Removing co-occurring contaminants of arsenic and vanadium with full-scale arsenic adsorptive media systems.

The U.S. Environmental Protection Agency conducted an Arsenic Demonstration Program (ADP) whereby 50 full, small-scale arsenic removal treatment systems were evaluated for removing arsenic to below the maximum contaminant level of 10 µg/L and their operating cost for a minimum of 1 year. The majority (28) of the systems installed were adsorptive media (AM) technology with the media replaced when exhausted. This paper reports on the results of two ADP projects and two laboratory rapid small-scale column tests (RSSCTs) using the iron-based media, Bayoxide E33 (E33) AM for the removal of arsenic (As) and the co-occurring contaminants (COCs) of vanadium and to a lesser degree fluoride (F) and nitrate (NO3). The ADP studies found that the AM effectively removed the COC of V, but with a lower removal capacity than of As. One ADP study found the AM to be ineffective for the removal of F and NO3. The RSSCT conducted on two other source waters also found vanadium to be removed by the same AM. The study results suggested the AM selectively sequence of As > V > F = N. The study also investigated the AM to achieve an As limit of 5 µg/L. The AM was found to reduce As to below 5 µg/L with around 30% shorter treatment run lengths.

Removing arsenic and co-occurring contaminants from drinking water by full-scale ion exchange and point-of-use/point-of-entry reverse osmosis systems.

This study investigated the performance of two full-scale ion exchange (IX) systems, one point-of-entry (POE) reverse osmosis (RO) system and nine point-of-use (POU) RO units for simultaneous removal of arsenic and several co-occurring contaminants from drinking water. The study was performed as part of the U.S. Environmental Protection Agency's Arsenic Treatment Demonstration Program. The IX systems, with strong base anionic (SBA) resins, effectively removed arsenic (As), nitrate (NO3-) and uranium (U) to below respective maximum contaminant levels and vanadium (V) and molybdenum (Mo) to below 2 µg/L. The useful run length, as determined by either 10-mg/L (as N) nitrate or 10-µg/L arsenic breakthrough, was approximately 400 bed volumes (BV) initially. However, it was decreased over time, e.g., by 15% in 13 months at one site and 33% in 7 months at another site, apparently caused by resin fouling due to the presence of 2-mg/L natural organic matter (NOM) in source waters. The use of dual resins ‒ an acrylic SBA resin underlain by a polystyrene SBA resin ‒ effectively removed NOM and allowed the system to perform at its baseline level through the 13-month study. Arsenic and nitrate peaking occurred when the resins were not regenerated timely. The removal of contaminants appeared to follow a selectivity sequence: U, Mo > V > SO42- > HAsO42- > NO3- > HCO3-. RO effectively removed arsenic, nitrate, antimony, uranium and vanadium, mostly with a >99% rejection rate. The POE RO coupled with dual plumbing (only treating a fraction of water for potable use) and POU RO in individual homes could be used as low-cost alternatives to traditional RO treatment.

Patterns of Arsenic Release in Drinking Water Distribution Systems.

Retrospective analysis of 20 water systems from the USEPA's Arsenic Demonstration Program revealed three patterns of arsenic levels at the tap, after arsenic treatment of the source well water. Following an initial destabilization period, Pattern A systems (6/20 with low iron/manganese in source water and plastic piping) had arsenic concentrations that did not change as water traveled to consumer taps (conservative contaminant behavior). Pattern B systems (8/20 with high iron/manganese in source water and iron piping) had consistently higher arsenic concentrations at consumer taps, above the arsenic content of incoming treated water, for months to more than a year after arsenic treatment (non-conservative behavior). Pattern C systems (6/20 with additional occasional arsenic treatment complications) experienced multiple arsenic spikes at consumer taps (non-conservative and unpredictable behavior). These field observations suggest that in some water distribution systems arsenic may linger long after it has been removed at its source.

Arsenic/Iron Removal From Groundwater With Elevated Ammonia and Natural Organic Matter.

Under the US Environmental Protection Agency's Arsenic Demonstration Program, an iron (Fe) removal process consisting of permanganate oxidation and greensand filtration was shown to be effective in removing soluble arsenic (As(III)) (24.1 µg/L on average) from Waynesville, Ill., groundwater, which also contained elevated ammonia (3.8 mg/L [as N] on average) and natural organic matter (NOM) (7.9 mg/L, on average, of total organic carbon). Chlorine was not used because it forms chloramines, which are not effective in oxidizing As(III). A permanganate dose over the stoichiometric amount was applied to overcome interference from NOM-based on jar testing conducted for this and another Fe removal system at Sauk Centre, Minn. These pressure filtration syste ms had no air contact, thus allowing simultaneous oxidation of As(III) and ferrous iron. Compliance monitoring data to date show consistently low arsenic (<4 µg/L) and Fe (<0.05 mg/L) since the commencement of system operation in July 2009.

