control. If this is feasible, suspended solids control will
To control corrosion of reverse osmosis product water,
be a post-treatment process at the electrodialysis-
add sodium silicate or sodium hexametaphosphate.
reversal facility. When it is impossible to centralize all
Corrosion control is affected by recalcification and
turbidity control, and electrodialysis reversal is still
Langelier Corrosion Index adjustment.
shown to be economical, both pretreatment and post-
treatment suspended solids control shall be designed.
Occasionally, remineralization of reverse osmosis
At high-current densities, electrodialysis reversal will
product water is not necessary. Since reverse osmosis
soften water by the selective removal of multivalent
increases the monovalent-to-divalent ion ratio,
ions. At low-current densities, electrodialysis reversal
can be used to selectively remove monovalent ions.
remineralization, will contribute to the production of a
This selective removal of monovalent ions at low-
pleasant tasting and healthful product water.
current densities can be economical for fluoride or other
c. Residual disinfection.
The use of calcium
exotic ion removal or control where total dissolved
solids and other ions are not a problem.
d. Reblending. Reblending of pretreated saline
electrodialysis-reversal product water is seldom
water is an acceptable remineralization procedure for
necessary. When recalcification is deemed desirable, it
reverse osmosis product water as long as the hardness
is generally best to make provisions for adding calcium
of the reblend can be shown to be healthful. All
reblended water must receive a full 30-minute chlorine
c. Residual disinfection. Disinfection chlorination
of electrodialysis-reversal product water should occur as
Post-treatment of electrodialysis-reversal
Electrodialysis reversal does not
desalination. Electrodialysis reversal will remove most
remove small suspended material. The pretreatment for
of the free available chlorine used in any pretreatment
electrodialysis reversal should remove any material that
chlorination. Electrodialysis reversal can lead to a
will plug a 10-micron filter.
Loosened scale and
greater concentration of bacteria in the product water
particulate matter may require postdesalination removal.
than was present in the feed stream due to osmotic loss
Micro-organisms are not removed by electrodialysis
of water to the waste brine stream. Chlorination of all
electrodialysis reversal waters will include a 30-minute
a. Suspended solids removal. Suspended solids
chlorine contact time after treatment. Pretreatment
removal during pretreatment is the preferred design for
chlorination will not be included as part of the 30-minute
contact time. The operation and design of electrolytic
suspended solids removes particulates, including micro-
hypochlorite production equipment, as listed in Chapter
organisms, which are prone to blind electrodialysis-
10, is similar to the electrodialysis process. This can
reversal membranes. This removal reduces the time
make electrolytic production an operational and strategic
advantage for electrodialysis-reversal desalination
product water turbidity cannot be controlled
economically by pretreatment, then an attempt will be
made to eliminate all pretreatment suspended solids