Groundwater Remediation For Drinking Applications
Chromium 6
Hexavalent chromium is a heavy metal that has been used in many industrial applications and also found naturally occurring throughout the environment. It has been shown to be carcinogenic and found in water supplies across the nation. In 2024 the California State Water Board reinstates 10 ppb MCL for drinking water standard after the challenge ruled by court in 2017. Large water system with more than 10,000 connections will have two years to meet the standard, while smaller water systems will have 3 to 4 years to comply. Weak base anion exchange (WBA) and strong base anion exchange (SBA) are two of the best available technologies. We had worked with City of Glendale CA to investigate two of our WBA and SBA.
The Study Highlight
About
The City of Glendale has tested three WBA resins in the past, which all had a high Cr(VI) capacity but each has challenges. SBA resins were also tested at Glendale in previous studies. All the resins had relatively short operational life before regeneration is required. AquaNano has two new resins under the trademark of Captymer® (one WBA, AQ208-WB and one SBA, AQ60-MP) that are in the development stage and showed promising performance in their bench-scale studies. A third-party and independent pilot study was conducted by Hazen and Sawyer at Glendale to evaluate the two new resins performance for Cr(VI) removal using the same methodologies as used for testing other resins. In addition, the resins were also evaluated for potential chemical leaching and spent resin characteristics for disposal. A complete report can be found at 638524950292730000 (glendaleca.gov)
Set up & Protocol
The two new resins were tested using a pilot skid that has been successfully used for previous resin testing at Glendale. WBA was tested with 2-minute EBCT for six months and then reduced to 1-minute EBCT to maximize bed volumes during the test period. SBA was tested with 2-minute and 4-minute EBCT as a single pass without regenerations. WBA received a stream of Glendale’s GS-3 water with carbon dioxide (adjusted pH to a target of 6.0) that was treated by the existing demonstration-scale WBA process. SBA received GS-3 raw water without pH adjustment. The field testing last for nine months.
WBA Result
Captymer® WBA resin had a high Cr(VI) capacity. Cr(VI) in the resin effluent was < 0.02 ppb until approximately 92,000 BVs. At that time, the EBCT was decreased from 2 minutes to 1 minute to maximize the bed volumes during the test period. The effluent Cr(VI) concentration slightly increased after the EBCT change and fluctuated in the range of <0.02 – 1.5 ppb. At the end of the field testing, Cr(VI) in the WBA resin effluent was 0.36 ppb at approximately 198,600 BVs. Cr(VI) in the 50% port samples was 6.4 ppb at approximately 397,000 BVs. The results suggest initial breakthrough occurred at approximately 98,200 BVs, which was possibly accelerated by the shorter EBCT, considering the slow rate of Cr(VI) removal by WBA in general. Compared with the other two WBA resins (Resin A and Resin B) tested at Glendale during 2012-2013, Captymer® WBA resin did not show an initial Cr(VI) leakage observed with other two resins. The influent Cr(VI) concentration was slightly lower than tested for other resins, although it was the same water source. The impact of influent Cr(VI) level on resin life has not been quantified. However, a Water Research Foundation study suggests the impact of Cr(VI) concentration on WBA resin life was minor (Najm et la., 2014). This study results suggest the new WBA resin likely has a greater Cr(VI) capacity compared with other two resins as well as PWA7 (no shown in the graphs).
SBA Result
The SBA results suggest significantly greater Cr(VI) capacity compared with other SBA resins (Resin C and Resin D) tested at Glendale during 2012-2013 as well as other previous studies. The test water contained relatively high sulfate (110 – 120 mg/L), which is expected to negatively impact SBA resin life. Resin effluent Cr(VI) reached 8 ppb at approximately 12,000 to 14,000 BVs (for the two columns tested), which is about 17 to 19 operational days with a 2-minute EBCT. By comparison, the other SBA resins had operational life of 4 to 5 days when tested with the same water. A greater EBCT (4 minutes) did not demonstrate improved Cr(VI) capacity or resin life for the new SBA resin. (Note: 2nd graph is a zoom-in of first 50K BV)
Breakthrough curves after prior regenerations at various breakthrough points
Breakthrough curves of regeneration cycles at 2ppb
Breakthrough curves of regeneration cycles at 2ppb using 2BV of recycles brine
In-House SBA Regeneration Studies
The field test showed that our SBA has close to 4 to 5 times capacity of commercial SBAs at 10ppb breakthrough. Even with a high influent sulfate concentration at 120 ppm, Our SBA has about 20 operation days before it needs to be regenerated. This laboratory study indicates it can be also effectively regenerated as the commercial resins are.
The study looked into factors including salt concentration, brine contact time, amount of brine needed, regeneration efficiency on spent resin of different chromium effluent concentration, deterioration of capacity over regeneration cycles and feasibility on partially reuse of brine. A synthetic water constituting of chromate 16 ppb, sulfate 20 ppm and nitrate 6.5 ppm at pH 8.0
was used to evaluate the performance with Dow SAR as a reference. Brine volume used is 5 BVs.
Key findings are: 12% salt brine with 60 min contact time is sufficient to regenerate the spent resin as literature indicated; The spent resin of various level (2ppb, 4ppb and 8ppb) chromium breakthrough all can be regenerated back to 90-100% of original capacity depending on the chromium breakthrough limit set by applications. With a spent resin at 2ppb chromate breakthrough (for make-up applications with untreated water) our resin lost about 10% capacity from the fresh one, however maintained a relatively stable afterward at this capacity over 33 regeneration cycles which is equal to about 900K BVs under this water conditions. Regeneration with 2 BVs used brine plus 3 BVs of fresh brine indicates similar efficiency with all 5 BVs fresh brine in a 8 cycles study