Understanding “parts per million”

What is a part per million, or a part per billion even a part per trillion?

In water quality reports and water analysis studies we see results of contaminants in our water expressed as parts per million, parts per billion and parts per trillion, and just what does this mean to you and I?


Nearly everyone we meet and speak with have difficulty understanding what these expressions actually mean. In the drinking water industry we take the concept of one in a million, billion or trillion very seriously, since this is how we measure very small traces of chemicals or contaminants in water.

The water we use daily for drinking, cooking and bathing must meet strict rules of purity. To help you better understand what a part per million, part per billion and parts per trillion actually are:

If you were to fill a 10 gallon aquarium using and eyedropper and added one drop at a time until the aquarium was full, it takes one million individual drops to do so, knowing this helps us understand that:

  • One part per million, or one milligram per liter (mg/l), would be equal to putting ONE drop of water from an eyedropper into 10 gallons of water.
  • One part per billion, or one microgram per liter (μg/l), would be equal to adding ONE drop of water to a 10,000 gallon swimming pool. (A part per billion is 1,000 times smaller than a part per
  • One part per trillion is 1,000 times smaller than a part per billion. Please notice the picture of an Olympic size pool at the beginning of this article. There is about 288,000 gallons of water in this pool. A single drop of water into this size pool is very close to a part per trillion.  Even closer to seeing this in your minds eye, if you were to take a single grain of granualted sugar and drop it in this Olympic pool, that is a part per trillion! Another very important point to remember about the grain of sugar, as with the drops of water, the grain of sugar would dissolve and would be dispersed into the entire area of the pool just as the water drop became part of the entire pool.


The Filtration Process

WFP      WFP3    Handout pictures 181  membrand cross section

Housed inside the St. Helens water filtration facility (picture to left), there are 228 Microfiltration modules (second picture) that make up the 5 racks of filters. Each rack has 52 individual modules. Inside each of these modules, there are 6,350 hollow fibers. ( third from left) The surface area inside this bundle of fibers is 538 sq. ft. If we were to take the fibers out of 107 of these modules and uniformly lay them out, the surface area of the fibers would completely cover an American football field from side line to side line to end zone to end zone! We have a little over two football fields of surface area that we are using to filter and produce 4 to 6 million gallons of fresh, high quality drinking water for the City of St. Helens.

How About Microns?

A micron is equal to one millionth of a meter, or about a tenth of the size of a droplet of mist or fog. It is a particulate smaller than the naked eye can see. To give you another idea for reference, one single strand of hair is approximately 20-100 microns in diameter. Now moving along to where the measurement of Microns comes into use. Pictured below are “critters” found in soil, lakes, rivers, streams, creeks and ponds and a human hair. Notice their sizes expressed in Microns or µm.


Giardia 8 – 15 µm microns Cryptosporidium 3 – 5 µm microns Human strand of hair 20 to 100 µm microns  E. Coli 0.2 µm microns

 The Micro-Filtration Process

The pictures below begin with  looking straight down at the END of a cluster of the 6,350 tiny hollow fibers bundled inside a module. The hollow center of the fiber is 0.7 of an inch in diameter, about the size of the shaft of an ordinary sewing pin.  The second picture from the left, is a magnified view of a single hollow fiber. The third picture is a magnification of the wall of the fiber in the middle photo. The pore size of the skin is 0.1 microns, smaller than even the E. Coli bacteria which is 0.2 microns. Notice that there is not a straight path from the outer wall of the fiber to the hollow center into which the filtered water flows, this is referred to as a “tortuous” path.  The tortuous path along with the small pore size of the micro fibers block and prevent not only E. Coli from being in the filtered water, but even Giardia, Cryptosporidium, Algae, along with sand and silt are trapped on the outside walls of the fibers.


The membrane skin has a 0.1 µm micron opening, smaller than even the E. Coli bacteria.  The membranes filter out the contaminants in your drinking water.


Any questions or comments? Please send me an e-mail howieb@www.ci.st-helens.or.us