New Water Tech Lowers Arsenic Treatment Costs

Posted in: Drinking Water News, United States Water News, Water Technology
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Trucks and trailers are used to haul the arsenic laden brine from the 13 Verde Village water reservoirs to the wastewater treatment plant inside the city limits of Cottonwood. This is a 30 to 40 hour per week task assigned to the wastewater division. (Photo Credit: City of Cottonwood)

Article courtesy of The Verde Independent | June 30, 2015 | The Verde Independent | Shared as educational material

COTTONWOOD — In 2006, the Environmental Protection Agency reduced the allowable levels of arsenic in drinking water from 50 parts per billion (ppb) to 10 ppb.

As a result, the City of Cottonwood was required to install arsenic removal systems at 13 well-sites outside the city and four sites within city limits to meet the new EPA safe drinking water standards for arsenic.

Arsenic is a naturally occurring element found in groundwater across much of the western United States. The naturally occurring levels of arsenic levels found in raw, untreated groundwater in the Verde Valley range from greater than 70 ppb to less than 10 ppb.

Well location, depth, geology and the amount of water withdrawn can affect the levels of arsenic found in our groundwater.

The removal of the arsenic is accomplished by passing water through a vessel that contains a manganese dioxide coated granular or resin media. Over time, the arsenic ions coat the media resulting in a reduction in the efficiency of the resin media’s ability to remove arsenic.

When the efficiency of arsenic removal drops to a preset point, the water flow through the vessel is reversed. This reversed flow, or backwash cycle, strips the arsenic coating from the media, which discharges to the sewer system and is pumped to the Mingus Avenue Wastewater Treatment Plant.

If no direct sewer connection is available, as in the Verde Villages, the arsenic laden backwash is hauled by trailer to a specifically designed sewer connection near the airport.

The purchase and installation of the arsenic removal systems, piping and required electrical and control upgrades cost $4.9 million with ongoing operations and maintenance expenses of $750,000 annually.

Approximately 22 million gallons of water per year are also required to backwash the treatment media.

Another issue resulting from the removal of arsenic is the added costs associated with the disposal of Biosolids. Arsenic coming into the WWTP concentrates in the treated Biosolids. Prior to the implementation of arsenic treatment, the Biosolids produced by the WWTP were applied to agricultural land growing crops such as sod or animal feed. The increased levels of arsenic in the Biosolids produced by the WWTP are now prohibited from being applied to agricultural lands and must be disposed of in landfills at an additional cost of $4-$5 per ton.

Recently, a new well, reservoir and arsenic treatment system were installed for the Mesquite Hills subdivision through a public/private partnership between the City and VRE Cottonwood LLC. This new system has the capability of removing arsenic from the backwash water before it is discharged to the sewer system.

The backwash water is held in a cone shaped tank while the arsenic solids settle to the bottom. The arsenic solids are then drained from the tank, dried and hauled to a landfill for disposal. The water in the tank is then returned to the start of the treatment process.

By employing this type of process, the volume of water required for arsenic removal is greatly reduced and the volume of arsenic going to the WWTP is also reduced.

By reducing the in-flow levels of arsenic to the WWTP, the levels of arsenic in the Biosolids can be maintained at a level that allows for disposal on agricultural lands, which is a cost reduction to the city. Utility Department Operations staff are closely monitoring the quality of the water produced, the arsenic levels of the backwash water, and the quantity of arsenic solids to be land filled at the Mesquite Hills well-site. If this process is successful, the Utility Department will begin installing similar equipment at other well-sites. The goal is to maintain compliance with the EPA safe drinking water requirements, while maximizing the disposal of Biosolids and the efficient use of precious water resources.

“Unfunded federal mandates often have unintended consequences on the local level,” said Cottonwood City Mayor Diane Joens. “When the arsenic mandate became law, it also impacted wastewater compliance, causing the city to landfill Biosolids rather than land applying them. This is an added cost to the ratepayers and doesn’t make sense as a long term solution. It is good that technology is going to help us return to land application. Filling up our landfills this way is not sustainable.”

 

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