United States Department of Agriculture 605-CPS-1

NRCS, NHCP

September 2020R

NRCS reviews and periodically updates conservation practice standards. To obtain the current 

version of this standard, contact your Natural Resources Conservation Service State office or 

visit the Field Office Technical Guide online by going to the NRCS website at 

https://www.nrcs.usda.gov/ and type FOTG in the search field. 

USDA is an equal opportunity provider, employer, and lender.

Natural Resources Conservation Service 

CONSERVATION PRACTICE STANDARD 

DENITRIFYING BIOREACTOR 

CODE 605 

(no)

DEFINITION 

A structure that uses a carbon source to reduce the concentration of nitrate nitrogen in subsurface

agricultural drainage flow through enhanced denitrification. 

PURPOSE 

This practice is used to achieve the following purpose: 

• Improve water quality by reducing the concentration of nitrate nitrogen in flow from subsurface

agricultural drainage systems 

CONDITIONS WHERE PRACTICE APPLIES 

This practice applies to sites where there is a need to reduce the concentration of nitrate nitrogen in the

flow from subsurface drainage systems. 

This practice does not apply to underground outlets from practices, such as terraces, where the drainage

source is primarily from surface inlets. 

CRITERIA 

General Criteria Applicable to All Purposes 

Performance and capacity 

Design the bioreactor to meet one of the following criteria: 

• Provide capacity to treat at least 15 percent of the peak flow from the drainage system. Disregard

flow from surface inlets when calculating design subsurface drain flow for capacity purposes.

Design the bioreactor hydraulic retention time for a minimum of 3 hours at the peak flow capacity.

Account for the porosity of the media and use the average depth of flow through the media. 

• Design capacity, hydraulic retention time, and nitrate removal performance to achieve an overall 20

percent nitrate reduction from the average annual flow of the subsurface drainage system, taking

into account bioreactor-treated flow and bypass flow. 

Media chamber 

Design the bioreactor carbon media for an expected life of at least 10 years. Select carbon media that are

relatively coarse, have a low percentage of fines, have a low content of soil or other contaminants, and

that will be durable enough to last for 10 years or can be removed and replaced, as needed. Before

placing the carbon media in the chamber, line the walls and floor with a geotextile or plastic membrane to

prevent migration of soil into the chamber. 

The most commonly used carbon media is wood chips. Use the following criteria when selecting wood

chips: 
• Wood species with low tannin content 

• Ninety percent of chips are 1 to 2 inches in length 

• Do not use rot-resistant woods such as cedar or redwood 

• Do not use wood that has been painted or treated for rot resistance 

• Do not use chips that contain significant amounts of sawdust, leaf litter, bark, or soil 

Design the media chamber to prevent development of preferential flow paths by distributing inflow across

the entire width of the chamber. For chambers with a length to width ratio wider than 4:1, use a multipleheader distribution system so the width served by each header is no greater than 25 percent of the

chamber length. 

Water control structures 

Design the bioreactor inlet and outlet water control structures to provide the required capacity and

hydraulic retention time. Use the criteria in NRCS Conservation Practice Standard (CPS) Structure for

Water Control (Code 587) to design the water control structures. 

Set the elevation of the upstream water control structure to prevent damage to upslope crops from an

elevated water table. If the topography is such that the planned water table upstream of the bioreactor will

negatively affect crop performance, manage water levels at the upstream end of the bioreactor using the

criteria in NRCS CPS Drainage Water Management (Code 554). 

Design the water control structure to safely bypass flows in excess of the design capacity. Include a lowlevel orifice in the outlet water control structure to assure the media chamber drains in a maximum of 48

hours during periods of low or no flow. In addition, provide an outlet to completely drain the media

chamber to facilitate operation and maintenance. 

Protection 

To minimize interference with cropping, locate the bioreactor far enough away from crop fields to allow

field equipment to operate without driving over the bioreactor chamber and at an elevation to allow proper

drainage of crop fields. To ensure that the carbon media is not compacted by equipment driving over it,

provide signage and/or fencing to prevent damage. 

Grade the area around the reactor to divert surface runoff away from the reactor and ensure that water

does not pond around the reactor. Use the criteria in NRCS CPS Critical Area Planting (Code 342) to

protect all noncrop areas disturbed by construction with seeding or mulching as soon as possible after

construction. For denitrifying bioreactors installed in existing filter strips or other conservation practices,

revegetate disturbed areas according to the seeding requirements of the conservation practice disturbed

by construction. 

