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340 Chapter 10 Introduction to Wastewater Systems
Manholes are channeled to improve flow, and the entrance of high-lying laterals is
eased by constructing drop manholes rather than lowering the last length of run, a wasteful
arrangement. In their upper reaches, most sewers receive so little flow that they are not self-
cleaning and must be flushed from time to time. This is done by (a) damming up the flow at
a lower manhole and releasing the stored waters after the sewer has almost filled; (b) sud-
denly pouring a large amount of water into an upstream manhole; (c) providing at the upper-
most end of the line a flushing manhole that can be filled with water through fire hose
attached to a nearby hydrant before a flap valve, shear gate, or similar quick-opening device
leading to the sewer is opened; and (d) installing an automatic flush tank that fills slowly
and discharges suddenly. Apart from cost and difficulties of maintenance, the danger of
backflow from the sewer into the water supply is a bad feature of automatic flush tanks.

EXAMPLE 10.1 CAPACITY OF SEWER
An 8-in. (200-mm) sewer is to flow full at a velocity of 2.5 ft/s (0.75 m/s).
3
3
1. What is its maximum capacity in ft /s and MGD using the U.S. Customary Units or m /s
and MLD using the SI Units?
2. How many people can it serve if the maximum per capita flow is 300 gpd (1,135 L/day)?
3. How many acres (or ha) will it drain if the population density is 50 per acre (124/ha)?

Solution 1 (U.S. Customary System):
1. Capacity Area Velocity
2
3
[( 8 )>(4 144)] 2.5 ft /s
3
0.87 ft /s
0.87 60 60 24 7.48 gpd
562,000 gpd.
0.562 MGD
2. People served 562,000>300 1,870.
3. Area drained 1,870>50 37.4 acres.

Solution 2 (SI System):
1. Capacity Area Velocity
2
3
[( 0.20 )>4] 0.75 m /s
3
0.0246 m /s
0.0246 1,000 1,440 60>1000,000 MLD
2.13 MLD
6
2. People served 2.13 10 >1,135 1,870
3. Area drained 1,870>124 15.1 ha

10.4 COLLECTION OF STORMWATER
Much of the suspended load of solids entering storm drains is sand and gravel. Because
fine sand is moved along at velocities of 1 ft/s (0.30 m/s) or more and gravel at 2 ft/s
(0.60 m/s) or more, recommended minimum velocities are 2.5 to 3 ft/s (0.75 to 0.90 m/s),