October 2012
7-5
7.2
Water Purification
the temperature and pH of the water; and the amount of time the contaminant is
exposed to the activated carbon.
5
Carbon filters can use either disposable cartridges
or packed columns. Disposable cartridges are disposed of once the adsorptive capac-
ity is exhausted. Alternatively, packed columns can be removed and regenerated off
site.
6
Water use is required to regenerate the columns; however, since regeneration is
typically done off site, no water is used at the facility level.
Deionization
Deionization is a physical process similar to water softening that exchanges cations
and anions present in the untreated water with hydrogen and hydroxide ions. Deion-
ization is not effective at removing particulates, but because the process is relatively
fast, it is commonly used in laboratory applications requiring a low level of water
purification. Regeneration of deionization resins often occurs off site.
7
Water use is
required to regenerate the resin; however, since the regeneration is done off site, no
water is used at the facility level.
Distillation
Distillation functions by boiling water to form steam condensate using either an
electric or gas still. Solid contaminants are left behind as the steam is generated, then
the steam is condensed into a purified water stream. Distillers can use large volumes
of water if once-through cooling water is used in the condenser, or if a reject stream
is discharged from the boiler to prevent scale buildup. These systems typically reject
15
to 25 percent of water entering the system.
8
Membrane Processes (Including Reverse Osmosis)
Membrane processes use a semi-permeable mem-
brane layer to separate purified water from con-
taminants. Several types of membranes are used
for water purification, including (from largest to
smallest size of particles removed) microfiltration,
ultrafiltration, nanofiltration, and reverse osmosis.
Because reverse osmosis is capable of removing
the smallest particles, it is used most often by
laboratory and medical facilities requiring very
pure water.
Reverse osmosis units use pressure to reverse os-
motic pressure and force water with a high solute
concentration through a membrane filter to create
purified (i.e., low solute) water. Reverse osmosis is
5
University of Minnesota | Extension. 1992. Treatment Systems for HouseholdWater Supplies: Activated Carbon Filtration (Clean Water Series).
6
East Bay Municipal Utility District (EBMUD). 2008.
WaterSmart Guidebook—A Water-Use Efficiency Plan Review Guide for New Businesses
.
Pages TREAT1-6.
7
Water Online. Deionization.
8
EBMUD,
op. cit
.
Reverse osmosis system
