It is important to be able to calculate the total flow of water through a plant. This figure will be used for a variety of purposes, including calculations of hydraulic loading.

The formula below can be used to calculate the total flow, in gallons per day, including recirculation

The "Recirculation" is the percent of the influent flow which
is recirculated through the trickling filter. The recirculation
must be expressed as a decimal. To express a percentage
as a decimal, simply divide the percentage by 100. So
if 2% of the influent flow was recirculated, the Recirculation
would be:

Let's consider a plant with an influent flow of 72 mgd and
a recirculation of 0.009. The total flow would be calculated
as follows:

Be sure to convert to the correct units, if necessary. I
converted 72 mgd to 72,000,000 gpd.

You may have also noticed that I added 0.009 to 1 before
I multiplied the total by 72,000,000 gpd. It is very
important to always perform any operation within parenthesis
before simplifying the rest of the equation. Otherwise, you
will get the wrong answer.