Collection of Wastewater 157Figure 8-5. Diagram of typical pumping station for wastewater. (Courtesy of Gorman-
RUPP.1
SEWER HYDRAULICS
Sewer design begins by selecting a reasonable layout and establishing the expected
flows within each pipe linking the manholes. For large systems, this often involves
the use of economic analysis to determine exactly what routing provides the optimal
system. For most smaller systems, this is an unnecessary refinement, and a reasonable
system may be estimated and sketched by hand.
The average discharge is estimated on the basis of the population served in the
drainage area, and the maximum and minimum flows are calculated as shown in
Fig. 8-2. Once this is done, the design is a search for the right pipe diameter and
grade (slope) that will allow the minimum flow to exceed a velocity necessary for
conveyance of solids, while keeping the velocity at maximum flow less than a limit
at which undue erosion and structural damage can occur to the pipes. The velocity is
usually held between the following limits:
0 minimum = 0.6 m/s (-2 ft/s)
0 maximum = 3.0m/s (-lOft/s).