Bioreactor: Redirects tile water to an underground bed of wood chips where nitrate is removed naturally by microorganisms. Vegetation on top of the bioreactor can provide other benefits such as wildlife habitat.
What is a bioreactor and how does it work?
A bioreactor consists of a buried pit filled with a carbon source such as wood chips, through which tile water is diverted. Nitrate is removed naturally by microorganisms. Bioreactors reduce nitrate by an average of 43 percent, according to the Iowa Nutrient Reduction Strategy. Vegetation on top of the bioreactor can provide other benefits such as wildlife habitat.
A bioreactor is an edge-of-field treatment process to reduce the amount of nitrate leaving the field from a tile line and therefore improving water quality of the receiving stream. Bioreactors rely on a natural process known as denitrification, in which bacteria in the soil ‘eat’ the carbon in the woodchips, and in the process transform nitrate in drainage water to harmless nitrogen gas (N2), releasing it to the atmosphere. Bioreactors take this naturally occurring process and implement it at a high rate on a small footprint to efficiently improve water quality.
How does tile water get into and out of a bioreactor?
Two control structures are used to divert tile water into the bioreactor, control the depth of water and to control how long each gallon of water stays in the bioreactor. The control structure at the upper end of the bioreactor determines the amount of water diverted. The structure intercepts or “T’s” into the existing field tile. When tile flow exceeds the bioreactor’s capacity, excess water bypasses the system and flows down the existing tile line, preventing any significant restriction of tile flow. The lower structure determines the depth of water within the bioreactor and the retention time.
Bioreactors are generally used to improve water quality where the drainage area is about 40 to 100 acres. The footprint is small, typically covering less than 0.05 acres. Most bioreactors installed in Iowa to date have been 100 to 120 feet long and 10 to 25 feet wide. Because they work well in existing filter strips, bioreactors usually require no land to be taken out of production.
The wood chips may need to be replaced every 10 to 15 years to maintain high levels of nitrate removal. It is estimated Iowa will need 10s of 1000s of bioreactors to reach the goals set out in the Iowa Nutrient Reduction Strategy.
Other resources about uses of bioreactors and water quality results:
- Denitrifying woodchip bioreactors - Iowa State
- How do bioreactors work? - Purdue University
- Bioreactors practice spotlight - NRCS
- Conservation media library - SWCS