National Drinking Water Clearinghouse
West Virginia University
PO Box 6893
New Methods for Removing Arsenic
By Vipin Bhardwaj
Editors note: Arsenic is a vexing problem for many small communities. Fortunately, there are different options for removing this naturally occurring chemical from drinking water. This article looks at four new technologies. Although certain brand names are mentioned in this article, their use here should not be construed as an endorsement.
The January 23, 2006, implementation date for the new 10 parts per billion (ppb) arsenic rule has spurred development of alternative treatment methods for arsenic removal from drinking water. Currently, the U.S. Environmental Protection Agency (EPA) categorizes seven emerging technologies: iron oxide-coated sand, granular ferric hydroxide, iron filings, sulfur-modified iron, greensand filtration, iron addition with microfiltration, and conventional iron/manganese removal. A discussion of four iron-based adsorption technologies is presented here. All have NSF Standard 61 certification; all are suitable for small systems. (For a comprehensive look at arsenic and drinking water, see the article All About Arsenic in the Summer 2002 On Tap.)
Photo Caption-Figure 1-Arrangenent of Vessels in Severn Trent Service's SORB33 Media for Arsenic Removal
GFH Arsenic Removal System
USFilters GFH (granular ferric hydroxide)Arsenic Removal System for drinking water is an adsorption process capable of removing arsenic and other heavy metals from raw water supplies. The process uses a ferric-based, non-regenerative media to absorb arsenic, selenium, uranium, chromium, and other heavy metals from drinking water. Like other adsorption processes, the water simply passes through the media to remove the contaminants. Once the media has depleted its adsorption capacity, it is removed from the vessel and fresh media is added. In many cases, the exhausted media can be discarded in landfills and classified as non-hazardous waste if toxicity characteristic and leaching procedure (TCLP) testing permits this disposal method. Onsite storage of regeneration chemicals and concentrated waste disposal issues are eliminated with the single use media.
The adsorption life of the media depends upon raw water pH, arsenic concentration levels, and operating cycles per day. GFH does not require preconditioning or pre-oxidation procedures, and the use of non-regenerative media are ideal design features for small and wellhead applications, particularly where no treatment currently exists. The GFH is available in a parallel or series operation, depending on the required removal concentrations. If a consistent 90 percent reduction is needed across the system, the series design is used. However, if the percentage is less than 90 percent, then the parallel design is typically applied.
GFH media operates as a fixed bed adsorber. Typical installation is in pressure vessels to allow a single pumping stage for the water treatment system. The standard GFH system consists of three vertical pressure vessels with factory-installed internals for distribution and collection of influent and backwash flows. GFH media is placed on a 12-inch gravel support bed. All face piping and valves are included in the standard system to streamline installation. Water continuously passes through the media bed where arsenic in water is adsorbed onto the media. The vessels are arranged in parallel or series arrangement depending on water parameters with an empty bed contact time of five minutes.
The vessels are backwashed once every two to six weeks to prevent bed compaction and to remove trapped particulates. The backwash process flow rate is typically 10 to 12 gallons per minute per square foot (gpm/sq.ft).
MEDIA G2 Adsorptive Filter System
ADI International Inc., of Fredericton, New Brunswick, Canada, developed MEDIA G2, an iron-based adsorptive filter media system. MEDIA G2 is a natural mineral, impregnated with a substantial quantity of ferric hydroxide. Used in a conventional filtering arrangement, arsenic-contaminated water flows downward through the filter bed where the iron on the media attracts and binds arsenic ions to the media substrate through a process of chemical adsorption. Tested and used in full-scale water treatment applications, MEDIA G2 will reduce arsenic from 1,200 ppb to less than five ppb. In most applications, the treated water will contain less than two ppb arsenic.
MEDIA G2 systems use an adsorptive process for arsenic removal, taking advantage of the natural affinity of iron to attract arsenic ions from the water. This takes place within the favorable pH range of 5.0 to 7.0. Extensive testing demonstrates the ability of the media to adsorb both trivalent (arsenite) and pentavalent (arsenate) arsenic. However, it is well recognized in the industry that arsenate is easier to adsorb than arsenite. MEDIA G2 also allows for the oxidation of arsenic on the surface of the media during the treatment process.
Iron-based adsorption and the MEDIA G2 process is suited to small systems because it is basically like a filtration process, requiring the least of unit operations, labor, and complexity. In California, for example, a Class 2 Operator is certified to operate a MEDIA G2 system.
