Sediment Filters: Which is Best?


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This page may help you decide which sediment cartridge to buy (but it may just add to the confusion).

In general, sediment filters—filters whose main purpose is to remove particles from water—are rated by a “micron” number. This refers to the particle size that will be trapped by the filter. A fifty micron filter traps particles of 50 microns or larger, while a 2 micron cartridge traps particles of two microns or larger.

They are further classified as “nominal” or “absolute.” A nominal 5 micron filter is one that traps 85 percent of the particles of five microns and larger. An absolute 5 micron filter is one that traps 99.9 percent of the particles of 5 microns and larger. For most purposes, nominal filters are fine, but when very high grade water is required, an absolute-rated filter may be needed.

Sediment filters can be made of a variety of materials. Wound string or cord, polypropylene, polyester, cellulose, ceramic, glass fiber, and cotton are among the most common.

The most common general-use sediment cartridges can be divided into two groups:

1. Surface Filters. These are thin, pleated filters that trap contaminants on their surface. Once the surface is filled, the filter is replaced. (Some high quality pleated filters can be cleaned and reused.) Surface filters are best if you are filtering sediment of similar-sized particles. If all particles are five micron, a pleated 5-micron filter works best because it has more surface area than other filters.

2. Depth Filters. These are the thick-walled common wound string or spun or blown cartridges that trap particles of larger size on the surface and smaller particles under the surface down to the center core. They are best when a variety of particle sizes are being filtered. As compared with pleated filters, they have a limited surface area, but they have the advantage of depth.

So ... which is best?

If you have a well with sediment, often the best way is to experiment. Since people usually don't know the particle size, it's usually easiest to try sediment cartridges until you find what gives you adequate water flow, an acceptable lifespan, and a good result in terms of particle filtration. Tightest isn't always best.

If you have a reverse osmosis drinking water unit, 5 micron filtration is the normal for the sediment prefilter. There's no reason to use a tighter cartridge, and it might cut pressure to the membrane and have a negative result. Spun or wound string? We use them interchangeably. Manufacturers speak of the superiority of one over the other, but they seem about the same to us.

If you're filtering a pathogenic agent—bacteria or cysts—an absolute rating is essential, just to be safe. Ceramic filters for bacteria removal are in general considered the gold standard. For cysts, an absolute 1-micron filter provides a margin of safety.

The important thing to know about sediment filters is that they only reduce sediment. They don't remove chemicals or heavy metals or make the water taste or smell better.

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