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,  here are three picture examples;

One part per million, or one milligram per liter (mg/l), would be equal to putting ONE drop of water into 10 gallons of water. return to top of page

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 million.) return to top of page

 

One part per trillion is 1,000 times smaller than a part per billion.  Please notice the picture of an Olympic size pool.  There is about 288,000 gallons of water in this pool.  Using a grain of sugar to represent one part per trillion in this pool, a single grain of sugar is about the size of the dot over this letter i.

To help us get a mental picture of how small a part per trillion is,  we found that there are  1,000 individual grains of sugar in a 1/4 of a teaspoon.   (Reference - http://www.vendian.org/envelope/dir0/grain_feel.html )

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.

As a point of interest, the City of St. Helens drinking water had a detection of 2.7 parts per trillion of Atrazine or almost the amount of 3 single grains of dissolved sugar in this pool.  The EPA allowable maximum contaminant  level (MCL) considered safe for consumption without any adverse health effects for this contaminant is 3 parts per BILLION!  That would be 3,000 parts per trillion!

We would have to add an additional 2,997 more grains of dissolved sugar into this pool, or 3/4 of a teaspoon, for our drinking water to even reach the Maximum Contaminant “safe” allowable threshold for Atrazine in drinking water.  St. Helens has some great drinking water!   

We are proud to make the statement that our drinking water is purer and safer than bottled water. return to top of page

AND NOW, WHAT 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 80-100 microns in particle size. 

Housed inside the St. Helens water filtration facility (picture upper left),  there are 228 Microfiltration modules (upper right) 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. ( upper center) 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.  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. 

Human hair 80 - 100  µm           Giardia  8 - 15  µm                 Cryptosporidium 3 - 5 µm         Amoeba 10 - 800 µm        Paramecium 100 - 350 µm        E. Coli 0.2 µm                              

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The picture below on the left is looking straight down at the END of just one 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 as seen in the picture of the pin cushion.  The third picture from the left,  is a magnified view of a section of the fiber in the first photo on the left.  The  pore size of the skin is 0.1 microns, smaller than 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 fourth picture from the left is a further magnification of the section of fiber wall showing just how crooked a path the water must flow through from the outer wall of the fiber to get into the hollow center of this tiny pipe like fiber. 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. To learn more about how the microfiltration modules are flushed, cleaned and maintained, follow the link below titled, "Module maintenance".

  Module Maintenance

  Return to the Source of St. Helens drinking water page

Analysis results of Pharmaceutical and Personal Care (PPCP's) products in St Helens drinking water             

  Definitions and uses of PPCP's tested for in St. Helens drinking water

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