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:

C

R

=

)))))

Q0.167

where:

R

= ratio of extreme peak flowrate to average flow

Q

= average daily flow or average hourly flowrate in liters per day (gallons per day),

or liters per hour (gallons per hour), and

C

= 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

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