. LRVs are reported using a percentage removal number based on an initial challenge. This number is commonly re- ported using a multiple of "9s" (example: 99.9%). Each "9" reported indicates the challenge level as well as the filter's re- moval capability. For example, a filter that reports four "9s" (99.99%) corresponds to a "4-log" removal and indicates the filter was challenged with at least 10,000 particles or organ- isms for every milliliter (mL) of water that was tested, and 0 particles or organisms were detected downstream. A mL of water is about the same size as a sugar cube. An understanding of industry standards for LRVs is crucial in determining if a filter is qualified for the intended use. A company may report a filter with 99.99% removal of bacteria, and to an uninformed user this value may look very impres- sive. But when compared to the industry standard for bacteria removal of ≥99.9999%, an informed user would realize the filter falls way short of the intended mark. In fact, when tested at >1,000,000 particles or organisms/mL required to achieve a 99.9999% rating, a 99.99% removal rating indicates that 100 particles/mL water still got through the filter. Industry standards are often cited as follows. Protozoan cysts: ≥99.9% removal ("3-log"). Bacteria: ≥99.9999% re- moval ("6-log"). Viruses: ≥99.99% removal ("4-log") Conclusion When a designer is considering the straining method for filtering water, it is vital to fully consider the physical and electrical state of the material to be removed and the water supply itself since almost no granular media can remove un- wanted material with a physical size of 10-20 microns and smaller, such as bacteria or viruses, without some type of chemical addition such as a coagulant, polymer, or filter aid. This may mean the water supply must undergo various lab- oratory and other tests to determine the best combination of media and chemicals to use. Often, these tests comprise a "jar test" where various chemicals are introduced to the water under a controlled set- ting to determine the best coagulant or polymer for a specific water. Media selection is also conducted based on the filtra- tion rate desired plus the size and chemical state of the unde- sirable material. In virtually all cases, a reduced scale test of a full scale operation, known as a pilot test, is warranted to ENGINEERING continues on page 46 ENGINEERING from page 43 Figure 3. Slow sand filtration strains through a single layer of sand with a gravel base to remove undesirable particles. Figure 4. The filtering and backwashing processes in a pressure filter. waterwelljournal.com 44 June 2016 WWJ