Water Well Journal
Issue link: http://read.dmtmag.com/i/681918
for automatic operation or a written schedule with a manual function. Sand separators are quite effective for solids removal in the proper application and design. First, the separator must be designed for the appropriate flow range of the device. For example, a 1-inch unit may have a stated flow range of 5-15 GPM—and it is critical the operation remain within that range for effective performance. Unlike most filters, a separator must be designed to function within this given flow range. It cannot be allowed to work below the stated minimum flow rate; otherwise, the needed centrifugal force and removal of solids will not occur. Second, the high range of flow must also be observed since exceeding the maximum flow rate will result in excessive wear and pressure drop of the unit, resulting in premature fail- ure due to sand erosion. Next, a sand separator must be designed for and function under a sustained and constant pressure drop of around 5-10 PSI. Any attempt to lessen or increase the pressure drop appreciably above or below these thresholds will negate the effectiveness of the separator's removal. Finally, a sand separator must be recognized for what it is and is not. It is not a filter or method of removing fine sedi- ment, silt, or particles from water. Remember the most effec- tive range of a separator is to remove up to 98% of 74 microns (and larger) material from a water supply. Although separators have been known to remove much larger sand and gravel from flow streams, they are essentially intended for sand and simi- larly sized particles between 74-100 microns. Gravels and large, abrasive materials have been known to cut through the upper centrifugal chamber in a short time, resulting in early failure of the device. For total effectiveness, all manu- facturers' guidelines for design and installation must be adhered to and the flow rate ranges and pressure loss through the unit followed. Conclusion The barrier method can be very ef- fective at removing larger material (>75 microns or 200 mesh), such as silt and sand, from most freshwater supplies at reasonable levels of the total particle volumes, flow rates, filter runs (time between backwashes or cleaning of the screen), and at a minimum of pressure drop (5-7 PSI). The most efficient bar- rier method uses a screening device of the proper mesh and inlet/outlet size to handle the flow rate and degree of parti- cle removal desired. The high end of the flow rate and removal efficiency through most barrier types of filtration is generally limited to 500 GPM or less and particle sizes of 50 microns and larger. Anything above that requires consideration of a granular media filter, possibly with chemical addition or a system with automatic backwash of the screen to avoid rapid clogging of the screen. Although the application of a barrier-method device is not as critical as that with the strainer method, it is still important to consider all aspects of the intended application and select a screen of the proper mesh, configuration, and size. This will wrap up part two of this series on solids separa- tion methods and techniques. Next month we will conclude this series with a discussion on the final two methods: adsorp- tion and chemical alteration techniques. Until then, work safe and smart. WWJ Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 35 years experience in the water well business, specializing in engineering and business management. He can be reached at epbpe@juno.com. References McDowell-Boyer, L., J. Hunt, and N. Sitar. 1986. Particle transport through porous media. Water Resources Research v. 22, no. 13, pp. 1901-1921. "Introduction to Filtration." AWWA Training Manual. Sand Separator and Filtration Data, Lakos Website and Lit- erature, 2016. Brown, Dennis. "Understanding Terminology in Regards to Filter Ratings." 2012. Aquamira Technologies. Twitter @WaterWellJournl PROBES WINCHES LOGGERS SOFTW ARE OUTSTANDING IN OUR FIELD mount sopris ins truments is a leading manufacturer of slimline bor ehole geophysical logging systems f or downhole groundwater applications. Learn more at mountsopris.com or call 303-279-3211 WWJ June 2016 49