An Introduction to Passive Nitrogen Removal

It is very likely passive nitrogen removal systems will become more popular in the future. Here’s what you need to know.

An Introduction to Passive Nitrogen Removal
Example passive test system with loamy sand overlying layer of sand/silt/sawdust (Photo credit Barnstable County Health)

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Conventional soil treatment areas are designed to have an aerobic environment for the removal of organic material (BOD) and pathogens. This aerobic environment is also very conducive to nitrification — the biological oxidation of NH4+ to NO3-. The nitrate formed by nitrification is, in the nitrogen cycle, used by plants as a nitrogen source (synthesis) or reduced to N2 gas through the process of denitrification. 

There are two key things needed for denitrification: food (usually carbon) and anoxic conditions. A large variety of heterotrophic bacteria can use nitrate in lieu of oxygen for the degradation of organic matter under anoxic conditions. If O2 is present, however, the bacteria will preferentially select it instead of NO3-. Thus, it is very important that anoxic conditions exist in order for NO3- to be used as the electron acceptor. A carbon source is required as the electron donor in the above equation for denitrification to occur.

Given the aerobic conditions in conventional soil treatment units, denitrification is usually limited. Until recently in residential onsite wastewater systems, denitrification systems were designed to provide an aerobic treatment step and included denitrification processes by recirculation of treated effluent back to the food and carbon-rich anoxic septic tank or processing tank.

Mound systems often have these required characteristics of an aerobic zone in the clean sand followed by a denitrification zone in the original topsoil. In two studies (Magdorf et al., 1974; Eastburn and Ritter, 1984), mound systems were found to reduce nitrogen concentrations ranging from 32 to 70 percent. It is important that the natural in-place soil not be removed as it often has sufficient carbon and anoxic conditions for denitrification to occur.

Passive nitrogen reduction systems are systems that reduce effluent nitrogen using reactive media for denitrification and a single liquid pump, if necessary. This process has been used successfully with agricultural runoff. Starting in the late ‘90s farmers began experimenting with wood chip filters to reduce nitrate in field runoff. In the onsite wastewater field, over the last 30 years, they have been studying such a system at the University of Waterloo. The application of this system has been primarily in commercial and cluster scale. With interest in nitrogen reduction increasing, similar passive systems are being developed and scaled down to the single-family home level. More recently, there has been a growing trend to add a reducing filtration medium, such as wood chips as a carbon source or sulfur/limestone as an alternative electron donor, to an aerobic treatment step.

These processes have been found to reduce nitrogen significantly to a concentration under 5 mg/L. This has typically occurred through a separate subsurface upflow filter that is filled with the carbon-rich media providing surface area for microbes in the anoxic environment. This process has also occurred, combining these systems in the subsurface that may offer lower operation and maintenance than previously utilized options.


The state of Florida has gone to great lengths to study passive systems. Units tested there were able to reduce nitrogen by 95 percent (Anderson and Hirst, 2016). An example of one of the systems studied is shown. For more information see:

Florida has now proposed rules that have a phased-in implementation of in-ground nitrogen-reducing biofilters as a prescriptive design and aerobic treatment units certified under NSF 245.

New York
The state of New York established the Center for Clean Water Technology that identified nitrogen-removing biofilters (NRBs) as a system potentially capable of meeting the goal of a wastewater treatment systems for individual onsite use that are affordable and highly efficient at removing nitrogen and other contaminants. NRBs are a form of passive wastewater treatment, which means they contain few moving parts (e.g., a single, low-pressure dosing pump) and operate largely by gravity, making them low-energy, low-maintenance and, thus, low-cost. Comprised of a sand-based “nitrification layer” underlain by a “denitrification layer” of sand mixed with finely ground wood, the system is installed following a standard septic tank/pump chamber combination and is intermittently dosed by a low-pressure distribution system. See for more information.

The Massachusetts Alternative Septic System Test Center has studied the passive system as well and due to the layering of material has labeled the system “Layer Cake." Their goal is to determine the simplest, most cost-effective modification of a soil treatment system to enhance nitrogen removal on Cape Cod’s “sand spit” where winter temperatures can create a challenge for nitrogen removal. The configurations included designs that incorporated saturated and unsaturated sand/lignocellulose mixtures along with finer textured media mixed with sawdust to test the performance of a moisture-holding denitrification layer in place of a liner.

The preliminary results from prototype and full-scale in-ground NRB suggest that at least 50 percent and up to 90 percent nitrogen removal occurred in the first year of the test. In Massachusetts’ colder climate, nitrogen removal performance degraded slightly in colder winter months for some of these trials. For more information:

Many studies have been completed demonstrating the robustness of the passive nitrogen removal and work continues on design optimization. There is also recent research showing denitrification can occur under aerobic conditions and that special microbes can convert ammonia directly into nitrogen gas (anammox) so we certainly have more to learn. In the future, it is likely passive nitrogen removal systems will become allowable options in local codes and consequently designers, installers and regulators will have another option in their arsenal.

About the author
Sara Heger, Ph.D., is an engineer, researcher and instructor in the Onsite Sewage Treatment Program in the Water Resources Center at the University of Minnesota. She presents at many local and national training events regarding the design, installation, and management of septic systems and related research. Heger is education chair of the Minnesota Onsite Wastewater Association and the National Onsite Wastewater Recycling Association, and she serves on the NSF International Committee on Wastewater Treatment Systems. Ask Heger questions about septic system maintenance and operation by sending an email to


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