Like many communities in the desert southwest, Phoenix, Arizona's water supply includes wells with arsenic concentrations exceeding 10 ppb. Long before the USEPA revised the arsenic rule from 50 parts per billion (ppb) to 10, the City of Phoenix began bench and pilot scale studies for arsenic removal options. Early investigations focused on well-known processes like activated alumina and ion exchange as well as less proven concepts such as coagulation-assisted microfiltration. Later, they added adsorption media technology to the study.
The City considered cost as well as other factors like ease of operation, lower required operator attention and reduced waste generation. They were leaning toward iron media adsorption as their technology of choice.
After carefully evaluating all of the factors, as well as bench-scale and pilot study data, the City selected a granular ferric hydroxide (GFH™) adsorption media to treat water at its well #280 site. Operated in a fixed-bed adsorber, the arsenic removal system uses 60,000 lbs. of ferric-based, non-regenerative media to primarily remove arsenic from Phoenix's well site. Other constituents are well below drinking water standards.
The total flow for well #280 is 2.53 million gallons per day (mgd). Only 1.5 mgd of the flow is treated, with the balance bypassing treatment and blending with the treated water. The ratio of blending is controlled so that a maximum combined effluent of 8 ppb or less is maintained outside of the plant.
The GFH media is installed in two 14-ft diameter pressure vessels, allowing the existing well pumps to be used. Piping to the contactors is arranged so that either can be the lead or lag vessel. During operation, water is simply passed through the vessels containing the GFH media where arsenic is adsorbed. Chemical pre-conditioning or pH adjustment is not required as part of the treatment process. Treated water is blended in a storage tank prior to distribution to the system.
Once the media reaches its adsorption capacity, media in the lead vessel is removed and replaced with new media. Spent media is stored in a storage area before being TCLP-tested and hauled away to landfill.
After a year of operation, the reported results of the arsenic removal plant are better than anticipated. More than 35,000 bed volumes have passed through the GFH media with an effluent arsenic concentration remaining below detection. In addition, there have been no backwash events based on headloss build-up during this time. Operational since September 2003, the facility requires approximately eight hours per week of city staff time for routine operation and maintenance.