The Village of Blissfield, Mich., located in southeast Lenawee County, experienced seasonal high nitrate levels in its drinking water source—the River Raisin. The elevated nitrate levels were caused by the combination of agricultural fertilizer used in the watershed and runoff
from storm events.
In March 2000, the village built a new water treatment plant to serve its 3,300 residents. The plant uses a conventional treatment process of flocculation, sedimentation, filtration and chlorination, but this process does not significantly reduce nitrate concentrations from the river water. At the time of plant construction, the nitrate problem was well understood, but priorities were set to first construct a new water treatment plant. Then in a separate follow-up project, the problem of nitrate removal would be addressed.
Nitrate is an inorganic compound that can be a natural or man-made contaminant in drinking water. Nitrate (NO3), and its chemical cousin Nitrite (NO2), can cause methemoglobinemia or “blue baby” disease.
The Federal Safe Drinking Water Act defines 10 mg/L parts per million (ppm) of nitrate concentration as the maximum contaminant level (MCL). Nitrate is considered a priority contaminant in concentrations above 10 mg/L. The Environment Protection Agency (EPA) requires public health advisories when this limit is exceeded. In the past, the Village of Blissfield was periodically in violation with the EPA for nitrate concentrations above 10 mg/L, a real nuisance as well as a health and safety hazard to residents. Thus, a means of reducing nitrates needed to be designed.
The initial treatment design was the construction of a raw water reservoir that would store safe but untreated water from the River Raisin. During high levels of nitrates in the river, pumps filling the reservoir would be shut off and the safe water stored in the reservoir would be used to supply the treatment plant. As nitrate levels
in the river subsided, pumps would
be turned back on, refilling the reservoir. The continuous use of
a reservoir also would enhance treatment plant operations by making the characteristics of the raw water more uniform.
Another treatment alternative studied was the use of elevated storage for treated water. In order to have a sufficient volume of treated water to serve the residents during high nitrate concentrations, at least five million gallons of available storage would have been required—an adequate amount of treated water to last approximately four to five days.
However, efforts by the village to secure a practical site for a reservoir were unsuccessful. A new elevated tank with four to five days worth of storage would be prohibitively expensive and very difficult to use effectively. Therefore, new alternatives were examined and considered by the village for feasibility.
Anion exchange was chosen by the village as the preferred method to treat River Raisin water for nitrates. Ion exchange is a proven technology commonly used in residential water softeners to treat hard water as well as in industrial processes to remove metals and other contaminants from wastewater. At Blissfield, anions (negatively charged nitrate and sulfate ions) are selectively removed in the treatment process.
According to Bethel Skinker, district engineer at the Michigan Department of Environmental Quality in Jackson, Mich., the use of anion exchange as a treatment process for nitrates in drinking water is unprecedented.
“It is a first-of-its-kind in the state
of Michigan,” she said.
Anion exchange systems work by using special resin media within pressurized tanks. Filtered water is pumped through tanks where the media removes the nitrates from the water. After the media removes a given amount of nitrate it becomes exhausted and must be regenerated using a salt solution that “exchanges” chloride ions with the nitrate and sulfate ions stuck to the media. This regeneration restores the media allowing it to
Sustainable Design Benefits
Anion exchange is a proven, relatively inexpensive water clean-up technology. It is useful and effective in reducing nitrates in a community drinking
“The Village of Blissfield is very satisfied with the design of the nitrate removal process,” said James Wonacott, administrator for Blissfield Village. “Our goal was to provide our citizens with the cleanest, safest drinking water possible and to meet the regulatory limits using the best available technology.”
According to Wonacott, another advantage of the design is that the system can be turned off and on during and after the seasonal high levels of nitrates in the River Raisin. “This flexibility allows the village to control operating costs,” he added.
The draw towards the use of anion exchange in this application is that it is not complex—merely an innovative use of old technology. Being a “tried and true” treatment technology meant that the village had limited training costs and time associated with the system start-up.
The use of this proven technology demonstrates that innovative designs often need not be complex.