AWWA ACE56254

In proactive anticipation of a more stringent arsenic standard, the City of Phoenix has been studyingarsenic 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 activatedalumina were tested on bench and pilot scale levels. In 1999, bench scale testing of sevenadsorption and chemical exchange media were tested. From the bench scale test results and adecision analysis evaluation of various treatment processes, four adsorptive media were selected forpilot testing. Adsorption media were preferred due to ease of operation, limited residuals handlingrequirements, potential for minimizing chemical handling, and process integrity. The preferredmedia that were pilot tested include granular ferric hydroxide, modified activated alumina coatedwith iron and other proprietary materials, and conventional activated alumina.The results from the pilot study were used to develop treatment system design criteria, equipmentlayouts and site configurations at 24 of the City's wells that will likely require treatment if a newMCL of 10 ppb is promulgated (under the aforementioned study). The estimated cost to treat allof the City's affected wells is approximately $26 million. Blending with treated surface water ispotentially 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 droughtreserve 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 costeffectivefor the City to design and construct one full-scale arsenic removal facility prior to widespreadimplementation of the process at several sites. In addition to the benefits to the City in termsof capital planning and implementation, this also represented a unique opportunity for the City tomake a significant contribution to the water industry, as the full scale facility would be one of thefirst of its kind in the US. The facility would assist many other utilities in implementing similarfacilities at their well sites.The selected treatment technology is granular ferric hydroxide or an equivalent absorbent that iscapable of long column run lengths without pH adjustment. As other adsorption media products aredeveloped in the future, these products could also be placed in the same removal facilities in placeof GFH. As a result, the proposed design was based on the use of an adsorption media, without pHadjustment, with two 2.5-minute contactors in series. The media would be used on a throw-awaybasis. 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 flowwould be blended back with the bypass flow from the well to meet a combined effluentconcentration of 8 ppb, prior to entry into the City's distribution system. Two 14-ft. diametercontactors are required for this process. Waste rinse water and backwash from the treatment plantwould be sent to the City's sewer system after being equalized. Equalization is necessary due to thelimited sewer capacity and the large backwash rate. The estimated construction cost of the facilityis $1.7 million. Includes table, figures.

Product Details

Edition:

Vol. - No.

Published:

06/16/2002

Number of Pages:

10

File Size:

1 file , 1 MB