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536 Chapter 14 Design of Sewer Systems
7. Excessive settling of solids in the manhole and sewer line may lead to obstruction,
blockage, or generation of undesired gases.
8. The diameter of the sewer line may be reduced by accumulation of slime, grease,
and viscous materials on the pipe walls, leading to blockage of the line.
9. Faulty, loose, or improperly fitted manhole covers can be a source of noise as well as
inflow. Ground shifting may cause cracks in manhole walls or pipe joints at the man-
hole, which become a source of infiltration or exfiltration. Debris (rags, sand, gravel,
sticks, etc.) may collect in the manhole and block the lines. Tree roots may enter
manholes through the cracks, joints, or a faulty cover, and cause serious blockages.

In arctic (permafrost) regions, sewers as well as water pipes may have to be placed in
utilidors. The transfer of all utilities to such structures is being considered, too, in the re-
building of old cities and the building of new ones. Effective inclusion of sanitary sewers
may call for the introduction of pressurized systems. Storm sewers can probably not be ac-
commodated economically.
Operation and maintenance costs are given in Fig. 14.19. Current 2010 costs are ob-
tained by multiplying the costs from the figure by a multiplier of 2.73 (Appendix 16).

14.15.2 Force Main and Lift Station
The operation of force main/lift station systems is usually automated and does not require
the presence of a continuous on-site operator. However, annual force main route inspec-
tions are recommended to ensure normal functioning and to identify potential problems.

10 0.1

1.0 0.01
Annual cost, USD/ft Annual cost, million USD/mile

0.1 0.001

0.01 0.0001
0.1 1.0 10 100
Wastewater flow, MGD

Figure 14.19 Operation and Maintenance Costs for Sewers. Conversion factors: 1 ft
3
0.3048 m; 1 mile 1.609 km; 1 MGD 3.785 MLD 0.0438 m /s 43.8 L/s.