WATER SOURCE SELECTION
(3) Turbidity and silt density index. These two
General. The selection of a water supply will be
characteristics provide two different measures of the
based on available quantity, quality, and cost of
amount of fine particulate matter in the water. Turbidity
development. Investigate usable fresh surface water
is measured in nephelometric turbidity units (a measure
of the amount of light scattered by a known water
sources requiring desalination.
When fresh water
sample thickness). Silt density index is a measure of
sources do not exist, consider saline water sources. The
the amount of 0.45-micron filter plugging caused by
most commonly used parameter to differentiate between
passing a sample of water through the filter for 15
saline water qualities is total dissolved solids (TDS).
Turbidity must be determined for all
The total dissolved solids is defined as the sum of the
desalination processes. Also, the silt density index must
dissolved organic materials and the inorganic salts.
be determined for water being considered for reverse
Fresh waters contain less than 1,000 milligrams per liter
of total dissolved solids. Brackish water contain 1,000-
20,000 milligrams per liter of total dissolved solids. Sea
constituents of the raw water must be determined to
water usually contains at least 20,000 milligrams per
provide information for treatment selection. Appendix B
liter of total dissolved solids. Quantities of potable water
shows the water testing analyses required for
needed will be determined by an analysis of the site in
accordance with TM 5-813-1. The quantity of saline
feed water necessary to produce the required quantity of
c. Bacteriological quality.
product water is addressed in Chapter 4.
testing of the raw water must include a type of a
coliform indicator organism count. Procedures for filter
Quality. The quality will be determined by the
and standard plate count, coliform organism
planned use. Physical, chemical, and bacteriological
bacteriological testing techniques can be found in
testing of source waters are required to determine the
Standard Methods for the Examination of Water and
level of treatment to supply the necessary water quality.
Wastewater and TB Med 576.
When the quantity withdrawn exceeds the recharge rate,
recommendations as to the media and procedures used
quality inherently decreases; therefore, this must be
to identify microbiological activity detrimental to the
considered during design.
operation of a particular desalination system shall be
a. Physical characteristics.
characteristics of the raw water source that must be
evaluated are total suspended solids (TSS) and
Selection versus rejection of potential raw
water sources. After the completion of physical,
temperature. Turbidity and silt density index (SDI).
chemical, and bacteriological testing, a final water
(1) Total suspended solids.
source may be selected. Extreme care must be taken in
suspended solids level of raw water sources must be
the selection of a source where the usage rate is greater
evaluated to determine the level of pretreatment
than the recharge rate. In most cases, selection will
Raw water having low total
involve choosing the brackish water with the lowest level
suspended solids levels generally requires less
of total dissolved solids. When brackish water is not
The source with the lowest total
available, use sea water or water as saline as sea water
suspended solids is preferred.
as the feed water source. When the coliform indicator
(2) Temperature. The temperature of the raw
organism count of a water is greater than 10,000 most
water source must be matched to the specific
probable number (MPN), then the water source should
desalination process. In extreme cases, the water
be rejected for sanitary reasons and a more saline water
temperature may control the desalination process
chosen (per Standard Methods for the Examination of
selection. A climatological survey must be made prior
Water and Wastewater and TB MED 576). If other
to finalization of process selection to determine the
water is available, a water containing more than 1,000
seasonal maximum and minimum water temperatures of
nephelometric turbidity units should be rejected on the
the proposed water sources.