Water Well Journal
Issue link: http://read.dmtmag.com/i/668983
Many of these larger particles, due to their specific gravity (weight) and/or surface tension, can often be readily settled from a static or moving fluid while the fine substances like clay are generally dissolved into the fluid and must be either filtered using a fine screening (barrier) device or chemically combined into larger particles for strainer filtration. When evaluating the various methods of solids separation, there are various terms important to understand for all meth- ods. With filtration the most misrepresented or confusing terms are pore size, removal rating, and porosity. Pore size refers to the actual opening size of the pores (holes) in a filter. This may be reported as a minimum pore size (smallest measurable hole), maximum pore size (largest measurable hole, which is the most meaningful classification), or a pore size range (5–10 microns). The actual pore size rating of a given filter may be much larger than the removal rating of a given filter in many cases. Removal rating is the statistical probability of the filter's ability to remove a certain size particle when challenged under controlled conditions. This should not be confused with the actual pore size of a filter. There are two types of ratings: nominal and absolute. These terms are misused to a great extent in filter claims and marketing literature and can mislead a designer or user. A nominal rating is attached to filters that can be shown under controlled conditions to remove an acceptable statistical amount of particles of a certain size, even though the actual pores or openings of the filter may be much larger than the particles being removed. The typical way a nominal filter rating is reported would be in the form of a statement such as: "Removes >99.9% of particles 3µm (microns) or larger." This means under field conditions a user can be confident the filter will remove greater than 99.9% of pathogenic organisms larger than 3 mi- crons. This information is obtained by challenging filters with test waters containing suspensions of 3-micron spheres. A similar Cryptosporidium or Giardia claim would require a challenge using live Cryptosporidium or Giardia organisms. Nominal ratings are usually applied to depth filters. An absolute rating can only be applied to a filter in which the end user can actually determine the size of the largest pore. Such a filter can be integrity tested using a nondestruc- tive test method and the data can be used to determine the actual size of the largest pore. Absolute ratings can only be applied to membrane filters due to the requirement of a defin- able pore. If a filter manufacturer applies an absolute rating to a filter, they should be able to provide the user with a nonde- structive test protocol allowing the user to verify the absolute rating. Porosity is the ratio of open space in a filter matrix to the amount of volume taken by the filter media itself. Typically, a filter with high porosity will have a more open structure, and therefore higher flow with lower pressure drops. High poros- ity does not necessarily mean the filter will remove particles better than a low porosity filter. Pressure drop of any treatment or filtration method is de- fined as the loss in pressure (usually in pounds per square inch) from the inlet to the outlet of the device. The pressure drop can vary based on the method of filtration, the area of filtration, the amount of plugging, and the flow rate per cross-sectional area. When using media filtration (the straining method), the pressure drop is maintained between 3–7 psi, with 5 psi a common value to initiate backwash. Pressure losses higher than 10 psi will often lead to channeling where the media be- comes upset due to the differential pressure occurring within the bed. Many screen (barrier) methods of filtration can en- dure pressure drops up to 15–20 psi depending on the operat- ing pressure, the precise method in use and type of device, and the manufacturer's guidelines. As you can see, solids separation doesn't always mean only removing the larger solids from water, but other methods also used to remove much finer material from the same water. We will discuss in this series all four of the methods in common use. We will go further into depth next month with a discussion on the various methods used for the straining and barrier removal of solids from water, including well screens. Part three will delve into the technology associated with the final two categories of solids separation: adsorption and chemical alteration. 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. Mesh Micron* Inches** (holes per linear inch) 20 850 .033 40 400 .016 60 250 .010 80 177 .007 100 150 .006 140 100 .004 200 75 .003 325 50 .002 500 25 .001 Figure 4. Relationship of microns, inches, and screen mesh Material Size (Microns) Coarse Sand 500-1000 Medium Sand 250-500 Fine Sand 100-250 Silt 2-50 Clay <2 Figure 5. Typical size ranges WWJ May 2016 37 Twitter @WaterWellJournl