Water Well Journal

June 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/681918

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Page 43 of 111

I n part one of this three-part series we introduced you to separating—or removing—solids and other harmful mate- rials from water, and the various technologies available to do it. This month, we expand upon this topic and provide an overview and detailed explanation of the first two of the four methods outlined in part one: straining and barrier removal methods. THE STRAINING METHOD The straining method generally involves the use of granu- lar filtration media, such as sand, to provide a physical, often combined with a chemical, method of removing undesirable substances from water. Although the title of this series is "Solids Separation," it is somewhat a misnomer for all the processes described can be used to remove dissolved sub- stances, such as ferrous iron, from water as long as the appro- priate pretreatment and chemical treatment is incorporated into the process. The straining method is very flexible and applicable to treatment of water in flows down to 5 GPM for residential app- lications and up to several thousand GPM for municipal or industrial projects. However, it is more commonly applied to water systems with flow rates of 100 GPM or more. The bar- rier method is more commonly used for lower flow rates. Here's how I try to remember the difference between the two methods. The strainer method uses a granular media to actually "strain" the water to achieve the removal of unwanted material. The barrier method uses more of a mechanical device that provides a "barrier," such as an inline screen, to accomplish the same task. The straining method often uses a combination of processes to remove particles or contaminants from a water supply. In addition to the basic filtration that occurs when water is directed through a filtration media, other processes can be used to assist with removal. Several assist with the filtration process by making the particles larger and heavier, changing the electro-chemical state of the particles so the opposite charge of the filter media attracts the charge of the offending particle, or by using multiple layers (up to three or four) of progressively smaller granular filter media. Regardless of the specific type of strainer method selected, the three most common methods of particle removal through this method are shown in Figure 1. Figure 1a demonstrates the use of a filter cake occurring on the upper surface of the media. A filter cake occurs as the unwanted material accumulates on the upper surface of the media, resulting in a progressively tighter, but thin, layer of combined filtered material, biological action, and chemicals, if used. This method is applicable to many water filtration technologies—diatomaceous earth systems, slow sand filtra- tion, and many adsorption type of filters. Figure 1b is a typical straining action by using granular media as a simple physical impediment to downward particle flow. Figure 1c indicates a combination of physical (straining) and chemical methods of filtration. This is the method most commonly used for potable and aesthetic water treatment since the average particle size, consisting of mostly very fine material, bacteria and viruses, are typically too small to effec- tively remove from water through strictly straining methods alone. By adding various chemicals to the flow stream, also known as coagulants, polymers, or filter aids, the unwanted material can chemically alter its electro-chemical state to physically combine with other material through a process known as flocculation. This results in a larger and more filter- able particle (referred to as a floc) in order to more easily be removed through the filtration media or chemically changed to a different electrical charge so the unwanted material then "sticks" or is attracted to the opposite charge of the media. In addition to the filtering action from straining the water and furnishing coagulants to create a condition more con- ducive to filtration, larger and heavier particles can also be removed from water through a process known as sedimentation. During sedimentation, the water is allowed to remain reasonably quiet or stationary within a chamber, re- ferred to as a sedimentation basin. In this basin, the quiet state of the water allows gravity to act on the larger and heavier ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS SOLIDS SEPARATION Part 2: Straining and barrier methods of removal waterwelljournal.com 42 June 2016 WWJ Figure 1. The most common methods of removing particles or contaminants from a water supply. 1a 1b 1c

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