Design of Sewer System...

Wastewater collection systems are normally designed to accommodate the maximum hourly flow rates. These are determined in the following manner. The quantity of domestic wastewater will vary in different sections of a community. Table shown below gives the average population densities for different types of districts. From such a table and community planning studies, future population densities for the areas to be served can be estimated. This information along whit that of anticipated average daily water demands and the expected percent of the latter being discharged into the collection system provides a working value of the future, average, daily wastewater flows. For collector sewers, the maximum hourly flowrate often is predicted by multiplying the future, average, daily waste flow by a factor of 4. For interceptors, the average flow is multiplied by a factor of 2.5. As a safety factor, the design flow usually includes an allowance for unavoidable infiltration due to faulty pipe joints. If significant quantities of industrial wastes are to be discharged into the system, an additional increment of flow increase must be considered. Normally, conditions at the 30- and 50-year horizon are assumed for the future.

Character of district Development Density of population (person/acre) Impervious surface percentage
Dense residental Two-family houses and six-family apartment buildings 55 34
Medium residential Mostly single-family houses 35 27
Light residential Single-family houses only, some on double lots 15 20
Mercantile - 14 100
Light commercial - 30 80
Industrial - 10 60

Pipe size is determined from the quantity of wastewater expected to be handled and either of two other factors - slope and velocity. If the ground surface is very flat, the minimum velocity will be used in order that the pipe does not lie too far underground. If the ground slope is greater than that necessary to produce the minimum velocity, then slope is the controlling factor, and the pipe is laid approximately parallel to the ground surface.

"General Outline of Sewer System"...

A minimum velocity of 0.5 m / sec is generally adopted to prevent the deposition of organic solids, unless the ground is very flat. In the latter case, a minimum velocity of 0.4 m / sec may be acceptable. Velocities greater than 2.5 m / sec should be avoided to minimize abrasion. Where excessive velocities would otherwise occur, resort is often made to drop manholes to reduce slopes. System design ordinarily begins with the collector system. A minimum depth of cover is adopted sufficient to permit the pipe to drain an ordinary basement. Design proceeds from the pipe farthest away from the discharge end of the system. Interceptor design is governed by the collector-system requirements. Info shown below is an outline of a procedure followed for the design of a domestic wastewater collection system.

  1. Select basic data
  2. Prepare a contour map of the area to be served, and draw a line to represent the pipeline in each street, place arrows on lines to indicate direction of flow
  3. Locate and number all manholes, place manholes at ;
  4. From a contour map, or from profile levels made in the field, prepare a surface profile for each street along the centerline of the proposed pipeline
  5. Sketch on the map the limits of area tributary to each manhole, i.e., the area served by each collector line between manholes
  6. Starting at the upper end of the system, design the system from manhole to manhole. Unless drop manholes are required by excessive differences in invert elevations, the inverts of all pipes at manholes should be at the same elevation. A pipe of one diameter should never discharge into another with a smaller diameter