Before You Buy a Home Water Filter

   05.19.17

Before You Buy a Home Water Filter

I recently came across a forum post that asked about water filters. I have seen this type of post before and it’s somewhat common. The question was asking if a certain type of home water filter will remove bacteria, cyst, or protozoa.

Municipal drinking water can become contaminated in several ways. During events like a hurricane, trees can uproot and either crack or break water lines.  The compromised lines allow contaminants to enter the water supply.

Then there are examples like Cryptosporidium that are resistant to chemical treatment.

When drinking water becomes contaminated, how do you know if a home water filter will make the water safe to drink?

Micron Size

The Center for Disease Control and Prevention has a page that I refer people to: Water Disinfection for Travelers.  All the way at the bottom of the page in section 2-10, the average sizes of waterborne organisms are listed.

Table 2-10. Microorganism size and susceptibility to filtration

ORGANISMAVERAGE SIZE (µm)MAXIMUM RECOMMENDED FILTER RATING (µm ABSOLUTE)
Viruses0.03Not specified (optimally 0.01, ultrafiltration)
Enteric bacteria (Escherichia coli)0.5 × 3.0–8.00.2–0.4 (microfiltration)
Cryptosporidium oocyst4–61 (microfiltration)
Giardia cyst6.0–10.0 × 8.0–15.03.0–5.0 (microfiltration)
Nematode eggs30 × 60Not specified; any microfilter
Schistosome larvae50 × 100Not specified; any microfilter

If the water filter manufacturer does not list the micron size, assume the filter does not remove cyst, protozoa, or bacteria.

There are a lot of “finishing” filters on the market.  They use the word filter, but actually just improve the taste of the water.

From the Brita website: what are micron ratings and why doesn’t Brita have them?

Brita® filters are not intended to purify water. Do not use with water that is microbiologically unsafe or of unknown quality without adequate disinfection.

If you want a home-based water filter, be sure to check the micron size and make sure the filter has a rating.

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