Water Well Journal

August 2016

Water Well Journal

Issue link: http://read.dmtmag.com/i/705618

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Page 39 of 71

T he first three parts of this series on solids separation methods have covered strainer, barrier, centrifugal sepa- rators, cartridges, adsorption, oxidation, and precipita- tion methods of solids removal. We've discussed the most common filtration methods of removing particulate matter from water as well as many additional procedures that assist with the filtration process. Now we'll wrap things up by delving into one of the most popular types of solids separation methods currently practiced for process, potable, and industrial water supplies today— membrane filtration. Introduction to Membrane Filtration Most of the earlier applications of membrane filtration in- troduced in the 1960s used reverse osmosis for the specific need to desalinate seawater or brackish water in order to create drinking water. Membrane filtration is now more commonly associated with and widely used for surface water supplies and treatment of potable and process water. Also, the many improvements seen in recent years in materials, technology, and types of membrane construction have increased the recent popularity in using these filtration devices in groundwater and other types of water and wastewater systems as well. Membrane filtration was at one time often considered a "last gasp" method for the removal of salts and small and fine particulates and contaminants from drinking water—including bacteria and viruses where conventional filtration methods either failed or were inadequate. However, the improvements in the filtration method in recent years—to prevent premature failure caused by plugging, overloading, or the breakthrough of contaminants, plus the enhancement and knowledge of the need for proper pre-filtration and treatment—now makes the units more of a primary filter. In reality, the recent improvements in the materials used, the improved design of membrane filtration cartridges, and using many units in parallel to lower individual flows makes the use of this system much less of a concern for plugging than in previous years. Membranes are now commonly used for the removal of various constituents including salinity, hardness and other dissolved inorganics, natural organic mat- ter, synthetic organic contaminants, disinfection byproducts, and other common water contaminants. In addition, selecting the proper membrane style and type for the specific application, along with implementing appro- priate pre-treatment or filtration ahead of the membranes where needed, now allows the designer to tailor the applica- tion and design of a membrane system to a specific raw and finished water—and with proper maintenance, greatly extends the life and removal efficiency of membranes far beyond what would have been experienced during the earlier years of their use. Membrane systems for filtration and desalination, with the appropriate application and design, are now available in a range of flow rates from less than a gallon per minute for a single unit all the way up to 30 million gallons per day or more for systems with thousands of units operating in parallel. Membrane Technology When comparing membrane filtration to the various solids separation methods we have previously explored, it becomes fairly obvious membrane filtration is a form of the barrier method. What it does is place a physical obstruction directly in the path of the water and particulate and removes the solid through a sieving-type process. Two main flow configurations are used as the sieves in membrane processes: cross-flow filtration and dead-end filtration. In cross-flow filtration (Figure 1), comprising the majority of water and wastewater applications, the feed flow is intro- duced tangential to the surface of the membrane, retentate is removed from the same side farther downstream, and the rejected material is removed from the fluid and withheld in the membrane. Finally, the permeate flow is developed on the other side of the membrane. Depending on the specific design of the membrane, the feed fluid can be directed to flow from the center through the membrane, with the permeate discharged on the exterior side. This is referred to as inside-out flow. ED BUTTS, PE, CPI ENGINEERING YOUR BUSINESS SOLIDS SEPARATION METHODS Part 4: Membrane filtration ENGINEERING continues on page 40 Figure 1. An illustration of cross-flow filtration. waterwelljournal.com 38 August 2016 WWJ

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