Page 551 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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14.4 Combined Collection of Wastewaters and Stormwaters 509
sewerage systems at a time when much sand and gravel were washed from unpaved streets.
Historically, too, the air in sewers, called sewer gas, was once deemed dangerous to health;
this is why catch basins were given water-sealed traps. Catch basins need much maintenance;
they should be cleaned after every major storm and may have to be oiled to prevent produc-
tion of large crops of mosquitoes. On the whole, there is little reason for continuing their use
in modern sewerage systems.

14.4 COMBINED COLLECTION OF WASTEWATERS AND STORMWATERS
In combined sewerage systems, a single set of sewers collects both domestic and industrial
wastewater and surface runoff from rainfall (Fig.10.1). Because stormwaters often exceed
sewage flows by 50 to 100 times, the accuracy with which rates of surface runoff can be es-
timated is generally less than the difference between rates of stormwater and combined-
sewage flows. Accordingly, most combined sewers are designed to serve principally as
storm drains. However, they must be placed as deep as sanitary sewers. The backing up
and overflow of combined sewage into basements and streets is obviously more objection-
able than the surcharge of drains that carry nothing but stormwater. Combined sewers are
given velocities up to 5.0 ft/s (1.5 m/s) to keep them clean.
The wide range of flows in combined sewers requires the solution of certain special
problems, among them choice of a cross-section that will ensure self-cleaning velocities
for both storm and dry-weather flows; design of self-cleaning inverted siphons—also
called sag pipes and depressed sewers—dipping beneath the hydraulic grade line as they
carry wastewater across a depression or under an obstruction; and provision of stormwater
overflows in intercepting systems.

14.4.1 Cross-Sections
Departures from circular cross-sections are prompted by hydraulic as well as structural and
economic considerations. Examples are the egg-shaped sections and cunettes illustrated in
Fig. 14.10. Two circular sewers, an underlying sanitary sewer, and an overlying storm
drain are fused into a single egg-shaped section. The resulting hydraulic radius is nearly
constant at all depths. Cunettes form troughs dimensioned to the dry-weather flow.
Rectangular sections are easy to construct and make for economical trenching with low
head-room requirements. Horseshoe sections are structurally very satisfactory; egg-shaped
sections are not. Large outfall sewers have been built as pressure tunnels.

14.4.2 Inverted Siphons

Siphons flow full and under pressure, and the velocities in them are relatively much more
variable than in open channels, where depth and cross-section change simultaneously with
flow. To keep velocities up and clogging by sediments down, two or more parallel pipes
are, therefore, thrown in and out of operation as flows rise and fall. The pipes dispatch
characteristic flows at self-cleaning velocities of 3 ft/s (0.90 m/s) for pipes carrying sani-
tary sewage and 5 ft/s (1.5 m/s) for pipes conveying storm or combined sewage. The small-
est pipe diameter is 6 in. (150 mm), and the choice of pipe material is adjusted to the
hydrostatic head under which it must operate.
Figure 14.11 shows a simple example: low dry-weather wastewater flows are passed
through the central siphon; high dry-weather flows and storm flows spill over weirs into lat-
eral siphons to right and left. The three siphons combine to equal the capacity of the approach
sewer. Weir heights are fixed at depths reached by characteristic flows in the approach sewer
and inlet structure. Flows are reunited in a chamber in advance of the outlet sewer.