Considerations for Single-Use Arsenic Adsorption Systems
When the revised arsenic rule reduces the acceptable level of arsenic in drinking water from 50 parts per billion (ppb) to 10 ppb in January 2006...
When the revised arsenic rule reduces the acceptable level of arsenic in drinking water from 50 parts per billion (ppb) to 10 ppb in January 2006, thousands of once-compliant water treatment systems will be required to further reduce their arsenic levels. To meet the revised rule, utilities face a two-fold challenge: identify and adopt an arsenic reduction method and properly dispose of any residuals. Arsenic removal systems requiring minimal operator intervention are preferred, due to labor and budget considerations.
Although a wide variety of treatment technologies and processes are available for arsenic removal, systems that use single-use iron-based adsorption media are proving to be one of the most cost-effective solutions. Such treatment systems pass water through a media bed where adsorption binds the arsenic to the media. Pre-treatment is not typically required, and regenerant chemicals and waste are eliminated. Moreover, minimal liquid waste is produced due to infrequent backwashes. This provides a simple to use system for operators that may not have already had prior treatment facilities.
The fully dewatered media is ready for shipment to a disposal facility.
Water quality greatly affects the adsorption media’s arsenic removal capacity. The operating cycle starts out with very low and often non-detectable effluent arsenic concentration. As the volume treated grows, the effluent arsenic concentration gradually increases to the point where adsorption capacity is met. Capacity is usually reached after several months, but it can sometimes take several years. Once exhausted, the media is removed from the treatment vessel and replaced with new media. Exhausted media is tested and then disposed of at an approved landfill.
Regardless of the removal method, testing for acceptance at the landfill must be carried out prior to disposal. The Toxicity Characteristic Leaching Procedure (TCLP) test is a common requirement for most landfills. However, disposal location options should be reviewed to determine which, if any, other tests need to be conducted prior to the media being disposed.
Direct vessel discharge and vacuum/slurry removal are the two most common methods for removing exhausted media from the treatment vessels. Media installation is accomplished by bulk placement or slurry transfer. Media handling considerations are important in the plant design phase since this affects vessel and plant construction details.
With direct vessel discharge, a media discharge connection must be located on the vessel side at the media and support gravel interface. A full throat valve is located at the connection to prevent media loss during operation. A discharge pipe connects the valve to a discharge pit where the exhausted media is sent. To remove the media, the vessel influent and effluent valves are closed and the media discharge valve is opened. Water and media contained in the vessel flow out the discharge pipe. A slow backwash is conducted to remove all media from the vessel. An access hatch on top of the vessel allows operators to confirm when all media is removed. Once in the discharge pit, water is allowed to drain out of the media to minimize weight hauled to the landfill. Two advantages to this method include minimal vessel unloading time and minimized direct media handling.
Spent arsenic removal media is discharged from a treatment tank.
Vacuum/slurry removal is used where open discharge pits are impractical such as in freezing climates or indoor installations. With vacuum removal, an access hatch on top of the vessel is opened, and a vacuum device (typically truck-mounted) is then used to remove the media. Contrary to the previous removal method, the vessel is drained prior to media removal. Slurry removal can also be used instead of vacuum removal. This type of removal incorporates an eductor system to draw the media from the vessel. Eductor systems are more portable and allow the people and equipment removing the media to be closer to the vessel. Either method is efficient for removal but compared to direct discharge, vacuum/slurry removal takes longer to remove all the media.
Once the media is removed, the vessel interior is inspected prior to media installation. Media placement in outdoor installations can be accomplished by using a top-mounted access hatch and chute to pour the media into the vessel. Bulk media containers typically consisting of large bags are positioned above the chute, and a bottom bag opening allows the media to flow into the cell. Overhead placement can be done at enclosed installations provided there is access in the building roof. If this is not available, an eductor system is used to slurry the media into the vessel in a manner similar to removal.
While media removal and replacement are fairly straightforward, many system owners prefer to contract these change-out services to system suppliers with equipment and operating expertise. The contracting of these services has been addressed in a number of ways. For instance, as the need for replacement drew closer, the City of Phoenix elected to bid the replacement media and services. The city removed the media from one of its well site vessels after two years of operation and contracted USFilter to supply and install new GFH® media. A second on-site vessel will be replaced in the coming months.
The media is allowed to drain until dry, to minimize shipping costs.
Conversely, the Dare County Water System in Kill Devil Hills, N.C., entered into a media service contract at the time of equipment procurement. This allowed the water system to obtain a set price for media and replacement services in a long-term contract. The six-year, renewable contract with USFilter helps Dare County determine the operating cost of the system and ensures that a skilled staff will carry out the services. Advancements in media design/operation are available under the contract agreement.
The operating benefits of single-use arsenic adsorption systems have been well documented and proven in full-scale operation. The disposable media minimizes on-site operator requirements. Media removal and replacement needs can be fulfilled using a number of methods best suited for specific project requirements. WW
About the Authors
Roman J. Aguirre is GFH product manager and western regional technical sales manager for municipal water treatment applications at USFilter. Based in Colorado Springs, CO, Aguirre joined USFilter nearly four years ago and has worked in the industry for 18 years. He can be reached at 719-622-5324 or email@example.com. Richard R. (Rich) Ross, PE, is eastern regional technical sales manager for conventional products at USFilter. Based in Chalfont, PA, Rich joined USFilter in 1992 as a project engineer/project manager on both municipal water and wastewater products. He can be reached at 215-712-7040 or firstname.lastname@example.org.