|Assessing Arsenic Removal by Metal (Hydr)Oxide Adsorptive Media Using Rapid Small Scale Column Tests (EPA/600/R-08/051) April 2008
The lowering of the maximum contaminant level for arsenic in drinking water from 50 to 10 micrograms per liter (µg/L) has posed significant technical and financial challenges to water treatment facilities throughout the United States. To assist small water systems (less than 10,000 customers) in meeting the new standard, EPA announced in October 2001 an initiative, the Arsenic Rule Implementation Research Program, to conduct a series of full-scale, on-site demonstrations of arsenic removal technologies, process modifications, and engineering approaches applicable to small systems. Of the 40 project sites under the Round 1 and Round 2 demonstration program, 23 selected adsorptive media technology because of its ease of operation.
The conventional way of selecting adsorptive media has been based on the results of long-term pilot-plant studies. To reduce the time and cost required, it was desirable to develop new or use existing rapid, small-scale methods to evaluate media performance. Preliminary studies have been recently conducted using a rapid small-scale column test (RSSCT) method that was originally developed for evaluating the performance of granular activated carbon. The results of these studies have shown that the RSSCT method, which usually requires only three to four weeks of testing, has the potential to predict the performance of full-scale systems. If the study results are proven to be true, this method would provide the water industry with a lower cost alternative for developing performance data necessary for full-scale system design.
Battelle was contracted by EPA to evaluate the usefulness of this short-term predictive method. Side-by-side tests were conducted using RSSCTs and pilot/full-scale systems either in the field or in the laboratory. The test locations included six EPA arsenic removal technology demonstration sites and one EPA pilot-scale test site. For each location, RSSCTs were conducted using at least three parallel test columns packed with different adsorptive media to compare arsenic breakthrough of the small-scale columns to the pilot/full-scale systems.
A total of eight commercially available, NSF International-certified adsorptive media were tested, including three iron oxide-based media (E33, ARM 200, and KemIron), one iron hydroxide-based media (granular ferric hydroxide [GFH]), one iron modified activated alumina (AAFS50), two titania-based media (MetsorbG and AdSorbsia granular titania oxide), and one hybrid ion exchange resin-based media (ArsenXnp). Virgin media were crushed and sieved to obtain the 100 × 140 mesh fraction, which was packed in 1.1 centimeter × 30.5 centimeter glass columns. The amount of media packed into each column was determined through the use of proportional diffusivity scaling equations. The columns were thoroughly backwashed to remove media fines and entrained air before use.
Of the media tested, full-scale performance data were available for direct comparison for AAFS50, E33, GFH, and ArsenXnp. RSSCTs proved to be a reasonably reliable approach for comparing media run lengths and adsorptive capacities for arsenate (arsenic [V]) but over-predicted the capacities to remove arsenite (arsenic [III]). Key operational parameters, including reduced Reynolds-Schmidt product (ReSc) values and empty bed contact times (EBCTs), were evaluated to minimize the run time and volume of water required to conduct RSSCTs. Under the conditions tested for most media, an ReSc value of 2,000 appeared to be appropriate for RSSCT column design. A reduced ReSc value of 1,000 was needed for titania-based media to cope with operational difficulties related to excessive pressure buildup, bed compaction (up to 50 percent), and media disintegration. RSSCT columns scaled to a reduced full-scale EBCT of 2.5 to 3.0 minutes could produce similar results as those scaled to the whole-length, full-scale EBCTs.
Water quality at each test site varied and, consequently, the ability to remove arsenic by a given media also varied. Arsenic occurred as arsenic (V) at concentrations of 21.5 to 61 μg/L in five of the seven source waters tested, and pH values were between 7.2 and 7.7. The two source waters containing arsenic (III) had pH values of 8.1 and 8.5 and arsenic (III) concentrations of 64 and 22.5 μg/L. (One site used a solid-phase oxidizing media to convert the arsenic [III] into arsenic [V] and RSSCTs were conducted to study both arsenic [V] and arsenic [III] removal.) All the waters contained arsenic plus at least one other important element of interest, such as uranium, antimony, vanadium, silica, or phosphate. Arsenic adsorption capacities across the different source waters and different media tested, based on reaching an RSSCT effluent concentration of 10 μg/L, ranged from 0.05 to 2.0 milligrams of arsenic per grams of dry media. The influent pH value as well as the presence of silica, vanadium, phosphate, or calcium all appeared to impact the relative performance of media on different source waters. By conducting side-by-side RSSCTs, significant differences were observed among the media to remove arsenic and these co-occurring elements.
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