AWWA WQTC65879

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 ofarsenic and/or nitrate. These resins included: a conventional sulfate-selective (SS) polystyrene Type 2resin (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 thesethree resins were obtained from previous studies. Various groundwater qualities, which exceeded arsenicand/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 resinprovided longer run lengths for the combined removal of arsenic and nitrate. Resin regeneration was alsomodeled. Due to the lack of selectivity reversal for the monovalent nitrate anion, nitrate removal fromboth 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/ft³ resin. Whenregenerating with 11 lbs of NaCl/ft³ resin, nitrate removal was approximately 33 percentand 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 significantreduction in salt consumption with a minimal impact on nitrate run length. Similar salt savings were alsoobtained using nitrate-selective resins. However, this came at the cost of a significant reduction in nitraterun length. Note that the use of nitrate-selective resins may be advantageous when considering denitrifiedbrine reuse with high sulfate (> 120 mg/L) waters because nitrate run lengths are not adversely affectedby sulfate build-up in the reused brine.When considering multi-contaminant removal, the overall run length is controlled by the contaminant thatfirst exceeds the MCL. This is true even when this contaminant is present below the MCL in the feedwater due to the phenomenon of chromatographic peaking, where the lesser-preferred contaminantsexhibit effluent concentrations higher than the influent concentration. The adverse impact of suchchromatographic peaking can be mitigated by the use of multiple parallel columns operating in astaggered fashion. Such parallel staggered operation results in an attenuation of extreme water qualitychanges in the ion exchange effluent and leads to a more uniform product water quality. Includes 13 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

11/01/2007

Number of Pages:

20

File Size:

1 file , 290 KB