Environmental Engineering FOURTH EDITION

156 ENVIRONMENTAL ENGINEERING

that run under the street. Collecting sewers are sized to cany the maximum anticipated
peak flows without surcharging (filling up) and are ordinarily made of clay, cement,
concrete, or cast iron pipe. They discharge in turn into intercepting savers, or inter-
ceptors, that collect from large ares and discharge finally into the wastewater treatment

Collecting and intercepting sewers must be placed at sufficient grade to allow
for adequate flow velocity during periods of low flow, but not so steep as to promote
excessively high velocities when flows are at their maximum. In addition, sewers must
have manholes, usually every 120 to 180 m (400 to 600 ft) to facilitate cleaning and
repair. Manholes are necessary whenever the sewer changes grade (slope), size, or
direction. mical manholes are shown in Fig. 8-4.
Gravity flow may be impossible in some locations, or may be uneconomical, so
that the wastewater must be pumped. A typical packaged pumping station is shown in
Fig. 8-5, and its use is indicated on the system layout in Fig. 8-3.

plant.

Section A-A Section B-B

B-J

Section C-C

BJ

Section A-A Section B-B Section C-C

Figure 8-4. Typical manholes used for collecting sewers. (Courtesy of ASCE.)