Page 210 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.46 CHAPTER FOUR

Acid injection. Some treatment plants must add acid injection somewhere in the sys-
tem. The purpose of the acid is primary for scale control. It is shown in Fig. 4.16 before the
heat exchanger because calcium carbonate is more likely to scale out when water is hot. If
there is little hardness (calcium, and magnesium ions) or alkalinity (hydroxide, bicarbonate
and carbonate ions) in the feedwater, acid may be eliminated. Another common reason to
use acid is to minimize damage to cellulose acetate membranes if it is a downstream RO
process component. These membranes are easily damaged at any pH level, but at a pH level
of 5.5 to 6, the damage is minimized.
Heat exchange. Feedwater is heated to lower the pumping costs for an RO unit because
the warmer the water, the less pressure is required to pump water through an RO membrane.
Another reason is that warmer water accelerates the rate of diffusion and chemical reac-
tions. Generally speaking, for every 18°F (10°C) rise in temperature, the speed of chemical
reactions doubles.
Softening. A water softener controls scale by removing the hard, scale-forming cat-
ions like calcium and magnesium, and exchanging (replacing) them with nonscale forming
sodium ions.
Activated carbon units. Often called activated carbon beds or activated carbon filters,
these filters remove chlorine or chlorimine compounds from the feedwater to protect both
RO membranes and DI resins from the oxidizing action of the chlorine and chlorimine com-
pounds. A second, less common reason is to remove certain organic compounds. Organic
compounds are molecules that always contain carbon, usually contain hydrogen, and fre-
quently contain other atoms. They are called organic because prior to human intervention
all organic compounds came from living organisms.
Cartridge filtration. This filter is installed upstream of the RO units as an additional
protection against suspended solids. RO membranes will foul if sufficient suspended solids
are not removed by the multimedia filter.
Reverse osmosis. RO is a membrane that removes a bulk of the suspended solids and
contaminants, which is, typically, 98 to 99+ percent of ionic contaminants and 95 to 99+
percent of dissolved TOC. A single pass through the RO will typically not meet PW or WFI
standards. A double pass system, where the processed water from one RO unit is processed
again through another unit, may be allowable. Distillation units are more commonly used
to provide PW and WFI quality water.
Ion exchange. Following an RO unit, it is generally required to further reduce the
contaminant level. This is done typically using a mixed-bed deionization unit. This could
be combined with electrodeionization (EDI), also known as continuous deionization (CDI)
to further polish RO water. The resultant water is not pyrogen free.
Distillation. In order to achieve PW and WFI quality, most treatment plants use dis-
tillation (stills) as a final step. A still heats the feedwater to the boiling point, and the
resulting steam condensate typically meets required standards. There are various types of
stills that require different feedwater quality; therefore, alternative upstream requirements
other than that shown in Fig. 4.16 may be necessary. Distillation produces water that is
pyrogen free.

FEEDWATER

Feedwater quality, which is source dependent, is the first parameter to be identified in the
design of a pure water system. The source strongly influences the pretreatment options and
may dictate the purification methods. There is a wide difference in types and concentra-
tions of various impurities possible, depending on whether the feedwater is obtained from
a public utility or privately, such as from a well or other surface or groundwater source.
Surface waters are usually high in particulates, colloids, and organics. Underground waters
tend to have low levels of particulates, colloids, and organics, and relatively high levels of

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