7-6
October 2012
able to remove a large portion of contaminants but recovers only a portion of the
incoming water. The recovery rate, defined as the ratio of the purified water (i.e.,
permeate) to feed (i.e., incoming) water, is used to depict the efficiency of a reverse
osmosis system. For commercial and institutional applications, reverse osmosis units
typically have recovery rates of 50 to 75 percent.
9
Thus, the systems reject 25 to 50
percent of water entering the system.
Water Softening
Water softening is used to remove hardness minerals, such as calcium and magne-
sium, from water. Cation exchange water softeners are the most common type of wa-
ter softening system, although other water purification technologies, such as reverse
osmosis and distillation systems, can also soften water.
In a cation exchange water softener, hard water with positively charged calcium and
magnesium ions passes through a mineral tank consisting of positively charged sodium
ions attached to a bed of negatively charged resin beads. The calcium and magnesium
ions are exchanged for the sodium ions on the resin beads, which causes the gradual
depletion of available ion exchange sites. Eventually, the water softener must be re-
generated to replenish the softening capacity. The regeneration process uses water to
purge and rinse the system and replenish the sodium ion supply on the resin beads. As
a result, the system generates sodium-rich wastewater that must be disposed.
The frequency of regeneration and the amount of water used by the water softening
process is dictated by the hardness of the incoming water, the rate of water con-
sumption, and the hardness removal capacity of the cation exchange water softener.
The most efficient cation exchange water softeners are demand-initiated, which base
the frequency of regeneration on the incoming water’s hardness or the demand for
softened water rather than a set regeneration schedule.
Other Technologies
Several less common technologies are also used to purify water. Chlorine com-
pounds, ozone, or hydrogen peroxide can be used to chemically disinfect water.
Ultraviolet light, heat, and extreme mechanical sheer can also be used to treat water
with contaminants. These technologies might not require the backwash phase used
by other water purification technologies, but they can require regular cleaning,
which can be water-intensive.
10
Chemical disinfection can use additional water if
chemicals are added in liquid or slurry form.
9
U.S. Environmental Protection Agency (EPA) and U.S. Energy Department (DOE), Energy Efficiency & Renewable Energy (EERE), Federal Energy Management
Program (FEMP). May 2005.
Laboratories for the 21
st
Century: Best Practices, Water Efficiency Guide for Laboratories
.
Page 5.
10
EBMUD,
op. cit
.
7.2
Water Purification
