Ask Jeff: WaterFed Filters—Making Sense of it All (Part 1)

So you’re all geared up and ready to take on the world with your new pure water window cleaning rig. Whether you have a Deionization-only (DI) setup or a fancy multi stage Reverse Osmosis/Deionization system (RO/DI), it makes sense for you to know exactly what your filters are there to do. This week we’ll cover what each filter does, and next week in part 2 we’ll cover the lifespan of filters and knowing when to replace them.

So let’s take the filters in the sequence of an RO/DI set-up. You DI-only guys feel free to skip to the end…

filter-flowSediment Filter

Pretty straightforward. The sediment filter is there to remove particulates from the water prior to the water passing on to the next stage of treatment. The most common particulate removal size rating used in RO/DI systems for our industry is a 5 micron sediment filter. These are usually either a wound string filter, a pleated filter, or a melt blown filter. The flow on these filters is usually from the exterior to the interior of the filter media.

Carbon Filter

The next stage in the RO/DI system is the carbon filter. This is crucial in the RO process because most RO membranes are not chlorine-tolerant and it is likely that the water you will be feeding your system will have chlorine in it. If you don’t have a carbon filter, you’ll kill your RO filter pretty quickly. The most common carbon filter is called a GAC carbon filter, which stands for granular activated carbon. Carbon filtration has been around for hundreds (if not thousands) of years as a water filtration method. It removes more than just chlorine—it improves the taste, removes unpleasant odors, and does remove some sediment. They are ineffective at removing minerals, salts or and dissolved inorganic compounds.

KDF Carbon Filter

An additional element added to many GAC carbon filters is KDF (Kinetic Degradation Fluxion Media). KDF was introduced in the 1980s and is one of the best “boosts” for the carbon filtration step. It is composed of an alloy of copper and zinc, usually flakes or granules. It kills algae and fungi, controls bacteria, and reduces chlorine, pesticides, iron, lead, rust, heavy metals and other organic compounds. In a way, KDF works like DI resin, which we will discuss later. As copper has a positive charge and zinc has a negative one, when water is passed through a KDF/carbon filter, it gets not only the benefits of the carbon filtration but the added benefits of the electrical & magnetic attraction generated by the charges of the copper & zinc attracting oppositely charged impurities and removing them from the water before it gets passed on to the next stage.

RO Membrane

RO-filter-cross-section

A cross-section of a reverse osmosis filter.

Reverse Osmosis is a process where water is forced through a thin (about as thick as cellophane) membrane that removes dissolved organic solids (commonly called (TDS or Total Dissolved Solids) from the water. The semi-permeable membrane allows only the water molecule to pass through it, not any impurities that are in that water.  RO is affected by the following factors: water pressure, either from the tap or use of or lack of a pump, water temperature, type and number of TDS in the feed water, and the type and quality of the RO membrane being used in the system. The holes in a typical RO membrane measure roughly between .0005 to .005 microns wide. To put that in perspective, the Polio virus is .005 microns, a red blood cell in a human being is 5 microns, the human eye can see a particle about 35 microns, and a human hair is about 80 microns. The most common RO membrane used is a spiral wound TFC/TFM (thin film composite material) which is not chlorine-tolerant. This is why a carbon pre-filter is so important in your RO/DI system. There are CTA (cellulose tri-acetate) membranes available which are chlorine-tolerant, but they generally do not operate at lower pressures needed in pumpless sytems window cleaners prefer, and can be cost prohibitive.

DI Resin

The final stage in the RO/DI system is the DI resin. Deionization removes the smallest of whatever has made it through everything else at the ionic level. DI resin is made up of small beads (called ion exchange resin or ion exchange beads) that have small electrical charges in them. Negatively charged beads are anions while positively charged beads are called cations. As we all learned in 6th grade science class, opposites attract. So when the water passes through the mixed bead resin, any impurity left in the water that has an opposite charge will be attracted to the corresponding DI resin bead, bond to it and be held back, leaving only pure water to pass through the outlet of your system, through the hose up the pole and onto the glass.

I hope this explanation of what each stage in the multi-stage pure water system has been informative and helps you understand exactly what’s happening in all those mysterious space-age looking housings and canisters when you hook up your hoses and turn on the water and switches.

Read part 2, where we’ll cover the lifespan of filters and how to know when to change them.

Author

Jeff Klass

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Jeff is a professional window cleaner and leading expert in pure water window cleaning. He runs First Klass Window Cleaning, and serves as abc's WaterFed guru.

Jeff KlassAsk Jeff: WaterFed Filters—Making Sense of it All (Part 1)

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