Page 212 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.48 CHAPTER FOUR

pH Adjustment. The selected membrane will have an optimum pH-operating range. When
using drinking water standards, this problem is rarely encountered. In order to achieve it, a
dilute acid is injected into the feedwater if necessary.
Filtration. Filters are used to remove suspended solids originating from any source. If
continuous production is required, a duplex arrangement should be installed so that the
filters can be backwashed or replaced with no interruption in service.
Sand and Multimedia Filters. A common initial method for gross particulate removal
from source water is a pressure multimedia sand filter if suspended solids are greater than
0.2 ppm. If lower, a 5- to 10-μm cartridge filter is often used. Experience has found that
cartridge replacement is not economical as compared to the cost of backwashing.
Cartridge Filters. Used to remove lesser amounts of particulates prior to the central
purification equipment, an in-line 5-μm cartridge depth filter should be installed to elimi-
nate any particulates that would clog or interfere with operation of the central purification
equipment. This filter is recommended if the SDI is less than 4, and is generally required
if the level is more than 4. If the level is more than 4, an additional l-μm cartridge filter is
recommended downstream of the main filter.
Carbon Filtration. Following the removal of gross particulates, a granulated carbon filter
is provided to remove residual disinfectants (chlorine, chlorimine, etc.), dissolved organics
(oils, pesticides, surfactants, etc.), and suspended organics (humic and fulvic acids, etc.).
2
Flow rates through this filter are usually in the range of 1.0 to 4.0 gpm/ft of filter area,
depending on the quantity of organics and chlorimines of the entering water. The higher the
quality, the higher the flow rate. Also the higher the quality of water, the slower the flow rate.
The problem with carbon filtration is its tendency to harbor microbial growth due to the
removal of chlorine. Frequent sanitizing is necessary, usually with potassium permanga-
nate, sodium hydroxide, or steam. If steam is selected, the tank should be constructed of
type 316L stainless steel.
Water Softening. If the hardness is high, it is necessary to provide a water softener to
reduce the calcium and magnesium present to the level required by the membrane selected.
Water softening is recommended if the iron content exceeds 0.4 ppm.
The softener is another device that harbors microbial growth. Sanitizing with potassium
permanganate is generally used.

Biological and TOC Reduction. UV units and ozone generators are generally used to
remove microorganisms and to remove TOC prior to the feedwater reaching the RO units.
Materials used in the system must be compatible with them. There is a reluctance to use
any chemical to remove microorganisms due to FDA restrictions against adding chemicals
to the feedwater. However, the addition of chlorine to disinfect the feedwater is commonly
used because it is cost effective and can be easily removed by GAC filters.
Biocide Removal A far greater source of feedwater for most facilities is potable water.
The water contains a residual amount of chlorine necessary to comply with code for drink-
ing water quality. To remove this residual chlorine, a granulated activated charcoal filter is
provided. An organic trap may also be required if organics are very high.

CENTRAL PURIFICATION EQUIPMENT

The basic methods used to produce high-purity water are distillation, ion exchange, and
reverse osmosis. The method used will depend on the purity desired and limitations on
initial or operating cost.

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