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tech update Fundamentals of Reverse By Diego Bonta his is the first in a series of five articles providing an overview often used in a series focusing on the larger contaminants first, followed by T of how point-of-use (POU) (RO) smaller and smaller impurities. Figure 1 shows the main available works in residential systems and the critical parameters to keep in for residential systems with the corre- sponding contaminants they remove. mind to optimize its performance. Out of the list of technologies, particle filtration and are An introduction to RO This article, the first in the series, This series is intended to assist the only ones that are not commonly covers the fundamentals of RO. The OEMs, dealers, installers and end- found as membranes. All of the other and its applications second article will look at different users in making better decisions in technologies may exist as flat sheet feedwater factors that can greatly optimizing a POU RO system and membranes, hollow fibers or even impact the performance of an RO avoiding failures in the field. ceramic candles. If the filter is made up filter. The third article will provide of membranes or hollow fibers, it can be POU system design guidelines by Technologies used in two modes of operation: dead- explaining how the major residential There are several technologies used end filtration or cross-flow filtration. POU components operate with one to remove impurities from residen- The schematic in Figure 2 demonstrates another. The fourth article will provide tial . Each technology how both modes of filtration work. guidance on preventing short lifetime focuses on different types of contami- failures. The final article will compare nants and can generally be categorized Dead-End Filtration RO filter quality and performance. by size. These removal technologies are In dead-end filtration, all of the

Figure 1. Contaminant Removal Technologies Figure 3. Reverse Osmosis Available for Residential Systems Angstroms Osmosis Reverse Osmosis 11010 100 1000 104 105 6 107 Applied Pressure Sugars Pollens Beach Viruses Bacteria Sand Aqueous Salts Colloids Particle

Microfiltration Flow Flow

Reverse Osmosis Concentrated Dilute Ion Exchange Solution

Figure 2. Membrane Operation Modes Figure 4. Cross-Flow Filtration Water Flow

Product

Feed

Concentrate

Dead-end filtration Cross-flow filtration Ion

12 PRODUCTS feedwater is pushed through the developing new membranes that allow The membrane allows water to Residential & Commercial Application membrane and becomes produced more water and fewer harmful contam- diffuse through the membrane but Development team at Dow Water & (permeate) water. The advantage of inants through. Some membranes are blocks the majority of the salt. The Process and a member of the this mode of operation is that no so specialized that they are developed most common mode of operation is WQP Editorial Advisory Board. Bonta wastewater is created. The disadvan- to specifically block ions of interest, cross-flow (see Figure 4 on page 12) can be reached at [email protected]. tage is that the impurities build up on such as nitrates, boron or silica. to enable the salt to be swept away the feed side of the membrane, creating In summary, the main principle from the membrane surface and not For more information on this subject a cake layer that decreases performance required for RO is the application build up. wqp write in 1004 on the reader service card and can require frequent filter changes. of sufficient pressure to push water or visit www.wqpmag.com/lm.cfm/ Dead-end filtration is used in microfil- through a semi-permeable membrane. Diego Bonta is a member of the Global wq021104. tration and ultrafiltration.

Cross-Flow Filtration Cross-flow filtration means that part of the feedwater is pushed through the membrane while the rest sweeps past the membrane, removing particulates and other foulants to extend the life- time of the filter. This mode of opera- tion is the most common for RO WaterWater ProductsProducts and nanofiltration. Along with differences in the mode of operation, filtration technologies also require different feed pressures. In general, the smaller the particles removed by the technology, the higher the operating feed pressure required. The table below shows the general operating pressures required for each technology. Ultrafiltration and micro- filtration operate at low pressures compared to RO because they operate independently from the osmotic pres- sure of the solution they are treating.

Membrane Typical Operating Process Pressure Range (psi) Reverse Osmosis Seawater 800 – 1,015 Brackish water 145 – 580 Nanofiltration 50 – 220 TM Ultrafiltration 30 – 100 PureWaterCoolers 1.5 – 45 Bottleless Water Dispensers with 2 and 3

Basics of RO If a semi-permeable membrane sepa- PureWaterMachines TM PureWaterConditioners TM rates a concentrated solution from a Gold Seal approved RO Systems Ion Exchange and Electronic Water Conditioners dilute solution, water will flow from the dilute solution to the concentrated side in order to bring the two solutions into equilibrium through a process called osmosis. However, if sufficient pressure is applied to the concentrated solution, it is possible to force water to flow into the opposite direction. This is the main principle on which RO is based (see Figure 3 on page 12). A semi-permeable membrane allows water to diffuse through the membrane and only allows a small amount of ions through. RO membrane rejects contaminants by three mechanisms at the surface: repul- sion from the ion charge and density; Full Range of Models at Affordable Prices. Contact us for details. dipolar interactions; and the size of the molecule being larger than the pore size of the membrane. (800) 627-2146 Companies that specialize in email: [email protected] local: (909) 626-2100 making RO membranes are continually website: www.VertexWater.com fax: (909) 626-3535 WATER PRODUCTS 5138 Brooks St. Montclair, Ca 91763

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