This truck and trailer haul
arsenic 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.
Allowable levels of arsenic in drinking water were reduced by the Environmental
Protection Agency in 2006 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 in 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of water withdrawn can affect the levels of arsenic found in our groundwater.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 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 (WWTP).
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
agriculture lands and must be disposed of in landfills at an additional cost of
$4-$5 per ton.
“Unfunded federal mandates often have unintended
consequences on the local level,” said 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,” the Mayor said.
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.
Mesquite Hills well site showing
arsenic remediation vessels, backwash settling tank and solids drying bin.
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 inflow 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.
By Roger Biggs, Utilities Manager City of Cottonwood
