A bioreactor is essentially a buried trench filled with a carbon source (commonly wood chips), through which tile water is allowed to flow. The carbon source provides material upon which microorganisms can colonize. Using wood chips as a food source, the microorganisms begin breaking down nitrate in the water and expelling the nitrate as dinitrogen gas (N2), a primary atmospheric component.
The bioreactor has no adverse effects on crop production and is designed in a way that it does not restrict drainage. A control structure determines the amount of tile flow that is diverted into the bioreactor. During periods of high flow, excess water bypasses the bioreactor and continues to flow through the existing field tile.
When used as part of a suite of solutions for achieving water quality goals in an agricultural watershed, the bioreactor offers many advantages for treating sub-surface drainage:
- In most locations, does not necessitate taking any land out of production, as it can be installed below filter strips at the edge of the field.
- Begins removing nitrate immediately upon completion with the first water flow.
- Can be targeted for placement to optimize impact.
- Is readily accepted by producers.
- Can be installed in landscapes in which wetlands cannot be built.
- Offers a cost savings compared with wetland installation, so, combined with targeted wetlands, can reduce the cost of subsurface drainage treatment in a watershed.
What about contaminants?
Methylmercury (MeHg), a water contaminant, has been detected in bioreactor demonstrations in other states. Although the high level of background mercury in the soils of these states is not found in Iowa soils, the issue is being monitored in the ACWA Bioreactor Demonstration project here in Iowa. Learning from previous demonstrations, ISA staff have implemented design and management strategies to prevent MeHg contaminants from entering bioreactor-treated water:
- Design modification: originally designed with field tile 1 ft. higher than the bioreactor floor, recent bioreactor installations kept the floor and field tiles at the same level, to allow more complete drainage and to minimize stagnant water at low flows.
- Management of flow and treatment times: Stop logs in the control structures are adjusted to get a high enough flow rate to not fully reduce nitrate, but leave at least 1-2 parts per million (ppm) in treated water. This limits sulfate reduction in the water, which follows complete nitrate removal and results in sulfate-reducing bacteria and possible methylation of mercury.
- Monitoring: Regular water monitoring takes place to ensure that there is not complete depletion of nitrate in the treated water.