EVALUATION OF EBCT ON ARSENIC ADSORPTION PERFORMANCE AND COMPETITIVEPRELOADING

摘要

Adsorption can be utilized as a treatment process to remove up to 99% of Arsenic in the feed water. Itrninvolves the separation of Arsenic from the liquid phase, and its accumulation or concentration at thernsurface of the adsorptive media. Physical adsorption is caused mainly by Van der Waals forces andrnelectrostatic attraction between the negatively charged Arsenate and the positively charged mediarnsurface. As constituents such as Sulfate, Phosphate, Silica and Vanadium adsorb onto the media, lessrnsites are available for Arsenic removal.rnTo date, there are several types of adsorbents being tested for Arsenic removal. Testing is primarilyrnbeing conducted with iron-based media, including iron-modified aluminas, granular iron oxides andrniron-modified zeolites. Newer adsorbents, such as titanium oxide and zirconium hydroxide, havernrecently entered the market place and are also being tested in the water industry.rnDamon S. Williams Associates (DSWA) contracted with the City of Mesa Arizona to evaluate thernperformance of various adsorptive media at the Falcon Field 4 (FF#4) well site. The FF#4 well wasrnchosen as a demonstration site based on its challenging water quality. Average water quality parametersrnare as follows: Arsenic = 32.3 μg/L (As[V] only), pH = 7.7, Silica = 35.5 mg/L, Sulfate = 47.0 mg/L,rnPhosphate = 0.014 mg/L as P, Vanadium = 13 μg/L.rnTen media were initially tested using laboratory Rapid Small Scale Column Tests (RSSCTs) and batchrntests. Three of the 10 media were selected for testing using a pilot-scale adsorption facility. The threernmedia that were pilot tested are as follows: US Filter supplied GFH, Severn Trent supplied BayoxidernE33, and Solmetex supplied As:Xnp.rnOne of the project goals in the pilot column design was to track Arsenic breakthrough along the mediarnbed for full-scale design consideration. To accomplish this goal, sample ports were installed along therndepth of the media bed. By installing sample ports along the length of the column, the Arsenicrnconcentration profile could be tracked as it travels down the column. Each sample port along therncolumn represents a different EBCT and the Arsenic results from each port can be interpreted asrnindividual column tests. By normalizing the results against the bed volume above each sample port,rninformation was gained about the importance of EBCT and competition on adsorption performance.rnIn general, the variable EBCT testing showed mixed results. For two media, GFH and E33, it appearsrnthat shortening the EBCT has a small positive effect on adsorption performance for FF#4 water. It isrnspeculated that this performance gain is caused by less preloading of competing contaminants. Itrnappears that for the FF#4 water, Silica, Barium, and Vanadium had a detrimental impact on adsorption.rnIt is theorized that this effect can be alleviated with shorter EBCTs, and further research is required tornvalidate the theory.