Simultaneous removal of arsenic and nitrate was studied using a staged-equilibrium ion-exchange model.
Three resins were selected for this modeling study, based on their reported efficiency for the removal of
arsenic and/or nitrate. These resins included: a conventional sulfate-selective (SS) polystyrene Type 2
resin (PS-2); a conventional sulfate-selective polyacrylic Type 1 (PA-1); and, a nitrate-selective
(NS) resin. Input parameters for the ion exchange model to simulate exhaustion and regeneration of these
three resins were obtained from previous studies. Various groundwater qualities, which exceeded arsenic
and/or nitrate maximum contaminant levels (MCLs), were modeled at varying sulfate concentrations.
Based on the modeling study, it was determined that the polystyrene sulfate-selective Type 2 resin
provided longer run lengths for the combined removal of arsenic and nitrate. Resin regeneration was also
modeled. Due to the lack of selectivity reversal for the monovalent nitrate anion, nitrate removal from
both conventional and nitrate-selective resins is more difficult than the removal of arsenic and sulfate.
Both arsenic and sulfate were easily removed using approximately 7 lbs of NaCl/ft<sup>3</sup> resin. When
regenerating with 11 lbs of NaCl/ft<sup>3</sup> resin, nitrate removal was approximately 33 percent
and 86 percent for the nitrate- and sulfate-selective resins, respectively.
The use of partial regeneration for the removal of nitrate using conventional resins results in a significant
reduction in salt consumption with a minimal impact on nitrate run length. Similar salt savings were also
obtained using nitrate-selective resins. However, this came at the cost of a significant reduction in nitrate
run length. Note that the use of nitrate-selective resins may be advantageous when considering denitrified
brine reuse with high sulfate (> 120 mg/L) waters because nitrate run lengths are not adversely affected
by sulfate build-up in the reused brine.
When considering multi-contaminant removal, the overall run length is controlled by the contaminant that
first exceeds the MCL. This is true even when this contaminant is present below the MCL in the feed
water due to the phenomenon of chromatographic peaking, where the lesser-preferred contaminants
exhibit effluent concentrations higher than the influent concentration. The adverse impact of such
chromatographic peaking can be mitigated by the use of multiple parallel columns operating in a
staggered fashion. Such parallel staggered operation results in an attenuation of extreme water quality
changes in the ion exchange effluent and leads to a more uniform product water quality. Includes 13 references, tables, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 290 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 20 |
| Published : | 11/01/2007 |
