How is detention time calculated in a water treatment basin?

Detention time in a water treatment basin refers to the amount of time water spends in the basin before it is treated and released. To calculate this, the correct method involves taking the volume of the basin (its capacity) and dividing it by the flow rate entering the basin, typically expressed in gallons per day. This calculation yields the detention time in days, reflecting how long the water is retained in the basin for treatment processes.

When dividing basin capacity by gallons per day, you are effectively determining how many days it would take to fill the basin at the given flow rate. This is crucial for ensuring that the basin is properly sized for treatment needs and can manage the expected inflow of water.

The other options do not accurately calculate detention time. For instance, dividing the flow into the basin by basin capacity would produce a dimensionless ratio rather than a time measurement. Multiplying basin capacity by flow rate does not yield a meaningful detention time value, and calculating total gallons used would not provide relevant information concerning the detention period in the basin.