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Milligrams Per Liter
(Parts Per Million)
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
The constant, 120,000, is always used in the ppm formula
when the chemicals used are weighed in pounds and the water measured
in gallons since 120,000 gallons weighs 1 million pounds.
Examples of the use of the formula
are given below:
 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
 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 ounces,
parts per million can be determined by:
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:
 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:
After making jar tests, suppose that the operator
knows how many ppm have been added to the water. How would he
determine how much chemical the machine should feed? Either of the
three formulas given may be used and need only to be changed to find
the amount of chemical needed:
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
