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12.30 CHAPTER TWELVE

boxylic groups, which therefore behave as anions. Carboxylic acids are moderately to
weakly ionized. Although they are exchanged as anions, they are kinetically slower and
bulkier than common inorganic ions. Therefore the ion exchange processing equipment
and regenerant delivery systems have to be designed for lower flow rates and longer re-
generant contact times than systems designed for exchanging inorganic ions.
The molecular weight distribution of naturally occurring organics varies widely, usu-
ally between 200 and 80,000 or more, depending on the source, age, and environmental
conditions. Typically over 80% of the naturally occurring substances that cause TOC have
molecular weights under 10,000. They are thus larger than inorganic ions but still small
enough to enter the gel phase of the ion exchange resins. The TOC values are typically
expressed as ppm as carbon. This can be converted to approximate ion exchange con-
centrations as calcium carbonate equivalents by multiplying the TOC values, ppm as car-
bon, by 1.5 to get approximate ppm as calcium carbonate.
The lower cross-linked, more highly porous "standard" gel resins designed for inor-
ganic exchange are somewhat better then the standard 8% DVB type but may still lack
enough gel-phase porosity to be effective. Special grades of resins, with extraordinary
high gel-phase porosities, are far more successful in this kind of service.

Macroporous versus Gel Resins. Macroporous resins were the first types widely used
for organics because initially it was believed that surface adsorption played a significant
role in the removal process. It has been shown that the best performing macroporous resins
are those that have the highest level of gel-phase porosity. It has also been shown that
gel-phase porosity is the most significant predictor of performance. Since over 95% of
the naturally occurring TOC matter is removed only by ion exchange and not adsorption,
macroporous structures cannot offer advantages unless combined with low cross-linked
gel phase. Activated carbon is effective in removing much of the 5% of naturally occur-
ring TOC substances that are nonionic. For best removal, activated carbon should be used
as a posttreatment, after the water passes through the resin.
The loading capacity and degree of breakthrough during the service cycle are affected
by competing ion ratios especially sulfates to TOC. Once sulfates begin to leak about 50%
of the influent, TOC will appear in the effluent. Therefore the service cycle should be ter-
minated when sulfate leakage increases if complete TOC removal is required. If the ser-
vice cycle is run past the sulfate break, the organics with the highest affinities will con-
tinue to load by displacing those with lower affinities and also sulfates. They will
accumulate on the resin and make it more difficult to regenerate the resin, which could
lead to premature fouling.
The naturally occurring TOC substances have affinities for the resins that are quite
similar to sulfates. Theoretically, they can be regenerated from a strong base resin in the
same type of process, as used for sulfates, i.e., sodium chloride. However, the regenera-
tion process is limited by kinetics. Salt regeneration can be made more effective by us-
ing a 10% concentration of warm brine and adding 1% to 2% sodium hydroxide to it. A
dose level of l0 lb/ft 3 applied over a 2-h contact time at 70 ° F is the minimum recom-
mended. Shorter contact times will produce proportionately lower organic removal; longer
contact times will add only about 3% per hour to the 2-h organic removal amount. The
TOC operating capacity is calculated based on sulfates and TOC as the exchanging ions.

DEMINERALIZA TION

Demineralization, which is also called deionization, is the name given to the removal of
ionic substances. It is the result of exchanging all the positively charged ions for hydro-