Principal Investigator: John Moncrief
Organization(s): University of Minnesota Department of Soil, Water, and Climate
Sponsor: Clean Water Legacy Act
Award Amount: $237,760
Start Date: 6/30/2009 | End Date: 6/30/2011
Project Manager(s): Mark Dittrich (Mark.Dittrich@state.mn.us)
FINAL REPORT: Field Evaluation of a Woodchip Bioreactor (PDF: 547 KB/ 24 pages)
This study examined two existing woodchip bioreactor systems located in the Cannon and Zumbro River watersheds to assess their ability to improve subsurface drainage water quality at the field scale. A woodchip bioreactor consists of a trench about 100 to 200 ft long, 4 to 6 ft deep, and 2.5 to 3 ft wide filled with woodchips. The water level is controlled to cover the entire woodchip depth to maintain anaerobic conditions (i.e. low oxygen). The trench can be covered with a layer of soil to allow limited field operations and crop growth.
This study built upon on data gathered at these two sites by extending the duration of monitoring, analyzing for additional contaminants, and establishing cause and effect for the efficacy of woodchip bioreactors.
Part 1: Denitrification in Field Anaerobic Bioreactor Under Rainfall and Snowmelt Regimes
Several conservation practices have been promoted to reduce or mitigate nitrate loss to waterways. Among them, a woodchip bioreactor system has been tested in several locations in the Midwest and has yielded reasonable reductions of nitrate concentration and load. The system is based on using woodchips as a carbon source and the ability of certain bacteria (facultative anaerobic denitrifiers) to use nitrate for respiration under low oxygen conditions. These microorganisms “eat” the carbon produced by the woodchips, and transform the nitrate from the water into nitrogen gas, which exits the bioreactor into the atmosphere.
Denitrification is a microbial metabolism through which nitrate is transformed to nitrogen gas with several intermediate compounds in absence of oxygen.
By facilitating this process, bioreactors have been shown to reduce the amount of nitrate that moves from subsurface drainage tile into adjacent streams and rivers.
A bioreactor was evaluated at the edge of an agricultural field in Dodge County Minnesota. Crop rotation included one year of soybeans followed by two years of corn. Patterned tile is at 80 ft. spacing and 4 ft. depth, which is common for subsurface tile in this area.
The monitored site has a contributing area of 26 acres with a bioreactor of 240 ft. long, 6 ft. deep, and 3.5 ft wide. The bioreactor trench is capped with 2.0 ft. of topsoil over 4.0 ft. of woodchips. The woodchips used in this experiment consist of a mix of maple and red oak. Red oak makes up 12.5% of woodchips by weight.
At this site, the inlet flow box contains three compartments. Flow from the field comes into the first compartment; second compartment directs flow into bioreactor; third compartment is connected to bypass system for high flow rates. The water in the bypass system was not treated by the bioreactor.
Automated equipment collected 8 sub-samples every 3 hours. For more information about the experiment design and monitoring equipment, please refer to the method section of the final report.
Water samples were analyzed for
- Nitrate and nitrite-nitrogen
- Total phosphorus
- Total suspended solids
- Nitrate and nitrite-nitrogen
- Total phosphorus
- Total suspended solids
Other parameters to assess bioreactor efficiency included: hydraulic residence time (HRT)*, temperature, pH, dissolved oxygen, and oxido-reduction potential.
*Hydraulic Residence Time (HRT) provides the length of time a parcel of water was flowing through the bioreactor. HRT is essentially the amount of time that water is held within the bioreactor and in contact with beneficial microorganisms.
The nitrate load reduction occurred during snowmelt, rainfall seasons, and late in the season. There was no difference for nitrate concentrations between soybean year (2009) and corn year (2010). The rainfall patterns and associated tile flow made more difference in terms of nitrate load reduction between those two years than any other parameters. On an annual basis, rainfall and flow quantity are inversely proportional to load reduction. During high flows, nitrate load reduction decreases significantly.
Monitoring included two snowmelt periods (2010 and 2011) and two rainfall seasons (2009 and 2010). Nitrate load reduction during snowmelt period was 26% (2010) and 10% (2011).
