Drinking Water Treatment in Arsenic ‘Hot Spot’
Situated approximately midway between the South Rim of the Grand Canyon and Phoenix lies the city of Sedona, Ariz. The city’s main attraction is its stunning array of red sandstone formations called the Red Rocks of Sedona. The formations appear to glow in brilliant orange and red when illuminated by the rising or setting sun.
But in recent years, this section of Arizona has become well known for another natural phenomenon: arsenic. The U.S. Environmental Protection Agency (EPA) first announced a new arsenic maximum contaminant level (MCL) of 10 ppb in drinking water in 2001.
Since then, the state of Arizona has become something of a testing ground for arsenic removal technologies with reverse osmosis, coagulation filtration, ion exchange and adsorption being among the most popularly evaluated treatment methods.
In January 2006, when the new federal arsenic rule came into effect, most of the state’s drinking water suppliers needed to take remedial action at some of their drinking water sources.
EPA Demonstration Site
In Valley Vista, Ariz., about seven miles south of Sedona, the Arizona Water Co. operated a drinking water well, Valley Vista Well #1, which produced groundwater with arsenic levels of 31 to 37 ppb.
In June 2004, the company worked with the EPA’s National Risk Management Research laboratory to install and operate an activated alumina adsorptive media system aimed at reducing the level of arsenic in the well.
The activated alumina system removed the arsenic to below the new arsenic MCL. But the system required periodic acid washing and reactivation of the media, which required more labor than Arizona Water had anticipated, raising the overall cost of operation.
After several years of utilizing the activated alumina system, the company tried different media to find the one that worked best. When Bayoxide arsenic removal media from Severn Trent Services was installed in the demonstration plant, “it worked flawlessly,” Arizona Water Co. Vice President of Engineering Fred Schneider said.
At this time, Valley Vista was also experiencing increased water demands that resulted in the closing of the first well, so a new larger-capacity well (Well #13) could be brought online as their drinking water source.
Water from the new well also contained high levels of arsenic contamination and would require treatment. A team of seven engineers and plant operators led by Schneider was tasked with evaluating various arsenic removal technologies.
A Noticeable Trend
A number of drinking water utilities in Chandler, Goodyear, Litchfield Park, Flowing Wells and Surprise, Ariz., had selected the same adsorption arsenic removal technology for use at their drinking water wells.
At the close of their evaluative process, Schneider and his team selected the SORB 33 fixed-bed arsenic treatment system from Severn Trent Services along with the Bayoxide media.
Schneider explained: “Throughout our service area, Arizona Water had been using a mixture of ion exchange, coagulation filtration and adsorptive arsenic removal treatment systems. After undergoing our evaluation of adsorptive treatment technologies to address arsenic contamination at Well #13, we found that we achieved the highest bed volumes with the Bayoxide media. And our operators preferred the adsorptive process because it was predictable and easy to operate. An additional benefit is that with adsorptive systems we didn’t incur the downtime that we had with other systems’ chemical pump malfunctions and the need to constantly deliver chemicals to, and haul waste from, the plants.”
In all SORB 33 systems, high arsenic concentration water enters the vessels and passes through a robust ferric oxide arsenic removal media: Bayoxide. As water passes through the media, arsenic is adsorbed and removed to a level below the MCL. The system requires no cleaning, no regeneration and no complex, labor-intensive steps.
Other constituents common to groundwater also have a high affinity for iron-based minerals. This creates competition among ions, resulting in less arsenic being adsorbed per volume of treated water. The Bayoxide media is specifically designed to adsorb arsenic while reducing competition with other ions, thus improving the arsenic-adsorbing potential of the media.
These characteristics enable systems using the Bayoxide media to achieve long operating cycles, reduce pressure drops and improve the operational costs. The media does not need to be replaced for six months to two years or more, and the spent media is sent to a non-hazardous landfill.
Full-Scale Operations Begin
In January 2010, Hennesy Mechanical Sales of Phoenix was awarded a contract to design, permit, construct and commission the SORB 33 system. Hennesy installed one 9-ft-diameter vessel, SORB 33 EAS Series, containing 189 cu ft of Bayoxide media designed to treat Well #13’s flow rate of 450 gal per minute (gpm).
The treatment objective for the arsenic effluent was set at less than 7 ppm. Severn Trent Services also submitted a written performance warranty to provide 65,400 bed volumes of treated water prior to the media becoming expended and requiring change. The system went into full-scale operation on May 27, 2010.
Water from Well #13 is chlorinated and flows into the adsorption vessel where a pump-and-treat process sends pressurized water through the filter vessel containing the Bayoxide media. As water is forced through the fixed bed, arsenic is attracted to the media.
At start up, arsenic was reduced to 0 ppb, and as the system continued to operate, arsenic levels would gradually rise to no more than 9 ppb until the media was spent. The media is designed to adsorb a large amount of arsenic to achieve long operating cycles, reduce pressure drops and improve the operating cost.
Pressure differential (ΔP) is monitored through the adsorber, and when ΔP exceeds the high ΔP setpoint of 10 psi, the system alerts the operator. After a 15-minute backwash is conducted to reclassify the compacted media, the adsorber is returned to service. During the backwash process, 100% of the water is captured and reused in the system.
Schneider said: “The system has operated flawlessly. So much so that we plan to install seven new arsenic treatment systems at various Arizona Water Co. wells in the next 12 to 18 months. Some will be brand new facilities, and in some we’ll be replacing ion exchange technology. The plants range from 200 to 450 gpm to larger, 900- to 1,200-gpm capacity configurations. Each of them will be an adsorptive system using Bayoxide media.”
Rich Dennis is separation process manager for Severn Trent Services. Dennis can be reached at [email protected] or 813.886.9331.