Frequently Asked Questions


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1. What is a wetland?
Wetlands provide natural ecological services, such as flood water storage, nutrient (nitrogen or phosphorus) storage, erosion control, improved water quality and habitat for wildlife. In regards to nitrogen pollution, wetlands can hold onto nitrogen or remove it completely through a process called denitrification, thereby preventing contamination of downstream waterways. Wetlands are transitional between terrestrial and aquatic ecosystems, where the water table is usually at or near the surface or the land is covered with shallow water.
2. What are constructed wetlands?
Constructed wetlands are artificial wetlands that are created to resemble natural systems and provide the same ecological services.  They are usually alongside ditches or streams and they allow for nitrate-nitrogen removal through denitrification to help protect downstream waterways.
- Constructed Wetlands for Wastewater Treatment and Wildlife Habitat, EPA 1993
3. What is a Denitrifying Bioreactor?
A denitrifying bioreactor is an artificially constructed system to mimic the functions of riparian zones and wetlands. The two main types are beds and walls places in the flow path of water leaving from the agricultural runoff. Both have an added carbon source(often woodchips) to support denitrifying microorganisms. The wood chips along with the water flowing through the bed or wall creates an anaerobic environment in which bacteria can transform the nitrogen in the water into nitrogen gas.

- Video describing Denitrification beds and walls by Science Learning and Louis Schipper. Posted July 2013.

denitrification wall and bed

3.1 Denitrification beds are large lined holes dug into the ground and filled with wood chips. Water from tile drains or treated effluent are fed into one end of the bed and removed from the other end. They can vary in size depending on the amount of nitrate that needs to be removed.


denitrification wall

3.2 Denitrification walls intercept diffuse groundwater flow high in nitrate concentrations. They are constructed by digging a trench down below the water table. The excavated soil is mixed with wood chips and returned to the trench so that groundwater flows through the soil/woodchip mix which promotes denitrification.
4. What is denitrification?
Denitrification is the microbial conversion of soluble nitrate –nitrogen to nitrogen gas. These micro-organisms require anaerobic conditions where there is little to no oxygen present and they also require carbon as the energy source.
5. Why is denitrification important?
When nitrogen is in excess of plant demand, water draining from agricultural fields can have high nitrogen concentrations.  Excess nitrogen decreases the quality of drinking water and contributes to the problem of eutrophication in surrounding waters, degrading natural ecosystems. The process of denitrification removes nitrogen from water by transforming it into nitrogen gas which is then released to the atmosphere.
6.  What is the purpose of denitrifying bioreactors?
The purpose of denitrifying bioreactors is to help improve water quality by reducing the nitrate-nitrogen content of groundwater or subsurface drain flow. These denitrifying bioreactors are installed in the subsurface where they will intercept flow high in nitrate-nitrogen.  In these bioreactors provide denitrifying bacteria with an energy source from a solid C substrate, often fragmented plant material such as wood chips.
7.  Why wood-chips?
Woodchips decay slowly and provide an energy source for the process of denitrification. Bioreactors constructed with wood chips will remove nitrate for 15 years or more. Large wood chips help maintain water flows with a decline in nitrate removal performance.
8.  How effective are bioreactors at reducing nitrogen levels?
Denitrification rates among bioreactors can vary based on the nitrogen concentration of the water being received, soil temperature, the carbon substrate used, the geometry of the structure, and the denitrifying organisms.  In general, beds tend to have higher removal rates than walls, but both can be very effective. Many bioreactors support nitrogen removal rates of over 90%. Bioreactors can also support denitrification over long periods of time. Currently, the longest bioreactor operating in the field has been in use for over 15 years and there are no known examples of bioreactors that have stopped working due to carbon depletion.
9.  Are they costly to install and maintain?
Denitrifying bioreactors are very cost effective when compared to other approached for managing nitrogen.  When bioreactors are installed on farms where farmers already have ready access to machinery and wood the installation costs can be greatly reduced. Once installed, bioreactors require minimal maintenance.
10. What is eutrophication?
Eutrophication of waterways results from increased nutrient levels that enhance algal and plant growth which subsequently leads to decreases in oxygen concentration. These conditions cause harm to the native species of the ecosystem. Some effects include increased incidences of toxic phytoplankton blooms, increased sea weeds, loss of sea grasses, increased disease in fish, crabs, and lobsters and increased production of greenhouse gases.
11. Why is excessive nitrogen addition a problem?
Nitrogen is a problem because:
a) Increased input of nitrogen through watersheds to coastal waters can cause eutrophication often noted by harmful algal blooms, declines in oxygen, and fish kills.
b) Increased productions of nitrous oxide, a greenhouse gas
c) Loss of crucial soil nutrients like calcium and potassium
d) Acidification of soils and of waters of streams and lakes
- Impacts of Atmospheric Pollution on Aquatic Ecosystems, Issues in Ecology 2004
- Water in a Changing World, Issues in Ecology 2001
- Nutrient Pollution of Coastal Rivers, Bays, and Seas, Issues in Ecology 2000
- Human Alteration of the Global Nitrogen Cycle: Causes and Consequencs, Issues in Ecology 1997
- Interactive graphic of the nitrogen cycle by Science Learning and Louis Schipper, July 2013.
12. What are hypoxic and anoxic conditions?
When the oxygen levels are low in marine ecosystems, the conditions caused for organisms are known as hypoxic conditions. Whereas if there is no oxygen left for organisms to utilize, such a condition is known as anoxic condition.
13.  What are dead zones?
The loss of fisheries, spawning habitats and creation of multitudes of low oxygen zones across the globe. These zones are known as “dead zones” and are often attributed to high nitrogen concentrations in coastal waters.
14. What is a Limiting Nutrient?
A limiting nutrient is the nutrient that controls the growth of a population.  In inland waterways, the limiting nutrient is phosphorus while in coastal waterways, it is nitrogen.
15. What is Point source of pollution?
Any discernible, confined, and discrete conveyance, including but not limited to, any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock concentrated animal feeding operation (CAFO), landfill leachate collection system, vessel or other floating craft from which pollutants are or may be discharged. This term does not include return flows from irrigated agriculture or agricultural stormwater runoff.
- Point source contamination, Science Learning 2009
- NPDES Glossary, EPA
16. What is a Non-point source of pollution?
Nonpoint source pollution results from a wide variety of human activities on the land. Each of us can contribute to the problem without even realizing it. Nonpoint source pollution generally results from land runoff, precipitation, atmospheric deposition, drainage, seepage or hydrologic modification.
- Non Point source contamination, Science Learning 2009
- What is Nonpoint Source Pollution? EPA
- Nonpoint Pollution of Surface Waters with Phosphorus and Nitrogen, Issues in Ecology 1998