Water Filtration

by Stephen Bernal on May 4, 2010

This article concludes our first multi-part discussion on home water quality. The first article highlighted some of the possible water contaminants that municipal water systems and well water drinkers face. The second feature compared the pros and cons of bottled water versus tap water. Finally, this article gives a very brief introduction to some of the home filtration options currently available to consumers.

No matter where your water comes from, whether it be a well or the city, you should consider filtering your water before you drink it. Water filtration adds an extra layer of protection for you and your family against any impurities or contaminants that might make it past your city’s water purification process or against the natural occurring contaminants in your well. Water filtration is the process of removing impurities by a physical barrier, a chemical process, or a biological process. There are water filters that work on many scales including pitcher filters, faucet filters, and whole house filtration. Some water filters are better for filtering municipally provided water and others are better for well water filtration. Whatever you water purification needs; there is a water filtration system to help you.

Activated Carbon Water Filtration

Brita activated carbon water filter

The most common point of use water filtration method is activated carbon. Activated carbon (AC) can come from various natural sources such as bituminous coal, coconut shells, wood, or lignite. This filtering material uses a process called adsorption to remove unwanted contaminants. Adsorption is a process where the unwanted contaminants adhere to the activated carbon while the water flows by. With only a few minor exceptions, activated carbon removes chlorine and chlorine byproducts as well as volatile organic compounds very efficiently.

Since the water needs to touch the activated carbon surface in order for the adsorption properties to work, surface area is extremely important with AC filters. That is why the most popular point of use water filters use granular activated carbon. A single teaspoon of granulated AC has the same surface area as a football field. It is also important that there is significant contact time between the water and the activated carbon surface. That is why you will notice this variety of water filter in pitchers where there is sufficient time for the water to run through the filter rather than in faucet filters that need to act under more strict time constraints.

While activated carbon is good for removing chlorine, chlorine byproducts called TTHMs, radon, and volatile organic compounds such as PCBs, they are not good for removing arsenic, fluoride, and some heavy metals. For those, you need another water filtration method known as reverse osmosis. In addition, AC filters will not remove bacteria. In fact, some carbon surfaces are the perfect place for bacteria to grow. That is why many AC filters will add silver. The silver helps to control bacterial growth for a short time. To get rid of bacteria, you need ultraviolet radiation filtration methods.

Reverse Osmosis Water Filtration

Reverse osmosis is a filtration method that uses a physical barrier to filter out not only insoluble contaminants, but also ions that are in solution. The process works like this: A selective membrane will only allow molecules through that meet a certain size criteria. The membrane is designed to let pure water molecules through but not the ions of heavy metals or other contaminants. The incoming stream of water is under pressure to force the pure solvent (water) through the membrane leaving behind the contaminants.

Pressure forces fresh water through the semipermeable membrane leaving contaminants behind

Reverse osmosis filters are not typically very efficient and the installation costs can be great. A common problem is when the membrane is clogged with buildup of water impurities or bacteria growth. As the membrane performance goes down, the flow of water through the system goes down. They need a lot of maintenance and upkeep to keep them working in good condition. The water you get out of a RO filter lacks any minerals and the minerals are what give water its taste. Many people think that RO filtered water taste flat or stale.

Because of the possibility of buildup of contaminants and bacterial growth, reverse osmosis systems are often installed as part of a water filtration system that uses activated carbon steps before reverse osmosis and ultraviolet radiation steps afterward.

Ultraviolet Radiation Water Filtration

The main purpose of an ultraviolet radiation filtration system is to kill pathogenic bacteria like salmonella, cholera, dysenteriae, typhoid, and others. There is no physical barrier in an ultraviolet radiation filter. Instead, an intense ultraviolet light is passed through the water to kill the bacteria. These types of water filters are rarely used alone because they are not effective at removing any dissolved contaminants other than bacteria.

Water Softeners

In addition to water filtration, you can also affect the quality of your home water with water softening systems. Water softeners work on the principle of ion exchange. Water is considered hard if it has a high amount of calcium and magnesium ions present. Ion exchange systems actually take out the calcium and magnesium ions and replace them with sodium ions, which do not contribute to water hardness.

The way this process is accomplished is by running the water through a bed of zeolite. Zeolite resin by itself carries a negative charge. When you start with a fresh bed of zeolite in your water softener system, it is covered with weakly positively charged sodium ions in order to make the system neutral. As the water passes over the zeolite with sodium ions attached, calcium and magnesium ions in the water take the place of the sodium ions because they have a larger positive charge than the sodium ions. Periodically, you need to regenerate the zeolite by knocking off the calcium and magnesium ions. The regeneration is achieved by running an extremely concentrated brine solution through the zeolite bed. The negative chlorine ions in the brine will take the calcium and magnesium away leaving more sodium behind.

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