1 March 1997
HYDRAULIC DESIGN OF SEWERS
3-1. QUANTITY OF WASTEWATER. For any segment of proposed sewer, the design
wastewater flow must be determined. Sanitary or domestic wastes based on the population
served by a given segment, extraneous infiltration/inflow, and contributing industrial flows must
be added to produce the design flow. Where existing flow records or data showing required flow
capacity are not available, the methods and criteria discussed below will be used to develop
a. Tributary area. This is the area contributing wastewater to a particular sewer segment.
The quantity of wastewater which is collected by a particular segment is dependent upon the
types of personnel and industrial activities which are regularly found in the area. Where no
information is available on existing areas to be served, a survey will be required to determine the
number and classification of personnel and the types of industries.
b. Sanitary or domestic wastes.
(1) Contributing population. Domestic wastewater quantities normally are to be computed
on a contributing population basis, except as noted in subparagraphs d and e below. The
population to be used in design depends upon the type of area which the sewer serves. If the
area is strictly residential, the design population is based on full occupancy of all housing and
quarters served. If the area served is entirely industrial, the design population is the greatest
number, military and civilian, employed in the area at any time, even though some of these
persons may also be included in the design of sewers in the residential area. For sewers serving
both residential and industrial areas, the design population includes residents and nonresidents,
but in the design of these sewers obviously no person should be counted more than once.
Allowances will be made for future population changes based on facility personnel requirements
(2) Average daily flow. Average daily per capita wastewater quantities for different types
of installations and buildings are given in table 3-1. The average daily flow will be computed by
multiplying the resident and nonresident contributing populations by the appropriate per capita
allowances and adding the two flows. The average daily flow represents the total waste volume
generated over a 24-hour period. However, it is not a realistic indicator of the rate of flow when
wastes are generated over shorter periods of 8, 10, 12 hours, etc. Thus, the average daily flow
will be used only for designing sewers to serve the entire installation, or large sections of the
installation, and where a major portion of the wastewater is generated by residents over a