Water Well Journal
Issue link: https://read.dmtmag.com/i/668983
T he last two Engineering Your Business columns provided an understanding of flow measurement techniques (flowmeters). This month, in part one of a three-part series, we will take a look at another important topic in the water industry: solids separation methods—a fancy term for what many people simply call filtration. Whether in air, liquids, gases, or other substances, the need to remove undesirable materials from an ongoing process is well known and universally conducted—and often performed by using filtration. We have filters in common use all around us. They're used in our automobiles, for treating the air we breathe, and the food we eat. Dealing with water, filtration is typically used as just one way to remove contaminants and undesirable materials from our water supplies. In almost universal applications around the world, filtration can be found in domestic, municipal, irrigation, commercial, and industrial water system settings. What Are Solids? Any discussion on solids separation from water must begin with a discussion regarding the size, scope, and type of mate- rial to be removed from the fluid stream. This generally starts at the physical size of the material. The ability to remove a given material, or particle, from a liquid is not only based on the physical dimension of the particle, but also on the angular shape, electrical charge (chemistry), consistency, surface tension, and weight (specific gravity) of the particle. Figure 1 illustrates various physical and biological types of particles and contaminants and their approximate size. Begin- ning at the top, the chart includes both inorganic-based and organic-based constituents and their relative sizes in microns. Of particular interest to water-related businesses is the third line down from the top that displays many of the inor- ganic particulates such as clay, silt, and sand and the approxi- mate physical size of each. The next two areas of particular import is the third line from the bottom, indicating the relative size of most proto- zoan cysts (2 to 50 microns). These include the Cryptosporid- ium cyst at a size around 2-3 microns and Giardia lamblia with a size generally between 4-6 microns. As you can readily observe, a filter system designed to re- move cysts with a size of 5 microns may easily remove most Giardia, but may not effectively do the same for Crypto- sporidium. Both cysts are within the overlap range for con- ventional particle filtration and microfiltration. This explains why many water treatment plants must be equipped with terti- ary filtration methods such as membrane filters for the effec- tive removal of these smaller organisms. The final line of particular interest to water is the lowest line on the chart. This displays the relative sizes of most viruses and bacteria—the primary disease-causing organisms in freshwater supplies. As you can see, most bacteria are typi- cally within a size range of 0.3 to 10 microns and can there- fore be readily removed using conventional particle filtration and microfiltration. However, viruses much smaller in physi- cal dimensions (0.005 to 0.1 microns) must be removed by using ultrafiltration and nanofiltration methods. This is a primary reason why additional or different methods for the deactivation of viruses, such as the use of chlorination or ultraviolet light, are required for totally effective water treatment. What Does Solids Separation Mean? It is not enough to write about solids separation by dis- cussing filtration as the only means available for taking con- taminants or other undesirable materials out of our water. To be sure, filtration is an old and established method of solids removal, but certainly not the only viable method we have available. As shown in Figure 2, there are several methods available to remove contaminants from water. In addition to the two common methods discussed in this column—strainers and barriers—removal of contaminants can also be accomplished through adsorption and chemical alteration. The strainer method simply means the particle is trapped between the pore spaces of granular media such as sand fil- ters, multi-media filters, and specialized media filters. The effectiveness of this process depends to a great degree on the cross-sectional flow rate through the media and the grain size of the filtering media itself. With the strainer method, chemicals such as aluminum sulfate, ferric chloride, and other filter aids are often added to the fluid stream to cause smaller particulate matter to electri- cally bond together, thus creating a larger size of material for more efficient media removal. These larger particles are usu- ally referred to in water treatment as a floc. The strainer method, while applicable to all flow ranges, is more often used with flow rates of 100 GPM or more. The barrier method uses physical obstructions to prevent the passage of particulates, including inline or suction screens, membrane or bag filters, and cartridge filters. Due to the ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS SOLIDS SEPARATION Part 1: Various methods of removal waterwelljournal.com 34 May 2016 WWJ