Page 175 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

WATER TREATMENT AND PURIFICATION 4.11

be repeated using a shorter overall elapsed time. A Millipore filter is the only membrane
currently approved by the ASTM for determining the SDI. The higher the SDI, the greater
potential for fouling.
Many manufacturers of RO cartridges recommend allowable SDI figures for feedwater. For
hollow-fiber modules the maximum SDI is 3, and for spiral-wound modules the maximum
SDI is 4. For continuous deionization, an SDI of 4 or less is recommended. In practice,
when water has an SDI greater than 4, a 4-μm depth prefilter is recommended. In addi-
tion to the 4-μm filter, an additional l-μm filter is recommended downstream. The use of a
4-μm filter in the feedwater stream is always recommended as a precaution against fouling
regardless of the potential SDI.

DEPOSITS AND CORROSION

The contaminants previously discussed will cause piping system fouling by depositing
materials on the walls of the pipe, thereby reducing the efficiency of the system and reduc-
ing the thickness of the pipe wall by corrosion, which will eventually cause failure of the
piping system. Following is a brief discussion of these categories and treatment methods
as they apply to most systems.

DEPOSITS

Scale and Sludge
Scale is a solid deposit on the walls of a pipe resulting from precipitation of dissolved solids in
the fluid stream. Scale reduces heat transfer and interferes with the flow of water by increas-
ing the friction of the fluid against the walls of the pipe. Boiler scale consists of calcium,
magnesium, iron, and silica minerals. It is prevented by pretreating water before it enters the
boiler to remove much of the scale-forming ingredients or by adding effective chemicals to
feedwater to adjust pH, prevent corrosion, and prevent deposits from occurring.
Sludge is a sticky and adherent deposit in the feedwater, which results from the settling
out of suspended matter from several sources. One source is an excess of iron, generally
iron oxide (rust) and iron carbonate (a corrosion product). Other sources are mud, dirt,
and clay that tend to collect and adhere in areas of low circulation. Sludge is prevented
by filtering the incoming feedwater and adding chemical dispersants to keep the solids in
suspension. They are removed by blow-down. Mud, dirt, and clay are rarely encountered
except where the feedwater comes from surface sources.
Condenser scale deposits consist of calcium carbonate, calcium sulfate, or silica miner-
als when their concentrations exceed their solubility or their pH exceeds saturation, causing
these minerals to come out of solution. This is prevented by controlling the pH, diluting
the circulating water to prevent concentration, and adding chemicals to inhibit and prevent
scale formation.

Biological Fouling

Microbiological fouling is caused by the growth of bacteria, algae, fungi, and other organ-
isms. Their growth is helped by a favorable water temperature, favorable pH, and the pres-
ence of oxygen and food. Slime is the buildup of microbes and their waste products and
also dust and other suspended matter.

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