AWWA WQTC58829

Damon S. Williams Associates (DSWA) was contracted by the City of Mesa to conduct arsenicpilot testing on their drinking water system, which has approximately 22 wells impacted by thenew Maximum Contaminant Level (MCL) of 10 parts per billion (ppb). DSWA teamed withArizona State University (ASU) to perform arsenic removal bench testing. The project focusedon testing and evaluating seven adsorptive arsenic removal media and one ion exchange resinsystem. Several of the media had not yet been tested on complex waters found in Arizona andthroughout the arid Southwest. All tests were conducted with a natural groundwater containingapproximately 33 ppb of arsenic (present as arsenate), an ambient pH of 7.7 and other parameters(e.g. silica, vanadium, phosphate), which are known to cause treatment difficulties for somemedia.The bench testing consisted of two phases. Jar tests were used in Phase I to screen seven mediafor arsenic removal efficiencies at three pHs. Phase II involvedRapid Small-Scale Column Testing (RSSCT), which focused on media kinetics and providedlaboratory generated breakthrough curves.DSWA generated a ranking system to determine which media to pilot test. Ranking systemvariables included the following: NSF approval status; results of the bench testing; capital costprojections; O cost projections; and operational issues (e.g., iron release or pH adjustmentrequirements). Pilot testing was conducted with the three most favorably ranked media: WRTZ33, GFH, and Bayoxide E33, utilizing the DSWA-designed arsenic removal pilot system. Thepilot systems contactors operate in parallel and series configurations, as well as in upflow ordownflow modes of operation to accommodate a wide variety of the media on the market today.These pilot tests were conducted in parallel until breakthrough. DSWA also tested the BasinWater ion exchange package pilot system as a comparison to adsorptive media.Competing contaminants (e.g. silica, vanadium, phosphate) were evaluated during piloting todetermine their impact on media life. Each media was also evaluated for potential contaminantreleases (e.g., iron), as well as potential increases in treated water turbidity, as they may createimpacts to the distribution system.This paper demonstrates the validity of the RSSCT results for this water through comparisonof pilot test results. Pilot testing results will be discussed, with an emphasis on the impacts ofcompeting and/or released contaminants and the biological impacts on media with intermittentwell use. The findings of this paper will be useful for all water systems that must considertreatment for arsenic removal to better understand the process that is required for selectingappropriate adsorptive media. Includes table, figures.



Keywords: Testing; Arsenic; pH; Ion Exchange; Phosphates; Mesa, Arizona; Adsorption