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When the U.S. EPA announced it was decreasing the acceptable amount of arsenic in drinking water from 50 to 10 ppb, many U.S. water purveyors were faced with the challenge of designing, constructing and operating new treatment facilities to meet the regulation.
The EPA’s Arsenic Rule, which went into effect on Jan. 23, 2006, is designed to prevent a number of potential health problems that can result from consuming water containing elevated levels of arsenic.
In order to comply with the new regulation, water purveyors must evaluate several alternative operating strategies and treatment options to determine which best fit their needs. This is a particular challenge for water systems in the desert Southwest, upper Midwest and Northeast regions of the country, where arsenic is most prevalent.
Some systems may be able to employ blending to comply with the new regulation. Blending is the process by which a water source that contains an excess amount of arsenic is combined with a water source that contains a minimal amount of arsenic, in essence creating a water supply with an acceptable amount of arsenic.
But for many communities, blending is not a feasible approach, so either new water sources will have to be found, or treatment to remove arsenic from existing sources will be required.
Methods of treatment
The EPA has identified seven different technologies as Best Available Technologies (BATs) for removing arsenic from drinking water. In addition, a number of adsorptive media have been developed in recent years that have been successfully used to remove arsenic from drinking water. Each technology has advantages and disadvantages that must be evaluated in light of the specific characteristics and circumstances of each individual system.
American Water owns subsidiaries that manage municipal water and wastewater systems throughout the U.S. These systems, in turn, supply drinking water and other services to communities in 29 different states.
Experts can help determine the best treatment alternative for each of their affected systems by researching such things as the supply availability, water quality characteristics, space availability, facility design configuration, construction costs, operating costs, and ability to handle and properly dispose of treatment residuals. These elements and more factor into the decision of what technology should be used to effectively and most efficiently remove the arsenic in a given system.
The activated alumina (AA) process involves passing arsenic-contaminated water through a bed of aluminum oxide media in a pressurized column or contactor. The positively charged media adsorbs the arsenic, thus removing it from the water supply.
AA systems require less space than some of the other treatment technologies. In addition, when the media becomes exhausted, either it can be regenerated with a concentrated caustic solution, or disposed of and replaced.
One negative aspect of the process is that regenerable AA technology results in waste that may be considered hazardous and could present disposal issues. Furthermore, while AA may be less expensive to install relative to other technologies, it has fairly limited adsorption capacity, so regeneration and disposal costs can offset the lower installation cost.
In the coagulation/filtration (CF) process, a chemical coagulant is added to the water to continuously provide fresh surface area for adsorption of arsenic. The resulting arsenic-bearing metal hydroxide precipitate is then removed via granular media filtration or microfiltration.
This process serves as an effective means of removing arsenic from groundwater sources, and the operating expense per unit volume of water treated may be lower than other treatment technologies.
High capital cost and space requirements are the main negatives of CF systems. Additionally, they can be more complex to operate than other technologies.
Granular iron media
Iron-based sorbents are becoming established technologies for arsenic removal. Similar to AA, this process involves passing water containing arsenic through a bed of iron-based media in a pressurized column or contactor.
Iron-based sorbents often have been demonstrated to have greater arsenic adsorption capacity than AA. In addition, the spent media consistently has been found to be below the Toxicity Characteristic Leaching Procedure threshold for hazardous waste.
Although not currently recommended as a BAT by the EPA, granular iron media is gaining popularity due to its simplicity, low capital cost relative to other technologies and ability to treat more bed volumes than AA.
Ion exchange (IX) is a physical-chemical process in which ions are exchanged between a liquid solution phase and a solid resin phase. The contaminated water passes, under pressure, through one or more columns packed with resin beads. As water passes through the resin, chloride anions are swapped for arsenic and other anions, thus removing the arsenic from the water supply.
IX is an accepted and fairly common technology used for potable water treatment. In addition, IX is capable of removing other contaminants, such as nitrate, that may be present in water supplies.
The efficiency of the IX process for arsenate removal is negatively affected by the presence of competing anions in raw water. In addition, IX regeneration produces a waste stream that is high in total dissolved solids and requires proper disposal. In fact, the concentration of arsenic in the brine waste can be high enough to classify the waste as hazardous, which may prevent IX from being a feasible option for many community water systems.
Reverse osmosis (RO) is a pressure-driven membrane separation process that removes dissolved contaminants by both physical and electrostatic means. In the RO process, pure water diffuses through semi-permeable membranes, which are enclosed in pressurized cartridges, and contaminants are continuously rejected from the upstream side of the membrane.
RO is very effective for arsenic removal, with typical removal rates of 95% or more. RO also is able to remove a multitude of other contaminants, such as total organic carbon, salts and other dissolved minerals.
RO has stringent pretreatment requirements, which can increase capital costs, while energy costs can result in relatively high operating costs. In addition, typically 20 to 40% of the total feedwater supply is wasted through this process, a particular concern where water supplies are limited. Disposal options for the high volume waste stream from RO also must be carefully considered.
Sun City West, Ariz.
Prior to passage of the Arsenic Rule, the only treatment required for many groundwater supplies in Arizona was the addition of chlorine. Such was the case in Arizona American Water’s Sun City West District, which was served by 10 wells that were evenly divided between two storage and distribution pumping stations. The concentration of arsenic in the individual well supplies ranges from 6 to 25 ppb.
After reviewing the advantages and disadvantages of each technology, as well as the availability of space, centralized versus wellhead treatment configurations, construction and operating costs, waste disposal issues and numerous other factors, American Water determined that the CF process was the best alternative for one of its Sun City West facilities and granular iron media was best for the other. The rated treatment capacities of the CF and granular iron media treatment systems are 7.5 and 2.5 mgd, respectively.
New Jersey American Water’s Frenchtown District is served by four wells, each of which pumps directly into the distribution system. Because of the distance between wells, American Water determined that individual wellhead treatment systems would be more cost-effective than construction of a centralized treatment facility.
A granular iron media treatment system was installed at the largest capacity well station. However, insufficient space was available to accommodate similar systems at the other well stations, so New Jersey American Water pilot tested a new kind of adsorption process that employs zirconium-based media within small cartridge vessels. This approach allowed New Jersey American Water to comply with the Arsenic Rule, while minimizing the cost of constructing and operating arsenic treatment facilities in its Frenchtown district.
California American Water’s Larkfield Water Treatment Plant was designed to remove manganese from well supplies in its Larkfield District. Through pilot testing, it was demonstrated that the existing treatment process could be modified to remove arsenic from the existing well supplies as well.
Specifically, ferric chloride was added to the raw water, which provided an adequate level of iron to facilitate arsenic removal through iron filtration. As a result, California American Water will be able to comply with the arsenic standard at a minimum cost to its customers.
One of the side effects of removing arsenic from drinking water supplies is that arsenic- bearing treatment residuals are produced. While some methods of arsenic removal produce more waste than others, waste is still a main concern for those responsible for disposing of it. Depending on future regulations, the cost of disposing of arsenic treatment wastes may increase, and certain treatment technologies may no longer be feasible for some systems.
There are a number of technology alternatives available for complying with the new arsenic MCL. However, no single technology is the best choice for every installation. Numerous factors need to be considered to determine the best treatment alternative for a given water supply, including the quality of a water supply and whether there are other contaminants or constituents present that may influence treatment efficiency.
Thorough planning and evaluation of alternatives will ensure that drinking water utilities will comply with the Arsenic Rule in the most reliable and cost-effective way possible.