Denitrifying bioreactors

What

Denitrifying bioreactors are an approach where solid carbon substrates are added into the flow path of contaminated water. These carbon substrates (often fragmented wood-products) act as a carbon and energy source to support denitrification, the conversion of nitrate to nitrogen gases.

There are many different types of denitrifying bioreactors and designs for different purposes. They can be retrofitted to existing drainage networks or implemented at time of construction. Some common types of denitrifying bioreactors are provided in the table below.

Examples of different types of denitrifying bioreactors. Table adapted from Shipper et al. (2010).
Name	Description	Objective	Setting	References
Denitrification wall	Carbon substrate placed into the upper 1–2 m of shallow groundwater in a trench perpendicular to groundwater flow path towards surface water.	Removal of nitrate from groundwater prior to surface water recharge.	Down gradient of localised sources of nitrate-enriched groundwater. At focal points of groundwater flow.	Robertson et al., 2000. Schipper et al., 2005. Fahrner, 2002.
Denitrification wall	Carbon substrate placed into the upper 1–2 m of shallow groundwater in a trench on either side of a subsurface drain.	Removal of nitrate from groundwater before entering drainage network.	In subsurface drained agricultural land.	Jaynes et al., 2008.
Denitrification bed or filter	Container (varied length and breadth dimensions but typically 1–2 m deep) filled with solid carbon substrate. Effluent or drainage water enters and exits in pipes. Beds may be lined.	Removal of nitrate from wastewaters or artificial subsurface drainage from agricultural fields.	Concentrated discharges that have high nitrate concentrations such as from subsurface drainage or treated wastewater.	van Driel et al., 2006a. Schipper et al., 2010.
Upflow bioreactors (subset of denitrification beds)	Carbon substrate in container with lined sides, open to groundwater flow at bottom. Groundwater flows towards carbon substrate with elevated saturated hydraulic conductivity and is discharged to adjacent stream via pipe.	Removal of nitrate from groundwater prior to surface water recharge.	Adjacent to surface water where groundwater is shallow and aquifers have lower conductivities than added carbon substrate.	van Driel et al., 2006b.
Stream-bed bioreactor (subset of denitrification beds)	Container (varied length and breadth dimensions but typically 1–2 m deep) filled with solid carbon substrate installed in the base of a stream.	Reducing nitrate concentrations in streams.	Streams and drainage ditches	Robertson and Merkley, 2009.
Denitrification layer	A horizontal layer of woodchips that receives nitrified effluent from above.	Reduce nitrate leaching vertically to groundwater.	Below septic wastewater drainage field that passed through a sand/gravel filter or other land-based effluent treatment system.	Robertson et al., 2000. Schipper and McGill, 2008.

Example of a denitrifying bioreactor bed to treat water discharged from an agricultural field with subsurface artificial drainage.
Image source: Carsten et al., 2020

Why

These structures provide the ideal conditions for soil microbes to remove nitrate from water through denitrification. Microbes use organic carbon to derive energy and under anoxic conditions, nitrate is removed in the process.

Temperature can be a limiting factor when it comes to denitrification rates. Denitrification occurs at all temperatures but is slower in colder conditions and faster as it gets warmer. The rate increases exponentially between 15 °C and 30 °C, and peaks between 23 °C and 27 °C.

The pH also affects both how fast denitrification can occur (rate) and the products of denitrification. See SCIENCE – CHEMICAL PROCESSES for more information.

Examples

Hauraki Plains

While denitrifying bioreactors are being used overseas (US, Europe) to reduce nitrate loss to surface waters, overseas designs may not be directly applicable to New Zealand conditions. This is largely due to differences in drainage system design, climate, and land use (particularly the predominance of all-year grazing in NZ). Research trials in New Zealand are looking at ways to adapt the design and operation of bioreactors to increase adoption in NZ for treating drainage waters.

Lincoln Agritech is currently working with ESR and AquaLinc Research Ltd on a science project in the Hauraki Plains. The project, as part of the larger “Enhanced Mitigation of Nitrate in Groundwater” programme, investigates using a woodchip denitrifying bioreactor to reduce nitrate loads from pastoral lands.

Waituna Lagoon

This trial in the Waituna Catchment by NIWA and DairyNZ found the efficacy of the woodchip treatment filter was strongly dependent on how fast water flowed through the denitrification bed, and to a lesser extent, temperature. The amount of water was strongly seasonally evident and resulted in varying treatment efficacy. The most amount of nitrate mobilisation from the soil zone coincided with high discharge rates. It may be possible to improve performance by buffering the flow or by temporarily retaining some of the drainage water in the landscape, within the drains themselves. Another approach could include a bypass system to limit inflow to the filter once a defined flow threshold was achieved.

References

Carstensen, M. V., Hashemi, F., Hoffmann, C. C., Zak, D., Audet, J., & Kronvang, B. (2020). Efficiency of mitigation measures targeting nutrient losses from agricultural drainage systems: A review. Ambio, 49, 1820-1837.

Fahrner, S. (2002). Groundwater Nitrate Removal using a Bioremediation Trench. Honours Thesis. University of Western Australia, Perth.

Hudson, N., McKergow, L., Tanner, C., Baddock, E., Burger, D., Scandrett, J., 2018. Denitrification bioreactor work in Waituna Lagoon catchment, Southland. In: Farm environmental planning – Science, policy and practice. (Eds L. D. Currie and C. L. Christensen). http://flrc.massey.ac.nz/publications.html. Occasional Report No. 31. Fertilizer and Lime Research Centre, Massey University, Palmerston North, New Zealand. 10 pages.

Jaynes, D.B., Thorp, K. (2008). Potential water quality impact of drainage water management in the Midwest cornbelt. In: ASABE Annual Meeting, Providence, RI. Paper No. 084566, Jun 29–July 2.

Robertson, W.D., Blowes, D.W., Ptacek, C.J., Cherry, J.A. (2000). Long-term of performance of in situ reactive barriers for nitrate remediation. Ground Water 38, 689–695.

Robertson, W.D. & Merkley, L.C. (2009). In-stream bioreactor for agricultural nitrate treatment. J. Environ. Qual. 38, 230–237.

Schipper, L.A. & McGill, A. (2008). Nitrogen transformation in a denitrification layer irrigated with dairy factory effluent. Water Res. 42, 2457–2464.

Schipper, L.A., Barkle, G.F., Vojvodic-Vukovic, M. (2005). Maximum rates of nitrate removal in a denitrification wall. J. Environ. Qual. 34, 1270–1276.

Schipper, L. A., Robertson, W. D., Gold, A. J., Jaynes, D. B., & Cameron, S. C. (2010). Denitrifying bioreactors - an approach for reducing nitrate loads to receiving waters. Ecological engineering, 36(11), 1532-1543.

van Driel, P.W., Robertson, W.D., Merkley, L.C. (2006a). Denitrification of agricultural drainage using wood-based reactors. Trans. ASAE 48, 121–128.

van Driel, P.W., Robertson, W.D., Merkley, L.C. (2006b). Upflow reactors for riparian zone denitrification. J. Environ. Qual. 35, 412–420.