Page 93 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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56 Chapter 2 Water Sources: Surface Water

Catchment area

Manhole
access
Roof washer

Water hauler’s
filler pipe
Line to house pump
(below frost line)
Screened
intake
Overflow

Figure 2.16 Rainwater Collection

semiarid regions devoid of satisfactory groundwater or surface-water supplies, and (b) to some
hard-water communities in which, because of its softness, roof drainage is employed princi-
pally for household laundry work and general washing purposes, while the public supply sat-
isfies all other requirements. In most hard-water communities, the installation and operation of
municipal water-softening plants can ordinarily be justified economically. Their introduction
is desirable and does away with the need for supplementary rainwater supplies and the associ-
ated objection of their possible cross-connection with the public supply.
For individual homesteads, rainwater running off the roof is led through gutters and
downspouts to a rain barrel or cistern situated on the ground or below it (Fig. 2.16). Barrel
or cistern storage converts the intermittent rainfall into a continuous supply. For municipal
service, roof water may be combined with water collected from sheds or catches on the
surface of ground that is naturally impervious or rendered so by grouting, cementing,
paving, or similar means.
The gross yield of rainwater supplies is proportional to the receiving area and the amount
of precipitation. Some rain, however, is blown off the roof by wind, evaporated, or lost in wet-
ting the collecting area and conduits and in filling depressions or improperly pitched gutters.
Also, the first flush of water contains most of the dust and other undesirable washings from the
catchment surfaces and may have to be wasted. The combined loss is particularly great during
the dry season of the year. A cutoff, switch, or deflector in the downspout permits selection of
the quality of water to be stored. Sand filters (Fig. 2.17) are successfully employed to cleanse
the water and prevent its deterioration (a) by growth of undesirable organisms and (b) by the
bacterial decomposition of organic materials, both of which may give rise to tastes, odors, and
other changes in the attractiveness and palatability of the water.