The primary purpose of a wood chip bioreactor is to remove nitrates from subsurface tile drainage water at the edge of a field prior to the water entering a ditch or a stream. Bioreactors do not interrupt any practices occurring in the field, nor do they change the effectiveness of tile drainage systems. A wood chip bioreactor is considered an edge-of-field practice.
Figure 51. A schematic of a wood chip bioreactor located at the edge of a farm field. Credit: Laura Christiansion, University of Illinois.
A bioreactor can be placed anywhere that a buffer strip and a subsurface drainage system exists. They should be placed where there is a high nitrogen runoff concern.
High nitrate levels can contaminate drinking water and impact the health of downstream waterbodies. High nitrogen levels have been associated with human health concerns, toxicity of harmful algal blooms in freshwater systems, and hypoxic (dead) zones in marine water systems.
• Annual wood chip bioreactor nitrate load removal averages approximately 33 percent in tile drainage applications (Christianson et al., 2012). Nitrogen removal rates average 4.7 gallons of nitrogen removed per bioreactor (Addy et al., 2016). The design life of denitrifying wood chip bioreactors is estimated to range from approximately seven to 15 years (Christianson et al., 2012).
• A study in Canada using wash water found that 6 percent to 16 percent of phosphorus was removed with a wood chip filter. Almost all of the TP removal was associated with PP (91 percent removal), and the dissolved phosphorus fraction was relatively unchanged. When coupled with a sediment basin, up to 91 percent of particulate phosphorus could be removed (Choudhury, et al., 2016).
- Two drainage control structures are needed to regulate water moving into and out of the bioreactor.
- Most bioreactors are less than 100 feet long and less than 50 feet wide. The bioreactor depth should be equivalent to the depth of the existing field tile.
- A number of materials can be used for a denitrifying bioreactor. Variances exist in regards to the costs and mixtures to remove phosphorus. Examples of other materials used to denitrify are straw, coal, flax harl, and sawdust.
- Lifespan estimates range from 10–30 years. Periodically, inspection should occur to ensure that the water and wood chip level are satisfactory. On occasion, wood chips decompose and require replenishment.
- Wood chip bioreactors can withstand moderate traffic over their surface, and they do not impede trafficability in the buffer zone.
Installation costs for a bioreactor average about $8,000, which includes control structures, wood chips, and excavation costs.
Figures 52a, 52b, 52c, and 52d. Examples of wood chip bioreactors during and after construction. Credit: go.osu.edu/CWaS.
A bioreactor is a covered pit filled with a high-carbon material such as woodchips. As excess water leaves the field through tile drainage, the water flows through the bioreactor before it completely exits the site.
Water enters the main part of the bioreactor with the wood chips and the denitrifying microorganisms. There, the nitrates are separated from the water as harmless nitrogen gas. The water with a reduced nitrogen load then exits the bioreactor through the outflow control structure, where it enters surface water, just as normal tile drainage would. Denitrifying microorganisms rely on the carbon in the wood chips as a food source, so the bioreactor is able to self-sustain its own denitrifying microorganism colony.
During times of high water flow, the bioreactor will fill to capacity. The remaining water will be diverted by the control structures, allowing excess drainage to enter surface waters untreated. This allows expedient drainage of the field, while still removing some of the nitrates.
Extensive design considerations can be found NRCS Standard 605: Denitrifying Bioreactor