AWWA ACE56254

In proactive anticipation of a more stringent arsenic standard, the City of Phoenix has been studying arsenic removal in surface water treatment plants and groundwater wells for the past several years. In 1996, technologies such as coagulation/microfiltration, ion exchange, and conventional activated alumina were tested on bench and pilot scale levels. In 1999, bench scale testing of seven adsorption and chemical exchange media were tested. From the bench scale test results and a decision analysis evaluation of various treatment processes, four adsorptive media were selected for pilot testing. Adsorption media were preferred due to ease of operation, limited residuals handling requirements, potential for minimizing chemical handling, and process integrity. The preferred media that were pilot tested include granular ferric hydroxide, modified activated alumina coated with iron and other proprietary materials, and conventional activated alumina. The results from the pilot study were used to develop treatment system design criteria, equipment layouts and site configurations at 24 of the City's wells that will likely require treatment if a new MCL of 10 ppb is promulgated (under the aforementioned study). The estimated cost to treat all of the City's affected wells is approximately $26 million. Blending with treated surface water is potentially feasible at several well sites, possibly eliminating the need for treatment. Regardless, treatment will be required to comply with an arsenic standard of 10 ppb, particularly at the drought reserve wells in North Phoenix. Currently, there are no large arsenic removal facilities operating in the US (greater than 60 gpm). Since no full scale criteria is currently available, the City felt it was prudent, beneficial, and costeffective for the City to design and construct one full-scale arsenic removal facility prior to widespread implementation of the process at several sites. In addition to the benefits to the City in terms of capital planning and implementation, this also represented a unique opportunity for the City to make a significant contribution to the water industry, as the full scale facility would be one of the first of its kind in the US. The facility would assist many other utilities in implementing similar facilities at their well sites. The selected treatment technology is granular ferric hydroxide or an equivalent absorbent that is capable of long column run lengths without pH adjustment. As other adsorption media products are developed in the future, these products could also be placed in the same removal facilities in place of GFH. As a result, the proposed design was based on the use of an adsorption media, without pH adjustment, with two 2.5-minute contactors in series. The media would be used on a throw-away basis. At the well 280 site, the total flow is 1,850 gallons per minute. To meet an MCL of 10 ppb, the treatment system flow would be approximately 1,050 gpm. The partial stream treated flow would be blended back with the bypass flow from the well to meet a combined effluent concentration of 8 ppb, prior to entry into the City's distribution system. Two 14-ft. diameter contactors are required for this process. Waste rinse water and backwash from the treatment plant would be sent to the City's sewer system after being equalized. Equalization is necessary due to the limited sewer capacity and the large backwash rate. The estimated construction cost of the facility is $1.7 million. Includes table, figures.