Backwash Water Volume

A backwash is a rise rate multiplied by an area multiplied by a clock. Fifteen gallons per minute per square foot over a 400 ft² filter is 6000 gpm, and ten minutes of that is 60,000 gallons of finished water sent to the washwater tank — a real and recurring cost, since that water was already coagulated, settled, filtered and often chlorinated before you turned it around and threw it up through the bed.

The rate is chosen to fluidise the media by 20–50% bed expansion, and it is temperature-dependent because cold water is more viscous and lifts harder: a rate set on an August afternoon can wash anthracite over the troughs in January. Two field checks are worth more than any calculation. First, measure the expansion with a clamped-on scoop rather than trusting the design figure. Second, keep the backwash percentage — this volume divided by the volume filtered in the run — down around 2%; anything above 4–5% usually means the run is being cut short by breakthrough or head loss, and the fix is upstream in the coagulation, not in the backwash valve.