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Water-Treatment Filters

Filters either trap, adsorb or modify contaminants. Made from a variety of materials, they are relatively inexpensive to purchase and maintain. Filters have a micron rating based on the smallest size contaminant they can remove. Most filter cartridges come in one of four standard sizes, offering the flexibility to custom design your own multi-filter water-purification system by attaching different types of filters to a common under-the-sink housing. Some manufacturers also offer all-in-one cartridges that combine several different filters in one unit, but they are less cost-effective over the long run. Here are six types of filters you might consider:

- Sediment filters use particle filtration. Often the first step of purification, these fine sieves allow subsequent filtering to work more efficiently.

- Activated carbon filters use adsorption technology: a "sticky" filtration, where the filter material latches onto certain contaminants. Block carbon (also called carbon briquettes) are the most effective of the carbon filter choices because they handle more water and their larger surface area means more contact time. Note: Toxic metals can build up in the water overnight, so run water for 30 seconds in the morning before use. Also, sudden changes in water pressure may break contaminants loose from pipes or the filter. Therefore, turn the water on gradually and use a flow stream narrower than a pencil.

- Activated alumina filters use adsorption technology with an aluminum compound.

- Redox (reduction-oxidization) is a chemical process that uses an alloy of copper and zinc to oxidize contaminants, converting them to harmless substances or particles that are then trapped on the filter. Redox works well for shower-head filtration, where activated carbon alone can't handle the volume and temperature of the water.

- Ceramic filters are made of a dense ceramic material combined with a silver compound. The surface can be scrubbed and used indefinitely.

- Membrane/Cross-Flow filters come in four levels, each one rejecting smaller "particles" due to increasingly smaller membrane holes: microfiltration, ultrafiltration (UF), nanofiltration (NF) and finally hyperfiltration /reverse osmosis (RO), which rejects contaminants right down to the ionic level.

Other Treatment Technologies

Reverse osmosis (RO) units apply cross-flow technology, using water pressure to force water molecules through a very fine membrane. Contaminants are discarded in wastewater through a drain line. RO systems are usually combined with at least one pre-filter (sediment) and post-filter (carbon) and store the final usable water in a small tank. On the plus side, RO units filter a large range of contaminants and use no electricity. But most units operate very slowly and waste 75 - 85 percent of the water in the process. Redirecting the drain line to flush toilets (or for other uses) can reduce the waste.

Distillation removes the widest variety of contaminants of any single method. Distillers boil the water in one chamber and then collect the water vapor as it condenses in a storage tank, leaving contaminants behind. They are very reliable, have no filters or membranes to replace and waste very little water (5 percent through evaporation). On the downside, they are also very slow, consume a tremendous amount of energy resources and generate a lot of heat.

Ultraviolet (UV) Light is a form of radiation that is higher in the spectrum than visible light. It is an effective way to remove microorganisms (though little else) and requires a pre-filter to first remove particles. Smaller UV lamps can fit inside the core of standard cartridges. Chris finds that it's difficult to get a decent flow rate and a good UV dose with these cartridges, and there are hidden costs for items such as special filters. A stand-alone UV lamp delivers a higher dose of radiation and uses a standard filter.

Ozone is a toxic form of oxygen (O3) that disinfects by chemically burning up contaminants. Though it is very expensive, it is also extremely effective and, unlike chlorine, has no long-term residual.

Ion exchange trades ions from the water onto a filtering material made of resin. Cation exchange is typically used for water softening and replaces minerals such as calcium and magnesium that make the water "hard" with sodium or potassium.

Aeration is the process of exposing water to air, which allows some contaminants to escape. It's a natural feature of surface water sources. A simple home version would be to let chlorinated tap water stand outside in an open container for 2 - 3 days — the length of time required for residual chlorine to volatilize into the air.

1. Activated carbon filters can be coated to remove toxic metals like lead and mercury. They can also come with a sub-micron strainer to remove bacteria.
2. Activated alumina is field proven for fluoride removal though not yet submitted and "listed" with NSF.
3. RO removal of arsenic requires pre-treatment with redox or with chlorine to oxidize it.
4. Distillers with venting features can address VOCs.
5. Some UV units with more intense exposure can address cyst removal.

* Though I do not recommend chlorine, Chris did include it in his top picks for bacteria and virus removal.

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