Basic Units of Measurement and Conversions - Basic Mathematics for Water and Wastewater Operators - page 40

If we assume that the plant filters 300 gpm, then we have 157,080 gal ÷ 300 gpm =
523 minutes, or roughly 9 hours of detention time. Stated another way, the deten-
tion time is the length of time theoretically required for the coagulated water to
flow through the basin. If chlorine is added to the water as it enters the basin, the
chlorine contact time would be 9 hours. To determine the CT value (concentration of
free chlorine residual × disinfectant contact time in minutes) used to determine the
effectiveness of chlorine, we must calculate detention time.
Note: If detention time is desired in minutes, then the flow rate used in the cal-
culation should have the same time frame (cfm or gpm, depending on whether
tank volume is expressed as cubic feet or gallons). If a detention time is desired in
hours, then the flow rate used in the calculation should be ft 3 /hr or gal/hr.

Note: True detention time is the T portion of the CT value.

Detention time is calculated in units of time. The most common are seconds, min-
utes, hours, and days. Examples of detention time equations where time and volume
units match include the following:

3

Volume of tank (ft )
Fl

Detentiontime(sec) = (2.7)

ow rate (cfs)

Volume of tank (gal)
Fl

Detentiontime(min) = (2.8)

ow rate (gpm)

Volume of tank (gal)
Flo

Detentiontime(hr) = (2.9)

wrate(gph)

Volume of tank (gal)
F

Detentiontime(days) = (2.10)

lowrate(gpd)

The simplest way to calculate detention time is to divide the volume of the container
by the flow rate into the container. The theoretical detention time of a container is the
same as the amount of time it would take to fill the container if it were empty. For vol-
ume, the most common units used are gallons, although occasionally cubic feet may
also be used. Time units will be in whatever the units are used to express the flow; for
example, if the flow is in gpm, the detention time will be in days. If, in the final result,
the detention time is in the wrong time units, simply convert to the appropriate units.

■ Example 2.93
Problem: The reservoir for the community holds 110,000 gal. The well will produce
60 gpm. What is the detention time in the reservoir in hours?

Solution:

110,000 gal
60 gpm

1 833 min

60 min/hr

Detentiontime = = 1833 min; = 30.6 hr