Page 545 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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14.2 Gravity Sewers 503
14.2.1 Applicability

Conventional gravity sewers are typically used in urban areas with consistently sloping
ground because excessively hilly or flat areas result in deep excavations and drive up con-
struction costs. Stoppage is frequent due to grease, sedimentation, tree root development,
and, in the case of combined sewers, debris. Excessive infiltration and inflow are the most
common problems for both old and new systems.

14.2.2 Advantages and Disadvantages
Conventional gravity sewer systems have been used for many years and procedures for
their design are well established. When properly designed and constructed, conventional
gravity systems perform reliably. Properly designed and constructed conventional gravity
sewers provide the following advantages:
1. Can handle grit and solids in sanitary sewage.
2. Can maintain a minimum velocity (at design flow), reducing the production of hy-
drogen sulfide and methane. This in turn reduces odors, blockages, pipe corrosion,
and the potential for explosion.
Disadvantages of conventional gravity sewers include the following:
1. The slope requirements to maintain gravity flow can require deep excavations in
hilly or flat terrain, driving up construction costs.
2. Sewage pumping or lift stations may be necessary as a result of the slope require-
ments for conventional gravity sewers, which result in a system terminus (i.e., low
spot) at the tail of the sewer, where sewage collects and must be pumped or lifted
to a collection system. Pumping and lift stations substantially increase the cost of
the collection system.
3. Manholes associated with conventional gravity sewers are a source of inflow and
infiltration, increasing the volume of wastewater to be carried, as well as the size of
pipes and lift/pumping stations, and, ultimately, increasing costs.

14.2.3 Collection of Spent Waters
Systems of sanitary sewers are shown in Chapter 10, Fig. 10.1. Because about 70% of the
water brought into a community must be removed as spent water, the average flow in san-
itary sewers is about 100 gpcd (gallons per capita per day), or 378 Lpcd (liters per capita
per day), in North America. Variations in water use step up the maximum hourly rate about
threefold. Illicit stormwater and groundwater magnify the required capacity still further,
and a design value of 500 gpcd (1,892 Lpcd) is not uncommon.
Sanitary sewers are fouled by the deposition of waste matters unless they impart self-
cleaning velocities of 2 to 2.5 ft/s (0.60 to 0.75 m/s). Except in unusually flat country,
sewer grades are made steep enough to generate these velocities when the sewers are run-
ning reasonably full (half full or more in circular sections, because the hydraulic radius of
a semicircle equals that of a circle). Nevertheless, there will be deposition of solids. To find
and remove them, sewers must be accessible for inspection and cleaning. Except in large
sewers, manholes are built at all junctions with other sewers, and at all changes in direction
or grade. Straight runs between manholes can then be rodded out effectively if intervening
distances are not too great. Maxima of 300 or 400 ft (91.4 or 121.9 m) for pipes less than
24 in. (600 mm) in diameter are generally specified, but effective cleaning is the essen-
tial criterion. For larger sewers, distances between manholes may be upped to as much as