The ability of MEDIA G2 to be regenerated, in place, offers many systems an economical solution, particularly where the arsenic concentration is more than 50 ppb and large through-puts are desired. MEDIA G2 can be regenerated up to five times before disposal and loses only 10 percent of its adsorption capacity per regeneration event. MEDIA G2 is unaffected by other contaminants at concentrations up to, or even exceeding, maximum allowable drinking water concentrations.
The most economical size range is 50 to 10,000 gpm to serve an estimated 100 to 50,000 population. MEDIA G2 systems produce very little residual waste: Typically, 99.8 percent of treated water (i.e., less than 0.2 percent wastage), which consists of the initial backwash and rinse of the media and normal backwashing requirements. The recovery can be increased to 99.99 percent with reclaim and recycle of backwash and regeneration water.
ADI supplies the main process equipment for a MEDIA G2 system as a package. Pressure vessels are 304 or 316 stainless steel, ASME code stamped. Manifold pipe and valves are pre-assembled at ADIs plant for fast bolting to pressure vessels at the site. Where chemical feed systems are required, pre-packaged systems can be supplied with automatic control and monitoring. Many MEDIA G2 plants can be operated with the minimum of controls or automation. As part of its supply, ADI provides on-site operator training, and a manual. MEDIA G2 systems operate under pressure of the existing well pump. Therefore, power requirements are very low.
Photo caption-This photo shows an ADI arsenic treatment system in Rose Hill Center, Holly, Michigan.
SORB 33 Uses Iron Hydroxide Granules
SORB 33 arsenic removal was developed in 1995 by Severn Trent Water (U.K.). The heart of the SORB 33 system is a fixed bed composed of iron hydroxide granules through which the polluted water passes. The iron oxide granules used are between 0.5 and 2 millimeters. The media has a high specific surface area of approximately 732,400 ft2/lb. The adsorption technology fixes the arsenic contaminant onto a solid media.
The SORB 33 process reduces arsenic levels to below 3 ppb across a complete range of large and small municipal drinking water systems, industrial wastewaters, and residential and commercial applications. The Bayoxide E33 media has a long life in the SORB 33 system, typically up to two years between replacement. Spent media fixed with adsorbed arsenic passes EPAs TCLP, making it suitable for simple landfill disposal.
This low-cost adsorption process has little flow interruption. The media will also adsorb other contaminants, including antimony, chromate, lead, selenium, and vanadium. No chemical usage is required for regeneration and the process produces very low residual (wastewater) effluents. The capacity of this media is more than other adsorbents such as activated carbon and activated aluminumoxide. Also, the media does not require regeneration like the ion exchange process. When the media is exhausted, it is discarded.
Aquabind Is Suitable for POU/POE
Another arsenic removal media is available from Apyron Technologies, Inc., based in Atlanta, Georgia. Apyrons custom-designed line of advanced adsorptive media products removes up to 99 percent of both arsenite and arsenate from drinking water. The product is suitable for point-of-use (POU), point-of-entry (POE), and small water systems. Recently, the National Drinking Water Advisory Council completed an EPA-commissioned cost analysis stating that POU applications should be given greater consideration as a method of tackling arsenic contamination. The councils cost evaluations show that communities as large as 10,000 may benefit financially from this approach.
Confirmed through independent laboratory analysis, each product is operational in a wide range of temperatures, water profiles, and pH ranges. The product can treat arsenic levels from 50 to 3,500 ppb and iron up to 15 ppm. Apyron has installed arsenic treatment units in Bangladesh and India, where they have been well received in the villages. The product is simple to use with no electricity or sophisticated controls needed, and can treat up to 4,500 gpd. Apyron also offers an arsenic test kit, a cost effective and simple method of informing potential customers of existing arsenic levels and validating the performance of the treatment option, once installed.
Conventional treatment technologies have been discussed in previous issues of On Tap (see the Summer 2002 issue). Some of these are suitable for arsenic removal. Please note, however, that no single treatment technology is best for every application. Water chemistry, such as arsenic concentration, speciation of arsenic (arsenite versus arsenate), feed water pH, presence of interfering/competing ions such as silica, fluoride, phosphate, sulfate, dissolved iron and manganese; dissolved oxygen content; daily water usage; and maximum flow rate are all factors that must be considered before selecting a technology.
The treatment technologies mentioned in this article will give communities additional options besides conventional treatment methods. Communities and water systems facing an arsenic problem are encouraged to explore different options in dealing with this issue.
The author wishes to thank M. Eric Winchester of ADI International and Rich Dennis and Nadia Abbott of Severn Trent Services, Inc. for providing information about their technologies.
About The Author
Vipin Bhardwaj, engineering scientist, has a master's degree in environmental engineering and a masters in agriculture from West Virginia University.