CONSIDERATIONS 

When planning, designing, and installing this practice, consider— 

• Denitrifying bioreactors can work effectively with other practices and management systems to

achieve further reductions in nitrate nitrogen levels leaving crop fields. Examples include— 

• NRCS CPS Nutrient Management (Code 590), 

• NRCS CPS Cover Crop (Code 340), and 

• NRCS CPS Drainage Water Management (Code 554). 

• Determining the normal nitrate levels in the subsurface drain discharge water prior to design work

will help to establish design parameters. This information will also be helpful in any monitoring that

occurs after installation. 

• Bioreactors work by supporting colonies of bacteria on the carbon media. Inoculating the media

during installation will hasten the startup process. Inoculation can be as simple as adding carbon

media and effluent from an existing bioreactor. 

-CPS-2

NRCS, NHCP

605

September 2020R
• Piping in the reactor can become clogged over time. Consider including vertical cleanout pipes to

allow for “jetting out” any clogged pipes. 

• It is important to maintain porosity in the carbon media to ensure that water flows evenly through

the media and does not short circuit the system. Mixing gravel with the required amount of carbon

media can help maintain the necessary porosity. 

• During startup, the flow from a denitrifying bioreactor can contain leached organics and some plant

nutrients. It may be necessary to take actions to mitigate any harmful effects if flow from the reactor

goes directly into a stream or other water body. 

• Denitirfying bioreactors can provide benefits year around, not just during the cropping season.

Therefore, if planned water elevations will not negatively affect crops or field operations, maintain

design water elevations throughout the year. 

• When revegetating areas disturbed by construction, consider including plant species that support

pollinators and other beneficial insects. 

PLANS AND SPECIFICATIONS 

Develop plans and specifications for the denitrifying bioreactor that describe the requirements for applying

the practice to achieve its intended purpose. The plans and specifications must be site-specific and as a

minimum include— 

• A plan view of the layout of the denitrifying bioreactor and associated components. 

• Typical cross sections of the bioreactor. 

• Profiles of the bioreactor, including inlets and outlets. 

• Details of required structures for water level control. 

• Material specifications for the bioreactor media. 

• Seeding requirements, if needed. 

• Construction specifications that describe in writing the site-specific installation requirements of the

bioreactor and associated components. 

OPERATION AND MAINTENANCE 

Provide an operation and maintenance plan and review this with the land manager. Specified actions

should include normal repetitive activities in the application and use of the practice, along with repair and

upkeep of the practice. The plan must be site-specific and as a minimum include— 

• Planned water level management and timing. 

• Inspection and maintenance requirements of the bioreactor and contributing drainage system,

especially upstream surface inlets. 

• Requirements for periodic monitoring of the bioreactor media and replacement or replenishment of

media as needed. 

• Any required monitoring and reporting criteria. 

REFERENCES 

Christianson, L. E., A. Bhandari, M.H. Helmers, and M. St. Clair. 2009. Denitrifying Bioreactors for

Treatment of Tile Drainage. In: Proceedings of World Environmental and Water Resources Congress, May

17–21, 2009. Reston, VA: ASCE Environmental and Water Resources Institute. 

Christianson, L., A. Bhandari, and M. Helmers. 2011. Potential Design Methodology for Agricultural

Drainage Denitrification Bioreactors. In: Proceedings of World Environmental and Water Resources

Congress, May 22–26, 2011. Reston, VA: ASCE Environmental and Water Resources Institute. 

-CPS-3

NRCS, NHCP

605

September 2020R
Christianson, L., M. Helmers, A. Bhandari, K. Kult, T. Sutphin, and R. Wolf. 2012. Performance Evaluation

of Four Field-scale Agricultural Drainage Denitrification Bioreactors in Iowa. Transactions of the ASABE

55(6):2163-2174. 

Cooke, R. and N. Bell. 2014. Protocol and Interactive Routine for the Design of Subsurface Bioreactors.

Applied Engineering in Agriculture 30(5): 761-771. 

Woli, K.P., M.B. David, R.A. Cooke, G.F. McIsaac, and C.A. Mitchell. 2010. Nitrogen Balance In and

Export from Agricultural Fields Associated with Controlled Drainage Systems and Denitrifying Bioreactors.

Ecological Engineering 36:1558-15

-CPS-4

NRCS, NHCP

605

September 2020R