Page 42 - Fair, Geyer, and Okun's Water and wastewater engineering : water supply and wastewater removal

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1.3 Sources of Water Supply 5
1.3 SOURCES OF WATER SUPPLY

The source of water commonly determines the nature of the collection, purification, trans-
mission, and distribution works. Common sources of freshwater and their development are
as follows:
1. Rainwater:
(a) From roofs, stored in cisterns, for small individual supplies.
(b) From larger, prepared watersheds, or catches, stored in reservoirs, for large
communal supplies.
2. Surface water:
(a) From streams, natural ponds, and lakes of sufficient size, by continuous draft.
(b) From streams with adequate flood flows, by intermittent, seasonal, or selective
draft of clean floodwaters, and their storage in reservoirs adjacent to the
streams, or otherwise readily accessible from them.
(c) From streams with low dry-weather flows but sufficient annual discharge, by
continuous draft through storage of necessary flows in excess of daily use in
one or more reservoirs impounded by dams thrown across the stream valleys.
(d) From brackish and seawater by desalination. Desalination is an artificial
process by which saline water is converted to freshwater. The most common
desalination processes are distillation and reverse osmosis. Desalination is
currently expensive compared to most alternative sources of water, and only a
small fraction of total human use is satisfied by desalination. It is only eco-
nomically practical for high-valued uses (such as household and industrial
uses) in arid areas. The most extensive use is in the Persian (Arabian) Gulf.
Mildly saline waters (brackish) are desalted most economically by reverse os-
mosis; strongly saline waters by evaporation and condensation.
3. Groundwater:
(a) From natural springs.
(b) From wells.
(c) From infiltration galleries, basins, or cribs.
(d) From wells, galleries, and, possibly, springs, with flows augmented from some
other source (i) spread on the surface of the gathering ground, (ii) carried into
charging basins or ditches, or (iii) led into diffusion galleries or wells.
(e) From wells or galleries with flows maintained by returning to the ground the
water previously withdrawn from the same aquifer for cooling or similar
purposes.
Several schemes have been proposed to make use of icebergs as a water source; to date,
however, this has only been done for novelty purposes. One of the serious moves toward the
practical use of icebergs is the formation of an Arabian-American investment group to search
for the optimal way to transport and melt icebergs for use as a source of drinking water sup-
ply without the need for on-land storage. Glacier runoff is considered to be surface water.
An iceberg is a large piece of freshwater ice that has broken off from a snow-formed
glacier or ice shelf and is floating in open water. Because the density of pure ice is about
3
3
920 kg/m , and that of sea water about 1,025 kg/m , typically only one-tenth of the volume of
an iceberg is above water. The shape of the rest of the iceberg under the water can be difficult
to surmise from looking at what is visible above the surface. Icebergs generally range from 1
to 75 m (about 3 to 250 ft) above sea level and weigh 100,000 to 200,000 metric tonne (about
110,000 to 220,000 short ton). The tallest known iceberg in the North Atlantic was 168 m
(about 551 ft) above sea level, making it the height of a 55-story building. Despite their size,