Water Well Journal
Issue link: http://read.dmtmag.com/i/705618
considering using bag filtration, it is imperative the designer doesn't try to match one brand of an approved filter bag with an unapproved canister. It will usually not work. The proce- dure for gaining NSF or UL certification typically requires the filter be challenged as an assembled unit. This is the only reliable way to be sure the filter system will effectively provide the log removal of the contaminant stated by the manufacturer. So What Should We Use? As we close out this series, there is one more question to be asked: "So what do we need to use for our system?" This is probably the single most asked question in filtering applica- tions—not which mesh or micron size, but which actual kind of filter should be used. Although numerous criteria can and should be applied when considering individual types of filter- ing units, let's look at the three most important considerations. 1. What are we taking out? Obviously, this is a primary consideration as this question often controls the device itself. For example, if removal of Giardia lamblia is the goal, then a filter capable of removing particles with the cyst's physical dimensions, or around 4-7 microns, should be the design criteria that controls the pore size or membrane capability. If multiple organisms or particu- late must be removed, the designer then must also incorporate the added burden from these particles or use an effective pre- treatment process. 2. What is the needed removal rate? It is simply not enough to state that giardia must be re- moved from a water supply. Regulatory agencies, specifically the EPA, has created and enforces a minimum level of the re- moval or deactivation of these cysts. Many filters may have the ability to remove 90% of giardia, but the normal require- ment is removal of 99.9% of the cysts. Due to the varying shape and concentration usually found in water, this level often requires either chemical addition to enable flocculation or extremely tight filtration to accomplish this. Regardless of what kind of organism needs to be removed, you must also know how many of the organisms must be taken out. 3. What is the flow rate? The design flow rate controls many elements of a filtration device, namely the inlet and outlet sizes, net screening or membrane filtration area, and whether the device is an inline pressurized unit for lower flow rates, a suction screen for a 1000 gpm irrigation pump, or a large gravity type of unit for larger flows. All three considerations and several more comprise the majority of the elements involved in selecting a filtration method. To do the job right, all these issues and more must be determined. We have just scratched the surface in the subject of solid separation processes. The primary element to take away from this discussion is no type of filtration should be taken lightly waterwelljournal.com 46 August 2016 WWJ ENGINEERING from page 45 Whether you are considering becoming certified through NGWA's voluntary program or are a professional in a state using NGWA exams for licensure, this updated study guide is the one resource you cannot do without. In addition to providing information on the NGWA certification program, answers to frequently asked questions, and sample test questions and answers, the new guide has an expansive section featuring math formulas and selected articles from Water Well Journal® that will aid in your exams. Spiral-bound book Catalog #K940 NGWA member price $40.00 Nonmember price $50.00 Order your copy today. Considering certification? Taking your state licensing exam? Here's help! Downloadable PDF Catalog #D940 NGWA member price $35.00 Nonmember price $45.00 NGWA.org/Bookstore • 800 551.7379 • 614 898.7791