What about Emissions?

As we examine the way bioreactors work - removing most of the nitrate from tile water they treat by turning it into gas - the question naturally arises: How extensive are those emissions? Are they toxic? Are they greenhouse gases?

Because bioreactors hold such promise for nitrogen removal from subserface flow, researchers at the USDA-ARS National Soil Tilth Lab have been studying this practice for almost a decade. An analysis of bioreactor emissions from the 10-year study is being prepared by Tom Moorman for publication in november.

Meanwhile, in the July-August 2009 issue of the Journal of Environmental Quality, "Denitrification in Wood Chip Bioreactors at Different Water Flows," Moorman joined authors Colin Greenan, Tim Parkin, Tom Kaspar, and Dan Jaynes USDA-ARS to report on a separate study whic included as one of its three objectives quantification of the production of N2O - far below most natural systems and much less than the amount that would be released if denitrification by the bioreactor had not taken place and the nitrate had been transported to the river system.

Scientists explain that the wood chip carbon source, the pH, and temperatures in the bioreactors seem to ensure that the nitrate passes through the N2O state of denitrification and allow the process to complete, primarily emitting benign N2 or nitrogen gas, which makes up 78% of Earth's atmosphere.

BIOREACTOR BASICS

Background: To make farming operations productive and viable, most agricultural land located in north central Iowa is artificially drained or tiled. As a result of the tile systems, high concentrations of soluble nutrients like nitrate-nitrogen are reaching Iowa's streams and rivers.

Edge-of-field treatment systems, such as buffer strips, have been installed to reduce contaminant loads in receiving waters. However, as research is showing, outside of large rain events, most water leaving agricultural fields is through subsurface tile flow and is never coming in contact with these filter systems at the surface. To capture subsurface flow, current practices such as wetlands and retention ponds exist, but in addition to their large costs they also require land to be taken out of production. A new practice gaining interest is the use of bioreactors to reduce the amount of nitrate-nitrogen reaching surface waters.

Bioreactor Definition: 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.

Advantages: 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:

What about contaminants?

Methylmercury (mm), 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 mm contaminants from entering bioreactor-treated water:

 

Biotech Dig

The yellow circles are the control structures. The yellow line is where the main field tile is located. The circle on the left side of the picture is the upper control structure which diverts flow from the main tile line into the bioreactor. The tile you see in the lower part of the picture collects the water after it flows through the reactor and feeds it into the lower control structure (circle on right hand side of picture), which then flows back into the main tile line.

Biotech Sketch

Computer model of bioreactor.

Bioreactor being filled with wood chips

Bioreactor as it is being filled with wood chips. The plastic on the side walls is to minimize water moving in and out of the bioreactor.

Bioreactor geo-fabric

After the bioreactor is filled with wood chips a geo-fabric is laid over the wood chips to prevent soil movement into the bioreactor. The soil movement into the bioreactor could potentially ???clog??? the reactor reducing its efficiency. Approximately 18??? of soil was laid over the top of the geo-fabric.

Looking down into the upper control structure, which diverts tile flow into the bioreactor

Looking down into the upper control structure, which diverts tile flow into the bioreactor. Red lines indicate flow direction. Once flow exceeds the bioreactor capacity, water flows over the 2nd set of stop logs and continues down the main field tile.

Looking down into the lower control structure

Looking down into the lower control structure, which controls the level of water within the bioreactor, depending on the height of the stop logs. The treated water flows over the top of the stop logs and back into the field tile.