Page 174 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.10 CHAPTER FOUR

comparator tube determination using formazin, called the formazin turbidity unit (FTU),
and the original test, the Jackson turbidity unit (JTU), named for the man who developed
the standard candle used to compare the color of the candlelight through the sample to a
color standard. The most accurate method of measuring solids is the gravimetric method, in
which a known quantity of water is evaporated and the resulting solids weighed.
The most effective devices for removing turbidity are filters and strainers. The equip-
ment chosen to accomplish this task depends for the most part on the size and type of solids
to be retained. Other factors include materials composing the device, the nature of the raw
water, flow rate requirements, particle removal target, initial and operating costs, and main-
tenance requirements. Filters and strainers are discussed in Chap. 3.

Total Dissolved Solids (TDS)
Often referred to as dissolved inorganics and mineral salts, TDS is the sum of all the dis-
solved minerals, including chlorides, sulfates, and carbonates. Dissolved solids contribute
to scale deposit and corrosion of piping and equipment. When dissolved in water, mineral
salts form positively charged ions, mostly sodium and calcium, and negatively charged
ions, mostly chlorides and sulfates.

Total Organic Carbon (TOC)
TOC is a measurement of the carbon level in water and is widely used to determine the level
of contamination of water by organic compounds. These compounds contribute to corro-
sion, cause problems in manufacturing, and usually indicate the presence of endotoxins in
water for pharmaceutical use.
The measurement is generally complicated and dependent on the expected level. For
high levels, the organic compound is first converted to carbon dioxide which is then mea-
sured by infrared absorption. Gas stripping is required to remove other forms of carbon ions
from dissolved mineral compounds. For PPB levels, photolytic oxidation is used and the
resultant carbon dioxide measured.

Silt Density Index (SDI)
The SDI is a measure of the fouling potential of a feedwater source. Since colloids and other
solids can be any size in the submicrometer range, there is no direct method to measure their
concentration in feedwater. The SDI is found by passing the feedwater through a 0.45-μm
rated Millipore filter at 30 psi (207 kPa), and then applying the following formula:

t
1 − 1 ×100
SDI = t 2 (4.1)
T

where t = initial time, in seconds, needed to collect 500-mL sample of water through
1
fresh 0.45-μm filter, 47 mm in diameter
t = time in seconds to filter and collect second 500-mL sample after exposing
2
same filter as above for 15 min to flow of feedwater
T = total test time, in minutes, typically 15 min; for high SDI, T may be less
To obtain an accurate test, the filter should not have become more than approximately
74 percent plugged by the end of the elapsed time. If this figure is exceeded, the test should

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