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12.16 CHAPTER TWELVE

There have been recent changes regarding arsenic levels. The proposed maximum
contamination level (MCL) for arsenic has been lowered to 10 ppb, from the current 50-
ppb acceptable level. The effective date for compliance with the rule is January 2006.
From the time the MCL review discussions began in earnest in the late 1990s, many ef-
forts to develop the product technology to meet the new standard economically have been
undertaken. Reports of new media for removal of arsenic have appeared and continue to
appear frequently in technical journals. Until recently, activated alumina (AA) and gran-
ular ferric hydroxide (GFH) had been identified as the two most likely adsorbents to be
employed for arsenic removal. However, other media have been developed with promise
of improved performance. Iron oxide-based media and most recently titanium oxide-
based media are showing what appears to be remarkable performance in producing very
low levels of arsenic at low operating costs. These media appear to be designed for one-
time use, are able to treat in excess of 10,000 and even 20,000 bed volumes before ex-
haustion, and can operate over a wide pH range. In some cases the new materials may
be able to remove both the pentavalent (arsenate) and more difficult to remove trivalent
(arsenite) forms of arsenic. Recent reports suggest that some of the inorganic-based me-
dia are able to pass the leach tests required for nonhazardous disposal once they become
exhausted.
Development work is ongoing and at a rapid pace. It is prudent to conduct literature
searches at the start of any new project as technology discussed here will likely have been
improved on or even made obsolete. Review what's available in technical approaches
when the time comes to select the technical approach for a new or upgraded water sys-
tem. For existing facilities, chemical addition, clarification, and filtration may already be
in place, requiring only feed rate and composition changes to meet the new requirements.
Conventional treatment will most likely remain the treatment method of choice for sur-
face water supplies.
The soluble forms of arsenic can be removed by strongly basic ion exchange resins,
activated alumina, or other adsorbent media. In either case, the process works best when
arsenic is present in the arsenate form. Proper pretreatment and testing should be em-
ployed to ensure that all the arsenic is oxidized to the arsenate form.
In oxygen-deprived waters, such as in well water, the arsenite species is usually 20%
to 50% of the total arsenic. Arsenite is a weak acid, and at pH levels below 9 it is un-
ionized and therefore not readily removed by ion exchange. However, arsenate is a rela-
tively strong acid and fully ionized at a pH levels above 6. Oxidative pretreatment with
chlorine is an effective way of converting arsenite to arsenate. Activated alumina exhibits
as much as an 80-times increase in throughput capacity for arsenate as for arsenite at the
same concentrations on otherwise identical waters.
Strongly basic anion exchange resins have the advantage over activated alumina of be-
ing regenerable in a single step and with common salt in much the same manner as a soft-
ener. Activated alumina needs to be regenerated in two steps, first with an alkali such as
sodium hydroxide and then the bed is neutralized with an acid rinse. However, anion resins
have a disturbing tendency to dump arsenic in preference for sulfate and have recently
fallen out of favor for small unmonitored systems.
The presence of iron or high levels of sulfate will interfere with and impair the re-
moval efficiency of arsenic in any form, as the sulfate ion is removed along with the ar-
senic and therefore increases the exchange load. Iron can be used to remove arsenic by
coprecipitation and may be used as a first-stage process with second-stage polishing via
ion exchange. Any remaining iron from the first stage can physically foul the polishing
media. Throughput capacities and resin volume calculations should include sulfates as ex-
changeable ions.
Arsenic is not detectable by normal human senses; toxic levels could exist without
warning. It is important that the treated effluent be properly monitored to ensure that ar-