Water/Wastewater Laboratory Calculations - Basic Mathematics for Water and Wastewater Operators - page 247

1. Determine what unit the question is asking you to find (lb or mg/L).

2. Physically cover or hide the area of the chart containing the desired unit.
Write the desired unit down alone on one side of the equal sign to begin the
necessary equation (e.g., lb =).

3. Look at the remaining uncovered areas of the circle. These exactly represent
the other side of the equals sign in the necessary equation. If the unit above
the center line is not covered, your equation will have a top (or numerator)
and a bottom (or denominator), just like the pie chart. Everything above the
center line goes in the numerator (the top of the equation) and everything
below the center lines goes in the denominator (the bottom of the equation).
Remember that all units below the line are always multiplied together ; for
example, if you are asked to find the dosage in mg/L, you would cover mg/L
in the pie chart and write it down on one side of the equal sign to start your
equation, like this:

mg/L =

The remaining portions of the pie chart are lb on top divided by MGD ×
8.34 lb/gal on the bottom and would be written like this:

lb

mg/L =

MGD × 8.34 lb/gal

If the area above the center line is covered, the right side of your equation
will be made up of only the units below the center line. Remember that all
units below the line are always multiplied together.

If you are asked, for example, to find the number of pounds needed, you would
cover lb in the pie chart and write it down on one side of the equal sign to start your
equation, like this:

lb =

All of the remaining areas of the pie chart are together on one line (below the center
line of the circle), multiplied together on the other side of the equal sign, and written
like this:

lb = mg/L × MGD × 8.34 lb/gal

C hlorine d osage
Chlorine is a powerful oxidizer commonly used in water treatment for purification
and in wastewater treatment for disinfection, odor control, bulking control, and other
applications. When chlorine is added to a unit process, we want to ensure that a mea-
sured amount is added. The amount of chemical added or required can be specified
in two ways: