Water Well Journal

May 2016

Water Well Journal

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lower flow rate usually associated with the barrier method, flow rates for potable water applications are generally lower than those with the strainer method, typically 200 GPM or less. This system, however, is popular for use in larger applica- tions for irrigation and industrial use, and systems for flows exceeding 2000 GPM are not rare. An example of the barrier method is illustrated in Figure 3. The feed fluid (usually water) containing undesirable particles is pushed through a screen with round openings where the screening element traps the particles in the openings. The finished fluid discharging from the screen is referred to as the filtrate. This process is applicable for the barrier method whether the screen element is a simple perforated screen with round openings, as shown in Figure 3, or uses a membrane type of filter, such as reverse osmosis. The basic difference is the size of material to be removed and the pressure required to push the fluid through the barrier to create the filtrate. The next process shown in Figure 2 is adsorption. This is a common water treatment process, often used for the removal of taste or odor-offending substances in drinking water. It uses activated carbon through which the molecules of the contami- nants are chemically attracted to the surface of a media. Ad- sorption is technically defined as the adhesion of a gas, vapor, or dissolved material onto the surface of a solid. One particle of activated carbon has an extremely large surface area due to its structure of pores similar to those found in a sponge. (The process of adsorption should not be confused with the similar term of absorption.) Using a sponge analogy, a sponge will readily absorb water containing a taste and odor. When the water is squeezed from the sponge, the taste and odor will generally still be present in the water. In adsorption using activated carbon, however, the water is brought into contact with the carbon particles (sponge), and when the water leaves the carbon particles, the taste and odor constituents remain with the carbon. This results in water free from taste and odor. The adsorptive capacity of activated carbon is directly re- lated to the surface area of the carbon particles that come into contact with the water. For this reason, manufacturers of acti- vated carbon try to maximize the pore space of carbon media. Lastly, chemical alteration involves processes in which a chemical reaction changes the chemical state of a substance from a dissolved state to a solid state or uses a method by which an exchange of ions occurs in the fluid. This method is popular in potable water applications and is found in many locations where the offending substance is dissolved into the water—such as iron, manganese, or calcium hardness. Included in chemical alteration processes are precipitation, oxidation, and ion exchange. ENGINEERING continues on page 36 Figure 1. Particle size chart WWJ May 2016 35 Twitter @WaterWellJournl

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