Page 601 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

556 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

p(pure water side) 0, we can simplify the equation, that is, The anode cell and the cathode cell are special cases. What
Dp¼ p(salt side) p(pure water side) ¼ p(salt side) 0. happens at each electrode is depicted also in Figure 17.17. As
Then simply let Dp ¼ p(salt side) ¼ p in order to simplify seen, an oxidation reaction occurs at the anode, that is, Cl
the nomenclature. loses electrons to form chlorine gas. Also water is dissociated
to form H and O 2 with a loss of electrons. The anode is a sink
þ
17.3.7 ELECTRODIALYSIS for these electrons. At the cathode, a reduction reaction occurs;
the cathode gives off electrons producing OH and H 2 gas.

The term, dialysis is the transfer of solute molecules across a
membrane by diffusion from a concentrated solution to a dilute 17.3.7.1 Applications
solution. The term electrodialysis refers to the transfer of ions Most ED systems are for desalinization of brackish waters. More
across membranes due to the influence of an electric field than 1000 ED plants have existed around the world (Lonsdale,
(Helferrich, 1962, p. 397). An electric field is applied between 1982), with 310 having capacities more than 95 m =day (25,000
3
two electrodes to mobilize ions to move in the direction of the gpd). The combined desalting capacity was about 272,000
oppositely charged plate across ion-selective membranes. The m =day (72 mgd). The ED technology has competed with RO
3
basic innovation goes back to the 1930s (Lonsdale, 1982), when for brackish water desalinization since the first plant in 1969
desalting was demonstrated in a three-compartment cell. The (Reahl, 2006, p. 1). Initially, c. 1969, ED was the only viable
multicompartment cell, the configuration of current technology, nondistillation type desalination technology. The 2008 share of
came in 1940 and is illustrated in Figure 17.17. As seen, the worldwide capacity was about 0.04 fraction (Section 17.1.2),
positive electrode plate is on the left and the negative plate is on having being reduced proportionately as RO membranes
the right. Therefore, the anions (represented by Cl ) are attracted became cheaper.

toward the left plate and the cations (represented by Na ) toward
þ
the right plate. The membranes are selective, passing anions or
17.3.8 FOULING
cations, respectively. For the system depicted, the center cell
becomes depleted of both anions and cations, and desalting A reduction in membrane flux density due to foreign material is
occurs. ED has been applied most often to desalination of called fouling. Almost all substances in water have the potential
brackish waters. to foul membranes and include (1) particles, (2) organic com-
Referring to Figure 17.17 again, a ‘‘cell-pair’’ is comprised of pounds, (3) inorganic compounds, and (4) biota that grow on the
a concentrating cell and an ion depleting cell; the cell includes membrane surface. Particles foul membranes by either collect-
‘‘spacers’’ that separate the membranes and permit flow to occur. ing inside membrane pores or by blocking pores due to surface
In practice, a membrane ‘‘stack’’ is composed of some several deposition. Organics, inorganics, and biota may adsorb to mem-
hundred cell pairs (Meller, 1984, p. 13). The system must be set brane surfaces and pores. The nature and extent of fouling is
up with a manifold system that collects the demineralized water influenced by the chemical and biological nature of the water,
and the saline water in separate flows; the anode and cathode the chemical composition of the membrane, solute–solute type
streams are minor and are waste also. interactions, and membrane–solute type interactions.

+ –
+ Cl 2 O 2 – H –
Cl – 2
+ Cl –
+ + Na + + – –
+ H Na (OH ) –
+ –
+ –
+ –
Cl – Cl –
+ –
+ Na + Na + –
+ –
+ Cation Anion Cation Anion –
+ selective selective selective selective –
+ membrane membrane membrane membrane –
+ –
Anode
Cathode
oxidation reduction
2Cl – Cl 2 +2e – Ion Ion Ion 2H O+ 2e –
2
–
2H O concentrating depleting concentrating 2(OH )+ H 2
2
+
4H +O 2 +4e – cell cell cell
FIGURE 17.17 Electrodialysis stack of five cells. (Adapted from Lonsdale, H.K., J. Membr. Sci., 10, 81, 1982.)