Locals build wetlands to treat leachate

Metro Waste Authority (MWA) of Des Moines, Iowa, has developed the state’s first constructed wetlands and prairie to naturally treat leachate. The system handles approximately 4.4 million gallons of wastewater each year and is projected to save more than $5 million over the next 30 years.

As a quasi-governmental organization serving Polk County and portions of adjoining counties, MWA is governed by city council members from 16 communities and a member of the board of supervisors from Polk County, Iowa’s most populated county. For 31 years, its core business has been operating the state’s largest landfill, Metro Park East (MPE).

In 1998, MWA’s board was investigating ways to reduce its operating costs. To comply with state law, the agency was treating the landfill’s leachate by hauling it to area wastewater treatment plants. However, the environmentally risky process of hauling the leachate cost approximately $250,000 per year. To eliminate hauling off-site and to reduce operating costs, MWA designed a Constructed Wetlands Leachate Treatment Facility. The facility is the only one of its kind in Iowa, and one of just a few in the nation.

“The cost of hauling the leachate and treating it at an off-site facility was higher than [the cost of] the wetlands treatment system,” says Mark Gray, a member of the MWA board of directors and a city council member from Ankeny. The money saved will help fund educational programs about recycling, household hazardous waste and other solid waste disposal and collection issues.

MWA’s wetlands facility has a piping system under the buried waste in the landfill to collect the leachate and pump it through a series of wetlands cells. Each cell contains a combination of plants that naturally removes contaminants in the wastewater.

After the leachate has been treated in the wetlands cells, the effluent is sprayed onto 4.5 acres of constructed prairie. The prairie plants finish the purification process and return the water back to the environment through a combination of evaporation and transpiration (the release of water vapors by plants). The wetlands facility does not discharge any of the treated leachate into surface or groundwater.

“Handling leachate creates environmental and financial problems because, even after it’s been processed at a treatment facility, the by-product has to be handled,” Gray says. “The wetlands treatment system promises to solve both these issues.”

When MPE is eventually closed, the wetlands system will reduce post-closure costs and help MWA avoid an increase in leachate disposal rates. “Because this is an entirely closed system, this is a long-term solution,” says Kip Headley, an MWA board member from Clive.

Weather conditions have discouraged other landfills in the northern United States from constructing similar facilities because the plants will be dormant for about five months of the year. MWA worked with local engineering firm Foth & Van Dyke along with Walkerton, Ind.-based wetlands consultant J. F. New & Associates to design a system in which the leachate is continuously routed through the wetlands cells during the winter and stored in three lagoons until spring. When the weather warms, the treated effluent can be applied to the prairie.

The wetlands facility, which took more than two years to plan and build, opened in June.