Environmental Engineering Reference
In-Depth Information
P
for a given salt solution of resistance
R
varies with the square of the current,
I
, lowing
through the system. To move the ions across the solution and membranes, a DC potential
at the electrodes has to be added to move ions through the solution,
η
solution
, across the
membrane, η
membranes
, and overcome the overpotentials created owing to Faradaic losses at
the electrodes (i.e., the transformation of electrons in the metal to ions in solution, usually
via the electrolysis of water), η
overpotential
. The power consumed can be expressed as
P = I
(η
solution
+
η
membranes
+
η
overpotential
) .
( 2 7.1)
The potentials across the solution and membranes (to the irst order) are simply pro-
portional to the current, or η
solution
=
C
J
I.
However, the overpotentials at the electrodes can
grow exponentially with the current, such that η
overpotential
=
C
F
I
(
n +
1)
,
where
n
≥ 1 and
C
F
>>
C
J
,
for concentrated salts. Therefore, the power consumption for desalination by electrodi-
alysis can increase rapidly, such that
P = C
J
I
2
+ C
F
I
(n + 2)
,
( 2 7. 2)
so that when either high luxes are desired or concentrated solutions are being desalted, or
both, the power consumption can greatly exceed both RO and thermal methods. However,
at low luxes and relatively low salt concentrations (<3000 ppm), electrodialysis can be
among the most energy-eficient methods.
Other challenges in electrodialysis are similar to those with all membrane processes and
relate to membrane fouling, membrane response to changing pH conditions when H
+
and
OH
−
ions are transported across membrane surfaces, and concentration polarization [23,38].
One key advantage of the electrodialysis process is its ability to deal with higher levels of
uncharged particles in comparison with RO processes, as these particles do not travel
through the membranes and are not driven to the membrane surface [23]. Electrodialysis
recovery rates for brackish water range from 80% to 90%, while RO recovery rates for
brackish water range from 65% to 75% [39]. Energy consumption of electrodialysis systems
ranges from 4.3 to 9 kJ/l for brackish water [39]. These membranes are also tolerant to up
to 0.5 ppm free chlorine dosing; can operate in a pH range 2-11; and tolerate high tempera-
ture cleaning, mechanical cleaning, and 5% hydrochloric acid cleaning [37].
Owing to the high product recovery, scaling and precipitation of salts (usually calcium
carbonate, calcium sulfate, or barium sulfate is the limiting salt but this depends on the
feedwater) can be a problem for electrodialysis systems [21,40]. To mitigate fouling
and precipitation issues, the EDR process was developed. The EDR process periodically
changes the polarity of the applied biases, allowing the system to be lushed [24]. The
polarity reversal of the EDR process changes the concentrate and product water sides,
making it possible to operate at a supersaturated state and increasing product recovery
without chemical additives such as antiscalants [40]. Avoiding the use of antiscalants is
generally preferable because feedwater must be closely monitored to achieve the correct
dosing. Correct dosing is important because too little additive results in scaling, while
excess doses result in membrane fouling. Hence, dosing must be changed in response to
feedwater quality changes [40]. Finally, since electrodialysis and EDR processing removes
charged particles, post-processing must be in place for potentially harmful electrically
uncharged biological species such as viruses and bacteria.
EDR plants are an economical solution to many niche water problems. For example,
the city of San Diego (California) uses reclaimed tertiary water for irrigation and indus-
trial processing. The reclaimed water was found to be acceptable for non-potable water
