AWWA WQTC58827

Pilot testing of a granular ferric hydroxide (GFH) adsorption system was conducted using groundwater from Scottsdale, Arizona Well #4 to determine the ability of the media to remove arsenic (As) at various pH values. The media was able to treat only 3,200 BV of water at the ambient pH of 8.7 - 9.2 before effluent As concentrations exceeded the new maximum contaminant level (MCL) of 10 Au/L. However, when the pH was decreased to pH 7.5 to 6.0, the media treated 33,600 BV and greater than 65,000 BV before reaching the 10 Aug/L limit. Subsequent testing with GFH that had been used to treat pH-adjusted waters indicated that continuing treatment with this media without pH control would cause previously adsorbed As to come off the media into the effluent at concentrations much higher than the influent. In two tests at pH 6.0 and pH 7.5, As was found to desorb at levels three to four fold higher than the approximately 30 Ag/L As influent, which is also above the new MCL, and that the desorption process took 7 to 18 days. In other words, should pH control in a system stop for any reason, it would not be long before As levels in the effluent would be unacceptably high. Adjusting the pH of the water from Scottsdale Wells #23 and #32 required approximately 40 mg/L of carbon dioxide gas to decrease the pH from 8.3 to 7.0 and a dose of 110 mg/L to reduce the pH down to 6.3. An air-stripping test showed that an air/water ratio of 4 was adequate to remove sufficient CO₂ from a supersaturated water to increase a pH 6.3 water to above 8.0. These results indicate that pH adjustment can substantially increase the adsorption capacity of GFH, but that the greatest capacity occurs at a pH range where the potential for lead and copper corrosion issues is very high. Any lapse in pH control can cause some of the previously adsorbed As to enter the effluent and potentially cause a violation of the new MCL. Finally, the tests showed that the use of carbon dioxide gas addition followed by air stripping is a viable alternative to the use of acid/base addition for pH control for adsorption systems. Includes 6 references, tables, figures.