Environmental Engineering Reference
In-Depth Information
−
,
HAsO
2−
, and
AsO
3−
with H
3
AsO
3
(arsenious acid) as the primary species in natural water. As
5+
exists in four forms in aque-
ous solution, depending on pH: H
3
AsO
4
,
HAsO
+
, H
3
AsO
3
,
HAsO
As
3+
exists in ive forms:
HAsO
4
3
2
3
−
,
HAsO
2−
, and
AsO
3−
. In raw water under
pH conditions of 6-9, As
5+
exists as an anion while As
3+
is fully protonated and exists as
an uncharged molecule.
5
At the end of the day, there are two basic forms of arsenic that
have to be removed to meet the 10 μg/L arsenic standard, As
3+
and As
5+
, with As
3+
being
more toxic.
Before new arsenic removal technologies were fully commercialized in 2006, state-of-the
art technologies for arsenic removal for small community water supplies were classiied
into several categories as discussed below.
2
4
9.2 Conventional Arsenic Removal Technologies
Conventional water treatment technologies include precipitation, alumina, ion exchange,
membrane iltration, and iron adsorption as follows:
1. Precipitate processes—coagulation/iltration, direct iltration, coagulation assisted
microiltration, enhanced coagulation, lime softening, and enhanced lime softening
2. Adsorptive process using activated alumina
3. Ion-exchange processes using anion exchange
4. Membrane iltration—reverse osmosis and electrodialysis ion exchange
5. Alternative treatment processes using granular ferric hydroxide (GFH) iltration
Evidently, these technologies were not meeting the need for small community water
supplies and there are clear reasons why not:
1. Precipitate processes—Requires large treatment and retention tanks for the pre-
cipitation reaction and settling to occur, a full time on-site operator, connection
to the sanitary sewer, and are still left with a hazardous waste for disposal. Most
small community arsenic removal systems do not have any sanitary sewer point
of connection and need a treatment system that is not active treatment but passive
as there is no requirement for an operator.
2. Adsorptive process using activated alumina—This was the irst adsorptive media
to remove both forms of ionized arsenic but the pH has to be adjusted to pH 5.5.
Incoming raw water is usually pH 6-9. Having a pH adjust system is an active
treatment and requires an operator and chemical storage, pH adjustment, and
monitoring of some kind.
3. Ion-exchange (IEX) resin is designed to adsorb an ion on the surface and exchange
it for another ion, in this case H
+
. The issue with IEX is that it requires regenera-
tion at a regulated RCRA (
Resource Conservation and Recovery Act
) Part B facility to
reuse the resin, and what do you do with an acidic arsenic solution? More treat-
ment to dispose of it, and liability still has not been terminated.
4. Membrane iltration depending on the micron rating of the membrane can have
arsenic permeate the membrane still requiring arsenic removal, and the membrane
