Optimizing Arsenic Treatment System Yields Significant Cost Savings
Since 1886, American Water has been “maintaining high water quality standards and dependable service” and “finding ways to do it better,” according to the company’s website. The country’s largest investor-owned water and wastewater utility company serves the needs of 16 million customers in more than 1,600 communities across the U.S.
Arizona American Water, a wholly owned subsidiary of America Water, is the largest investor owned utility in Arizona, serving a population of approximately 350,000 northwest of Phoenix. When the Surprise, Ariz., company committed to meeting the January 2006 federal arsenic Maximum Contaminant Level (MCL) of 10 parts-per-billion, those responsible for choosing the arsenic removal technology stayed true to the parent company’s culture. They piloted and selected two distinctly different technologies and then worked to optimize each through thorough testing and evaluation.
Arsenic, of course, is common throughout Arizona, and many water and wastewater utilities have installed a variety of arsenic removal technologies, including reverse osmosis, coagulation filtration, ion exchange and adsorption. Arizona American Water decided to pilot one coagulation filtration system and five adsorption systems to treat arsenic levels ranging from 12 to 82 µg/L. The adsorption system selected was the SORB 33 fixed-bed arsenic treatment system and Bayoxide E33 arsenic removal media from Severn Trent Services .
Arsenic Treatment System Design Criteria
With a combined capacity of 27.1 million gal per day (mgd), the six Arizona American Water water treatment facilities serve a significant portion of the utility’s customer base. In order to minimize the rate impact on customers, the company selected the two treatment technologies based on lowest capital, operating and maintenance requirements. The design criteria for the systems, whose arsenic treatment goal was less than 8 µg/L, included:
- Pre-oxidation to be used at all facilities;
- Silica, phosphate, manganese and vanadium are present in the water supplies and must be monitored for interference with the arsenic removal;
- The pH must be adjusted as necessary;
- Incorporating blending vs. 100% source flows to maximize system efficiency; and
- The adsorption system would incorporate a lead/lag design.
In order to maximize each system’s performance, Arizona American Water implemented a sampling schedule that included biweekly sampling of treated and combined water and quarterly sampling for regulatory requirements. Dosage and bypass sampling results would be used to optimize system operations.
Arizona American Water’s waste management strategy for the coagulation filtration system was to maintain a consistent concentration of discharge into its sewer system and to optimize solids handling processes through polymer dosing and mixing. For the SORB system, backwash water would be recycled when possible, and fines in the backwash effluent discharge would be minimized by increasing settling time.
At the adsorptive treatment plants, which became operational in February and March 2006, plant capacity ranged from the 3.1-mgd Agua Fria Water Plant 5, where arsenic levels measured from 6 to 82 µg/L, to the 8.0-mgd Sun City West Water Plant 2, where arsenic levels were found to be 6 to 25 µg/L. Blending was required at some of the plants to accommodate high levels of fluoride and/or nitrates, while arsenic levels in the 6.8-mgd Agua Fria Water Plant 2’s source water were low enough that 100% bypass flows were possible.
The Adsorptive Process
The SORB system employs a simple “pump-and-treat” process that flows pressurized well or spring water through a fixed-bed pressure vessel containing the iron-oxide media where the arsenic removal occurs. Bayoxide is a granular ferric oxide media, and arsenic has a high affinity for iron-oxide-based minerals, adsorbing quickly to the surface of the media. This makes granular iron oxide media, such as Bayoxide, excellent for arsenic removal.
Other contaminants 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. Bayoxide E33 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 dry, crystalline granular media to achieve long operating cycles, reduce pressure drops and improve the operational cost. The media does not need to be replaced for six months to two years, and the spent media is sent to a nonhazardous landfill.
Evaluating the Arsenic Treatment Systems
As Arizona American Water staff completed monthly and quarterly milestones with the coagulation filtration and adsorption systems, they were impressed with the differences between the two systems. Operation of the coagulation filtration system was more labor intensive than the adsorption systems, requiring more chemicals, more instrumentation on site that needed monitoring and significantly more maintenance time each day. As staff became more familiar with the coagulation filtration system, they identified several operational improvements, including maintaining a more consistent concentration of sludge, preventing the sludge from “caking” in the collection system. This was done by continuously running the recycle pumps rather than operating them in normal duty. In addition, the staff increased the frequency of cleaning the clarifiers to semi-annually.
“By contrast, adsorption is a pretty simple process that was easily adopted by the staff,” said Jeremiah Mecham, operations superintendent for Arizona American Water. “And that’s what we expected based upon the system’s reputation and the experience of others.”
Among the enhancements Arizona American Water staff recommended for the adsorption systems was installing high-pressure relief valves to replace rupture discs for pressure relief. “Two of our sites are below grade, and a ruptured disc would allow water to continue to flow from the vessels, potentially flooding the treatment area,” Mecham said. “In addition, we installed piping to carry any water that was released by the pressure relief valves outside the treatment containment area, further preventing possible flooding.”
“As we continued to monitor the systems with our optimized processes, we were able to nearly double the bed volumes compared to the Severn Trent guarantee, primarily by bypassing more of the water while still achieving arsenic levels of less than 8 µg/L. This has resulted in a significant savings in media replacement costs.”
“Complying with the new arsenic MCL in the Agua Fria District was made relatively simple through the implementation of the SORB systems,” Mecham said. “The systems have exceeded our expectations by enabling us to provide clean, safe, EPA-compliant water to our customers at a reasonable cost to Arizona American Water—and ultimately to ratepayers.”