A part per million is equal to a milligram per liter.  Parts per million (abbreviated ppm) is one part in a million parts.  Parts per million refers to a ratio of weights.  One part per million is one pound in a million pounds, one ounce in a million ounces, one gram in a million grams, etc.  Therefore:

1 ppm = 1 pound per million pounds, or, since

120,000 gallons of water = 1,000,000 pounds

1 ppm = 1 pound per 120,000 gallons of water
 

To visualize a part per million, locate and look at 75,000;  100,000; 125,000;  150,000;  500,000; or a million gallon tank.  A 125,000 gallon tank would contain a million pounds of water.  If one pound of sugar is dissolved in 125,000 gallons of water, this would be one part per million.  A ten pound bag of sugar in a million gallon tank of water would be a little over one part per million, 1.2 ppm.

One pound of sugar in 120,000 gallons of water is one part per million, or one ten thousandth of a percent.  Parts per million is an easier, better term to visualize than percentages for concentrations in water.

Milligrams Per Liter

Milligrams per liter also refers to a weight ratio.  One milligram per liter is the same as one part per million when the reference volume is water because,

1 gram (weight) = 1,000 milligrams  and

1 liter of water weighs 1,000 grams;

therefore,

1 liter of water = 1,000,000 milligrams (1,000  x  1,000)

so,

1 milligram in one liter of water = 1 milligram per liter or
one part in a million parts

    Examples of the use of the formula are given below:

1. A plant which treats 200 gpm operated for 15 hours and used 30 pounds of alum. How many ppm of alum was used:

                pounds of chemical (alum) = 30
                constant = 120,000
                number of gallons = 200 (gpm)  x  60 (min)  x  15 (hrs) = 180,000 gallons
 

2. A tank holds 150,000 gallons and it is desired to disinfect it with 50 ppm of chlorine. HTH is to be used (HTH is 70% chlorine).  How much HTH will be needed?

               Y = amount of chlorine needed (pounds)

                120,000 = constant
                150,000 gallons = amount of water to be treated
                50 = ppm
 

The HTH is only 70% chlorine.  Therefore,

If the weight of chemicals is measured in grams, parts per million can be determined by:

An example of the use of these formulas is shown below:

1. A dry feeder added 5 ounces of chemical per minute and the rate of filtration was 1.0 mgd;  what was the dosage in ppm?

   First, find the number of gallons treated in 1 minute.  Since 1 million gallons are treated in a day and there are 1440 minutes in a day,

   

    

   By using the ppm formula for ounces of chemical, the desired results are obtained without converting the ounces to fractions of a pound:

   

Suppose the operator wishes to add 1 ppm of fluoride to the water and the plant operates at a rate of 1.5 mgd.  Sodium silicofluoride is used, and contains 60 percent fluoride.  Note that the strength or purity of the chemicals must be taken into consideration in calculations.
 

The operator wishes to know how many grams of the powdered chemical he must feed per minute in order to add 1 ppm of fluoride to 15 mgd.

Using formula "C" above:

This indicates 3.9 grams per minute would be needed if the chemical were 100 percent pure fluoride.  Since it is only 60 percent fluoride, the amount needed may be found by:

Thus 6.5 grams per minute of sodium silicofluoride is necessary to get 1 ppm of fluoride in the water which is being filtered at the rate of 1.5 mgd.

Grains per gallons is refered to frequently in literature and therefore, for reference there are 7,000 grains in one pound.  One grain per gallon is the same dosage rate as 1,000,000 grains in 1,000,000 gallons of water.

 

One grain per gallon  =  143 pounds per million gallons
 

1,000,000 gallons (water)  =  8,340 pounds
 

 

Therefore, one grain per gallon  =  17.1 ppm, or