Introduction

The consumption of water containing low levels of arsenic has been linked to an increased risk of lung cancer, bladder cancer and other human ailments. In January 2006, this risk prompted the United States Environmental Protection Agency (EPA) to reduce the maximum contamination level for arsenic in drinking water from 50 µg/L to 10 µg/L, bringing the United States in line with the European Union and the recommended World Health Organization (WHO) level. Naturally occurring arsenic is found in the groundwater in many parts of the United States and there are also areas where extended pesticide use has led to the anthropogenic contamination of groundwaters. The EPA has estimated that approximately 4,000 water providers and several million domestic wells were not in compliance with this new lower arsenic MCL

There are a number of methods to remove arsenic from groundwater that have successfully been applied on a commercial scale, including ion exchange, adsorption and coagulation-filtration technologies. Of these methods, probably adsorption has proven the most popular due to simplicity of operation, low levels of waste residuals and cost of treatment. The most common adsorbents commercially available consist of oxides of iron, titanium, aluminum and zirconium, though the use of aluminum is decreasing following the development of new media with greater arsenic capacity and better pH tolerance. Granular media are used in packed beds and arsenic-contaminated water is passed through, either up or down flow, arsenic is adsorbed onto the media and treated water sent through to the distribution system. The advantage of adsorbent media over ion exchange is that common anions in water, such as bicarbonate, sulfate and chloride, are not adsorbed by the media so the taste and other aesthetical qualities are not impacted. The operational life of adsorptive media is long and can vary from tens to hundreds of thousands of bed volumes (BVs) of treated water which can equate to months or years of use.

When arsenic breakthrough into the treated water is detected, the adsorbent media must be replaced with fresh media. Although the arsenic can readily be stripped from most of the adsorbents using strong caustic, the low physical integrity of the granules means that the regeneration process will cause significant media attrition leading to the generation of fines and an overall reduction in particle size. Consequently, the regeneration of most adsorptive media is not considered to be economically viable. Currently, spent media are disposed of once breakthrough occurs, leading to the generation of large volumes of arsenic-tainted solid waste. Although this waste passes the current Toxicity Characteristic Leaching Procedure (TCLP) and can therefore be disposed of in a non-hazardous waste landfill, recent research is suggesting that arsenic may be remobilized under landfill conditions.

To overcome the limitations of granular adsorbents, SolmeteX in collaboration with Lehigh University developed ArsenX. This media consists of hydrous iron oxide particles immobilized throughout a strong base anion exchange resin. The media is manufactured in the mixed sulfate/bicarbonate form and removes arsenic by adsorption; the ion exchange component serving mostly as a rigid support while the removal is performed by the iron oxide. Due to the improved physically integrity supplied by the polymer bead, this media can be efficiently regenerated using warm caustic solution and reused, thus minimizing the generation of solid waste. Field and laboratory studies have shown that minimal arsenic capacity is lost during the regeneration process making regeneration and reuse and attractive economic option.

Field Study

The performance of ArsenX in the field was evaluated at Paxton, MA using a 3 liter (0.79 gallons) bed of media. The media was packed into a two inch diameter column giving a column height of approximately 5 feet. The water chemistry at this site is shown below in Table 1. All of the arsenic was present as As(V) (arsenate).

Water was passed down through the resin at a flow rate of 1 liter per minute, giving an empty bed contact time (EBCT) of 3 minutes. Sampling points were located along the length of the media column to allow the arsenic breakthrough to be monitored as it progressed down the column, allowing data to be gathered on the kinetics of arsenic adsorption. At the conclusion of the trial, no arsenic was detected in the column effluent and it was still below the 10 ppb MCL at sampling point S-4, equivalent to an EBCT of 2.2 minutes. The breakthrough curves for arsenic at S-4 and S-2, equivalent to an EBCT of just 0.73 minutes is shown below in Figure 1. Both curves overlap each other indicating that the arsenic uptake kinetics for ArsenX are extremely rapid.

Figure 1. Breakthrough Curves for ArsenX

A full scale arsenic removal system is currently being installed at this site and consists of 2 separate 100 gpm skid-based systems connected in parallel. Each individual skid consists of 2 resin tanks in series, each tank containing 29 cubic feet (217 gallons of media). This system is designed to allow each skid to operate on one tank only to allow spent media to be regenerated off site without interrupting the well operation.  Based upon the pilot data shown in Figure 1, it is estimated that each column of ArsenX will treat in excess of 100,000 BVs of water before requiring regeneration. The treatment system is expected to become operational in late May of 2008.