Achieving Arsenic Removal
Shortly after the U.S. Environmental Protection Agency (EPA) lowered the maximum acceptable level of arsenic from 50 μg/L to 10 μg/L, EPA unveiled its National Arsenic Treatment Demonstration Program. Twelve demonstration sites were chosen where iron- and alumina-based adsorptive media systems, anion exchange systems, iron removal systems and system modifications would be employed to test their efficacy for achieving compliance with the new federal standard.
Among the first demonstration sites was Brown City, Mich., located about 60 miles north of Detroit. One of 160 communities in eastern Michigan that was affected by arsenic, Brown City’s water system serves a population of 1,300 with water from two wells, No. 3 and No. 4.
Prior to the demonstration study, both wells provided water on an alternating basis. But during the demonstration, well No. 3 was reserved for emergency backup and well No. 4 became the sole source of water. The well has a capacity of 640 gal per minute (gpm), and the water is delivered to a storage tank at a pressure of 60 psig. The well water had an arsenic (As) level of 19 to 25 μg/L, mostly in the form of reduced As(III), and also contained a modest level of iron, typical of Michigan well waters. The pump building required a significant increase in floor space to accommodate the arsenic removal system.
The EPA and its team of consultants on the Demonstration Program panel selected Severn Trent Services’ SORB 33 arsenic adsorption system for use at the Brown City site. The program would validate the reliability of Severn Trent’s technology for small systems (EPA estimated that nearly 97% of the water systems affected by the new standard were systems serving fewer than 10,000 people); gauge the simplicity of its operation, maintenance and required operator skills; determine its cost-effectiveness; and characterize its treatment residuals.
Severn Trent Services proposed to install two of its automated SORB 33 APU 300 vessels, each with a capacity to treat 320 gpm or half of the well pump’s total flow. The 16-ft-long by 6-ft-wide by 9-ft-high skid-mounted units each consisted of two 51/4-ft-diameter FRP adsorbers and 152 cu ft of Bayoxide E33 iron oxide media, which were installed as modules after the building was completed. Each unit included 10 automatic valves, five for each adsorber, which were controlled by individual PLCs. The four adsorbers operated in parallel flow configuration. Piping for the system was schedule 80 PVC with PVC butterfly valves.
In the SORB 33 system, contaminated water enters the vessels and passes through the robust ferric oxide media. As water passes through the media, arsenic is adsorbed and removed to a level below the EPA standard. The system requires no cleaning, no regeneration and no complex, labor-intensive steps. The dry Bayoxide media was designed with a high capacity for both As(III) and As(V). The media’s life expectancy is dependent on site-specific water quality and operating levels with change-out typically needed between nine and 36 months. The exhausted media is non-hazardous and can be sent to a landfill, passing Toxicity Characteristic Leaching Procedures.
After the pump building was expanded to house the additional equipment, the two APU systems were installed and began operation in April 2004, and their performance was monitored by EPA’s contractor.
The SORB 33 system was designed to treat Brown City’s unchlorinated water directly from the well pump, although an injection point was installed upstream of the adsorbers during construction. Spent backwash water was discharged through a swale to a nearby county storm water drain. Backwash was initiated by the operator, although the system had features for automatic backwash on a timed schedule or on high differential pressure across the adsorber vessel.
After several months of operation, As(III) began to break through because the adsorbed reduced iron, Fe(II), lowered the media’s ability to oxidize and adsorb As(III). To remedy the breakthrough, the chlorination injection point was moved upstream of the SORB 33 system. As a result, the effectiveness of the Bayoxide E33 media improved, reducing arsenic to less than 2 μg/L in the treated water, and performance remained consistent once prechlorination was implemented. Manganese was also removed in the process.
With the successful removal of arsenic at well No. 4, Brown City officials decided to build a second arsenic removal facility for well No. 3 and selected the SORB 33 system. Construction of the new facility was completed in 2007, and both systems continue to operate well.