Table 1. Nitrate load and load reduction for snowmelt in 2010 and 2011. Bioreactor reduction refers to the load difference between the inflow and outflow loads. Watershed load reduction refers to the amount reduced through the bioreactor reported to the load at inflow and bypass.
Table 2. Flow depth through the bioreactor during snowmelt in 2010 and 2011. Bypass is the difference between inflow and outflow depths.
Impact of Precipitation
Total precipitation for 2009 was 30.4 in., which is less than the 30-year normal precipitation for this area. Total precipitation for 2010 was 41.7 in., which is 8.1 in. greater than 30-year normal.
Total bioreactor flow for 2009 was 9.9 in. with 5.5 in. through the bypass and 4.5 in. through the bioreactor. Total flow for 2010 was 25.3 in. with 15.8 in. through the bypass and 9.5 in. through the bioreactor.
During high flows, nitrate load reduction decreases significantly. From April to November 2009, nitrate load reduction was 48%, but it fell to 21% in 2010 during the same time period due to a significant increase in rainfall.
Rainfall amount in 2010 was 11.3 in. greater than that of 2009. Flow depth and precipitation were extremely high from September to November 2010 decreasing the percent load reduction.
Table 3. Nitrate load for the 2009 and 2010 growing seasons.
The HRT was much smaller in 2010 (14.4 hours) compared to that of 2009 (21 hours); flow rate was faster in 2010 and the nitrate load reduction was 50% smaller. Overall, shorter HRT was associated with smaller load reductions.
Longevity of Woodchips
The longevity of the woodchips is related to the continuous presence of water in the bioreactor; the key is to keep as much woodchip volume as possible under water, as long as possible. The upper 12-18” of 48” woodchip depth experiences the greatest frequency of fluctuating water level, and therefore the highest degradation rate. The bioreactor was established in 2007, and by 2011 the carbon to nitrogen (C/N) ratio of the upper layer 12-18” decreased by 25%. However, the bottom layer decreased only by 1.5% for the same period.
Part 2: Atrazine and Acetochlor Dissipation in A Bioreactor Under Flowing Conditions: Adsorption Reactor Model
The herbicides Atrazine and Acetochlor are commonly used with corn-soybean farming system in the Midwest to control weeds. This experiment examined the ability for a bioreactor to breakdown and remove Atrazine and Acetochlor, and their breakdown products, introduced to the bioreactor at known concentrations.
This project used adsorption reactor models to assess the ability of the bioreactor to dissipate herbicides. It is assumed that the primary mechanism of herbicide transformation is adsorption or adhesion of molecules to the surface of the woodchips.
This experiment explores the adsorbing capacity and defines the limitations of woodchip material with Atrazine and Acetochlor. It provides both field data and computer modeling to confirm the relationship between these two herbicides and woodchip material properties.
This study was located on an agricultural field in Rice County Minnesota. The bioreactor is in a relatively flat field, and is part of a managed pattern-tiled drainage system that captures water from an area of 6.7 acres.
The bioreactor is 90 ft long, 3 ft wide and 6.0 ft deep (2.0 ft top soil and 4.0 ft of woodchip). This field is in a corn-soybean rotation, with soybeans in 2007, 2009 and 2011.
For the herbicide experiment, nearby ditch water was filtered and pumped at a flow rate of 4.0 gpm into the woodchip bioreactor. A known concentration of herbicide was injected into the bioreactor for three cycles each of one-week duration during September-October 2010.
Based on bioreactor volume (5284 gal), the flow rate insured a 24-hour residence time (HRT) before discharging water back into the ditch.
For the computer modeling component, the models selected were the Bohart-Adams and Yoon and Nelson. For justification regarding experimental design and the model selection, please refer to the final report.
Nitrate, Total Phosphorus, and Herbicide Load Reductions
For more details regarding the adsorption model results and conclusions, please refer to the final report.
Supervisor, Clean Water Tecnhical Unit
Conservation Drainage Specialist