Water Well Journal
Issue link: http://read.dmtmag.com/i/681918
particles in the water to settle to the bottom of the basin. This action can drastically reduce the impact upon the downstream filters by lowering the filtration burden. In all three cases shown in Figure 1 a sand, anthracite coal, or layers of other types of granular media are used to remove undesirable material from the water. This is the primary dis- tinction between the strainer method and the barrier method. The barrier method uses a solid physical impediment, such as a perforated screen, with a known size of round openings or slots, to remove undesirable material. The strainer method uses a single layer or a series of layers of granular filter media to accomplish the same objective. This action is shown in Figure 2 where the various physical and electro-chemical reactions that occur in a granular media are illustrated. Particles become trapped or stick between the openings of the various grains of the media, which over time causes the flow rate through the filter to decrease and head losses through the bed to increase as more and more particles become trapped as they collect in the higher regions of the filter and travel lower and lower through the filter bed. The filtration rate and efficiency through a barrier filter is fairly predictable up to the point where enough openings be- come clogged to reduce the device's efficiency. But the filtra- tion rate and removal efficiency through a granular media is highly dependent on the type and size of the media used; the type, size, and volume of the material removed; as well as the unit flow rate through the media. Sand in various grain sizes is the most common type of filter media used. Other types of specialty filter medias such as anthracite (coal), manganese dioxide, and manganese greensand are also used for removing specific compounds and elements, particularly from potable water. Even though several methods are used to evaluate a filtra- tion media for possible use, the two most common and impor- tant criteria are (1) the effective size, and (2) the uniformity coefficient. Effective Size The effective size of a granular media is defined by the size of screen opening where 90% of a sample of granular media is retained (held back) on the screen and 10% passes through the screen (referred to as D 10 ). The larger the grain size, the faster the water or wastewater moves through the sand and the more water can thus be filtered. But if the grain size is too large, treatment efficiency will be reduced. For wastewater, larger breakthroughs of unoxidized matter are observed due to short retention times and an instantaneous lack of oxygen within the filter bed when applying relatively large hydraulic loads to filter media with a coarse grain size, especially above 1 mm (millimeter). Often, the ideal sand for intermittent sand filters receiving domestic water or waste- water is a coarse sand with an effective size between 0.3 and 0.5 mm. Clogging becomes a major concern when using sand with an effective size of less than 0.3 mm. Therefore it is important filters using this size of sand be lightly loaded (<1.2 GPM/ft 2 ). Clogging is generally less a concern when using coarser sand. In a field evaluation of sand filter systems, it was found sands with too many fines have a greater chance of clogging than sand with a particle size between 0.3 and 0.5 mm, assuming other similar characteristics. Uniformity Coefficient The uniformity coefficient (UC) is a numeric estimate of how sand is graded and is a dimensionless number—in other words, it has no units. The term "graded" relates to regions where the concentrations of sand particles are located by size. Sand with all the particles in two size ranges would be defined as narrowly graded sand and would have a low UC. Sand with near equal proportions in all the fractions would be defined as widely graded sand and would have a high UC value. The UC is calculated by dividing D 60 (the size of screen opening where 60% of a sample passes and 40% is retained) by D 10 (the effective particle size: that size of screen opening where 10% of a sample passes and 90% is retained). The larger the UC, the less uniform the sand. For water filtration it is important the sand grains all be about the same size— that is, relatively uniform. A UC of 4 or less is recommended for all filter media. This recommendation is intended to avoid clogging of the filter bed at higher loading rates. Sands from most natural sources are widely graded containing a variety of grain sizes, which re- sults in a high UC. If the grain sizes vary greatly, the smaller ones will fill the spaces between the larger particles, making it easier for the filter to clog Filters Two types of filters are used in current practice: gravity (or "open") filters and pressure filters. Gravity filtration, as it is often an open system, uses only the head of water above the filter media to create the force necessary to drive the water through the media bed. The amount of head in a gravity filter varies with the application. Nonetheless, 3 to 5 feet is fairly common. Gravity filtration can take many forms, but the most com- mon methods are slow sand and rapid sand. The slow sand filtration method (Figure 3) uses a combination of straining through a single layer of sand with a gravel support base, ENGINEERING continues on page 44 Figure 2. Physical and electro-chemical reactions that occur in a granular media. WWJ June 2016 43 Twitter @WaterWellJournl