Regenerating an Arsenic Removal Iron-Bed Adsorptive Media System, Part 2: Performance and Cost.

Replacement of exhausted, adsorptive media used to remove arsenic from drinking water accounts for approximately 80% of total operational and maintenance costs of this commonly used small system technology. Results of three full-scale system studies of an onsite media regeneration process (discussed in the first article of this two-part series) showed it to be effective in stripping arsenic and other contaminants from a granular ferric oxide (GFO) exhausted adsorptive media. This second article details the performance of the regenerated media to remove arsenic through multiple regeneration cycles and the approximate cost savings of regeneration over media replacement. Results indicated that media regeneration did not appear to have a major detrimental effect on the performance of the GFO media, and the regeneration cost was potentially less than the media replacement cost. Therefore, onsite regeneration offers small systems a possible alternative to media replacement when removing arsenic from drinking water using iron-based adsorptive media technology.

Regenerating an Arsenic Removal Iron-Bed Adsorptive Media System, Part 1: The Regeneration Process.

Adsorptive media technology is frequently used by small water systems to remove arsenic because of its simplicity and efficiency. Current practice is to replace the media when it no longer reduces arsenic below the maximum contaminant level of 10 µg/L that the US Environmental Protection Agency has set for drinking water. Media replacement typically accounts for approximately 80% of the total operational and maintenance costs. One potential option to reduce the cost is onsite regeneration and reuse of the media. To evaluate the regeneration option, three consecutive regeneration studies were conducted on a full-scale adsorptive media system. This article, the first of a two- part series, describes the regeneration process and its efficacy in stripping arsenic and other contaminants from exhausted media. Study results found that a three- step regeneration process of media backwash, caustic regeneration, and acid neutralization conditioning proved effective for stripping arsenic and other contaminants from the exhausted media.

Regeneration of iron-based adsorptive media used for removing arsenic from groundwater.

Adsorptive media technology is regarded as a simple, low cost method of removing arsenic from drinking water particularly for small systems. Currently, when the effluent of a treatment system reaches the USEPA maximum contaminant level (MCL) of 10 ug/L, the exhausted media is removed and replaced by new virgin media. Although the commonly used iron-based media products are reasonable in price, the replacement cost accounts for around 80% of the systems total operational costs. One option to media replacement is on-site regeneration and reuse of the exhausted media. To determine whether an iron based media can be successfully regenerated and reused, laboratory batch and column regeneration tests were conducted on six exhausted iron-based media products obtained from six full scale arsenic removal treatment systems. Batch tests conducted on three of the media products to evaluate the effectiveness of 1-6% caustic regenerant solutions found that arsenic desorption increased until around 4%. Using 4% caustic solutions, the columns tests on the six exhausted media products showed arsenic removals ranged from 25 to 90% with the best results obtained with the Severn Trent E33 media. Exposing the media to caustic (pH ≥ 13) and acid (pH ≤ 2) solutions found minimal media loss with the caustic solution, but significant media dissolution with a pH 2 acid solution. A six column pilot plant test at an Ohio test site with the lab regenerated media products found that the regenerated media could achieve arsenic removals somewhat similar to virgin media.

Arsenic species in drinking water wells in the USA with high arsenic concentrations.

Arsenic exists in ground water as oxyanions having two oxidation states, As(III) and As(V), and its concentrations vary widely and regionally across the United States (USA). Because of the difference in toxicity and removability of As(III) and As(V), arsenic speciation is important in the selection and design of an arsenic treatment systems. Identifying the arsenic species is also helpful in explaining and understanding the behavior and characteristics of arsenic in the environment. Although laboratory methods exist for speciating arsenic in water samples, the lack of a universal preservation method has led to the predominant use of field separation methods that are somewhat complex and costly. Thus, very few studies have incorporated arsenic speciation. A U.S. Environmental protection Agency (EPA) arsenic treatment research program provided a unique opportunity to speciate the naturally occurring arsenic in 65 well waters scattered across the USA with many of them being speciated monthly for up to three years. Speciation test data showed that 31 wells had predominantly As(V), 29 had predominantly As(III) and five had a mixture of both. A general pattern was found where As(III) was the dominant species in midwest ground waters where anoxic conditions and elevated iron concentrations prevailed and the well waters in the east, west and farwest had either As(III) or As(V) as the dominant species. The monthly (12-36) speciation tests results at many of these sites also found no major changes in arsenic species over time.