Page 382 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

JWCL344_ch10_333-356.qxd 8/2/10 9:02 PM Page 342

342 Chapter 10 Introduction to Wastewater Systems
of street inlets govern the degree of freedom from flooding of traffic ways and pedestrian
crossings. To permit inspection and cleaning, it is preferable to discharge street inlets di-
rectly into manholes. Catch basins are, in a sense, enlarged and trapped street inlets in which
debris and heavy solids are held back or settle out. Historically, they antedate street inlets
and were devised to protect combined 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 production of large crops of mosquitoes. On the whole,
there is little reason for continuing their use in modern sewerage systems.

EXAMPLE 10.2 SIZE OF STORM DRAIN
A storm sewer needs to drain an area of 37.4 acres or 15.1 ha (the area drained by an 8-in. or 200-mm
sanitary sewer in Example 10.1). How large must this drain be if it is to carry away rain falling at a
rate of 2 in./h (51 mm/h) during 30 min, the time needed for the entire drainage area to become
tributary to the sewer? The required velocity of flow is to be 3 ft/s (0.9 m/s), and the ratio of the
peak rate of runoff to the rate of rainfall on the area is assumed to be 0.6.

Solution 1 (U.S. Customary System):
3
3
2
1. 1 acre-in./h 43,560 ft 1>12 ft>(60 60) 1.0083 ft /s 1.0 ft /s closely enough.
This is a fact to remember.
3
2. Rate of runoff 2 in./h 0.6 37.4 acre 45 acre-in./h 45 ft /s.
3. Cross-sectional area of drain flow/velocity 45>3.0 15 ft 2
4. Diameter of drain = 1224 * 15>p = 53 in.
5. Ratio of storm runoff to sanitary wastewater (Example 10.1): 45.0:0.87 52:1; that is,
sanitary wastewater, if admitted, would constitute less than 2% of the combined flow.
Solution 2 (SI System):
3
2
1. 1 ha-mm/h [10,000 m 1>1,000 m>(60 min 60 s)] m /s 1,000 2.78 L/s. This is
a fact ro remember.
3
2. Rate of runoff (51 mm/h 15.1 ha) mm-ha/h 2.78 0.6/1,000 1.25 m /s
3. Cross-sectional are of drain flow/velocity 1.25/0.90 1.39 m 2
4. Diameter of drain = 24 * 1.39>p 1.33 m. Use 1,400 mm
5. Ratio of storm runoff to sanitary wastewater (Example 10.1) 1.25: 0.0246 51:1; that
is, sanitary wastewater, if admitted, would constitute less than 2% of the combined
flow.

10.5 COLLECTION OF COMBINED WASTEWATER
In combined sewerage systems, stormwaters often exceed sanitary wastewater by 50 to
100 times (Example 10.2), and the accuracy with which rates of surface runoff can be esti-
mated is generally less than the difference between rates of stormwater and combined
wastewater flows. Accordingly, most combined sewers are designed to serve principally as
storm drains. Understandably, however, they are placed as deep as sanitary sewers.
Surcharge and overflow of combined sewers are obviously more objectionable than the