1 March 1997
the average daily flow would still be 115,000 L/d, or 115,000 liters in 24 hours, but the sewer
must be designed hydraulically to carry the 115,000 liters in 8 hours, not 24 hours.
(4) Peak diurnal flowrate. The normal daily range of the rate of flow, or the diurnal
pattern, is from approximately 40 percent to 250 percent of the average daily flow. The peak
daily or diurnal flowrate is an important factor in sewer design, especially when minimum
velocities are to be provided on a daily basis. The peak diurnal flowrate will be taken as one half
of the extreme peak flowrate.
(5) Extreme peak flowrate. Extreme peak rates of flow occur occasionally and must be
considered. Sewers will be designed with adequate capacity to handle these extreme peak
flowrates. Ratios of extreme peak flowrates to average flows will be calculated with the use of
the following formula:
= ratio of extreme peak flowrate to average flow
= average daily flow or average hourly flowrate in liters per day (gallons per day),
or liters per hour (gallons per hour), and
= constant, 47.71 for L/d (38.2 for gpd) or 28.10 for L/h (22.5 for gph)
When designing sewers to serve the entire installation, or large areas of the installation, and
where a major portion of the wastewater is generated by residents over a 24-hour period, the
average daily flow will be used in the formula, and the extreme peak flowrate will be computed
by multiplying the average daily flow by the ratio R. However, for sewers serving small areas of
the installation where several buildings or a group of buildings are being considered, and where
the majority of wastewater is generated by nonresidents or other short term occupants, the
average hourly flowrate will be used in the formula, and the extreme peak flowrate will be
computed by multiplying the average hourly flowrate by the ratio R. Examples illustrating the
use of the above formula are provided in appendix B.
c. Infiltration and inflow. Extraneous flows from groundwater infiltration enter the sewer
system through defective pipe, joints, fittings and manhole walls. Sources of inflow include
connections from roof leaders, yard drains, storm sewers, cooling water discharges and
foundation drains, in addition to submerged manhole covers.
(1) In computing wastewater flows for new sewers, design allowances for groundwater
infiltration will be 45 to 95 liters/day per millimeter diameter per kilometer of pipe (500 to 1000
gallons/day per inch diameter per mile of pipe), and will be added to the peak rate of flow. For
design of Air Force facilities, use 45 Lpd/mm/km (500 gpd/in/mi). Acceptance tests required for
newly constructed sewers normally limit leakage to 45 Lpd/mm/km (500 gpd/in/mi).
(2) Where infiltration/inflow must be calculated from an existing collection system,
attempts must be made to obtain flow records from treatment facilities or pumping stations which
will provide information on the magnitude of I/I quantities. In the absence of such flow data, and
depending on the scope of the project, it may be necessary to measure flows in the existing
system. Where this is not possible or feasible, allowances of 23,500 to 235,000 Lpd/km (10,000