What is the formula for calculating parts per million by weight (PPMW) in water?

The formula for calculating parts per million by weight (PPMW) in water involves understanding how to express the concentration of an active ingredient in relation to the volume of water treated. The correct formula is designed to convert the total amount of active ingredient applied into a mass-to-volume ratio that can be expressed in parts per million.

The selected formula (total gal x lb. of active ingredient per gal x 1,000,000) / (CFS x 3,744 x minutes applied) effectively determines the concentration of the active ingredient in the treated water.

Here’s how it works:

1. The numerator calculates the total weight of the active ingredient applied:

• This is done by multiplying the total gallons of water treated by the pounds of active ingredient per gallon, which gives the total weight of the active ingredient.

2. The factor of 1,000,000 converts the weight concentration into parts per million, as PPM is a way to express very dilute concentrations.

3. The denominator accounts for the total volume of water flow over time, expressed by multiplying the cubic feet per second (CFS) of flow by 3,744 (the number of seconds in 1 hour times 24 hours), adjusting for the duration