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12.26 CHAPTER TWELVE

Selenium
The MCL for selenium is 0.01 mg/L. When selenium is found in potable water, it is usu-
ally as a divalent anion. The selenate ion (SeO42-, Se 6+) is more highly preferred than
the selenite form (SeO32-, Se n+) by strongly basic anion exchange resins. The operating
capacity of a strongly basic anion exchange resin will be greatest when all the selenium
is present as selenate, SeO4. The selenate ion is more preferred than any of the ions com-
monly found in potable water including sulfate, whereas selenite ion is poorly exchanged
and is less preferred than sulfates.
When a resin loaded with selenite is exhausted, the sulfate will continue to be loaded
on the resin and displace previously loaded selenite ions. As this happens, the concentra-
tion of selenite in the effluent can approach the sum of the sulfate plus selenite ions in
the raw water. The sulfate level in the raw water is usually many times higher than the
allowable limit of selenium. Therefore, the danger exists of selenium dumping and ap-
pearing in the effluent at levels much higher than in the influent, if the resin bed is over-
run. Dumping does not occur if the selenium is all converted to the selenate ion prior to
the ion exchange vessel because selenate is preferred over sulfate.
Oxidation of selenites to selenates by chlorine occurs quite readily in the pH range of
6.5 to 7.5. A retention time of 5 min will ensure that more than 70% of the Se 4+ is con-
verted to Se 6+ when the free chlorine level is maintained at 5 mg/L or more. Free chlo-
rine is a much more effective agent than either potassium permanganate or hydrogen per-
oxide. Over 99% of the selenite can be converted to the selenate in 15 min with a 5-ppm
free chlorine residual. But a 2-ppm chlorine residual takes 4 times as long. Both pH and
the chlorine residual level must be controlled to maintain stable and effective operation.
For example, by letting the pH rise to 8.3 and the chlorine residual drop to only 1 ppm,
only 80% of the selenite may be converted to selenate in 30 min. Each installation should
be evaluated on its own to determine the necessary parameters for proper chlorination.
Even though the selenate ion has a higher affinity for strongly basic resins than sul-
fates, their relative affinities are sufficiently close that when the sulfate breaks through,
it causes an increase in the selenate level in the effluent. Selenates will begin to rise grad-
ually once sulfate breaks through, usually within 10% of the throughput at which sulfate
breakthrough occurs. The effluent concentration of selenium will soon rise above maxi-
mum allowable levels. This could happen without notice unless the effluent is carefully
monitored. For this reason it is considered standard practice to end the service cycle and
regenerate the resin at or before the sulfate breakthrough.
Operating Capacities and Parameters. Since selenium is only present in trace quanti-
ties, its concentration alone will have little effect on the resin's throughput capacity. It is
prudent that any anion exchange system designed for selenium removal be designed to
run as a sulfate removal system to a sulfate leakage endpoint. The system should be sized
on the basis of the water analysis with the highest sulfate concentration. The ion exchange
vessel and regeneration equipment for removing sulfates and selenium are the same as
used for dealkalization, except for the monitoring equipment.
The relative affinities of divalent ions such as sulfates and selenates against monova-
lent chlorides drops at the higher ionic concentration during regeneration. To carry out a
proper regeneration, the anion resin should be regenerated with sodium chloride at con-
centrations of at least 5% at a flow rate that will allow at least 30-min contact time. This
can be achieved with a salt dose of 5 to 10 lb/ft 3 injected at a concentration of at least
5%, to a maximum concentration of 15%. The selenium and sulfate are pushed from the
resin bed. Any residual selenium will be found at the bottom of the bed after the regen-
eration cycle. It will resist leakage until another divalent ion such as sulfate begins to leak
through at the end of the next service cycle.