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Bioreactors — Key considerations

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Bioreactor walls are constructed to intercept groundwater while bioreactor beds receive surface water runoff or are connected to subsurface drains.

Critical features:

Bioreactor beds:

  • Carbon source (e.g. woodchips)
  • Sizing sufficient to remove the desired quantity of nitrogen.
  • Sediment trap upstream to avoid clogging
  • Careful design of inlet and outlet structures to ensure sufficient residence time and hydraulic efficiency whilst maintaining flooded (anaerobic) conditions
  • Excess flow bypass and scour protection for higher stormwater flows.


In-line bioreactor bed. Image by Queensland Government.


Offline bioreactor bed. Image by Queensland Government.


Bioreactor walls:

  • Ensure the bioreactor wall is located to intercept shallow groundwater flow.
  • Confining layer (e.g. clay) at the base of the bioreactor to prevent groundwater bypassing under the wall.
  • Wall positioned perpendicular to the groundwater flow to avoid bypassing, requiring good understanding of groundwater flows.
  • Shallow groundwater to maintain anoxic conditions.
  • Depth up to 3m to avoid walls falling in during construction, deeper walls will require wider excavation, or suitable engineering solutions.
  • Sufficient volume of carbon source must be provided. The nitrogen removal rate is approximately 5g N m-3 d-1. Rates tend to increase with increasing nitrogen concentration in the inflow[3].
  • Sediment trap to avoid clogging of bioreactor bed by turbid surface water.
  • Walls need to be built in permeable soil overlying a confining layer (aquitard) of low permeability, such as clay to prevent short circuiting. They need to intercept the groundwater, so correct siting is essential[3].
  • Dissolved organic carbon can be released immediately after fresh wood chips are installed and needs to be managed[3]. This can be mitigated by pre-washing the woodchips prior to construction[1]. This is more pronounced with hard wood chips than soft wood chips.
  • Detention time should be sufficient to allow for microbial denitrification. Desired period is 6 to 24 hours.
  • The outlet should be approximately level with the top of the bed, to maintain flooded conditions within the bioreactor, so that the woodchip is saturated with water ensuring low or no available oxygen (e.g. anoxic or anaerobic). However, making the outlet level adjustable can facilitate draining of the bed and ensure the outlet collects water from the full depth of the bioreactor, to minimise dead spots within the bed[2].


Bioreactor wall. Image by Queensland Government.


Suitability and limitations

Bioreactors are common in agricultural regions of the US Midwest and also in New Zealand. Bioreactors are now being used in a variety of agricultural settings for nitrate mitigation from subtropical climates to areas with snow cover.

Trials are underway in South East Queensland (pineapples) and North Queensland (sugarcane).

In the USA bioreactor beds are commonly connected to tile drainage networks and they are particularly suited to subsurface drainage systems, which are used in some production systems in parts of Queensland. Bioreactor walls are better suited to areas with shallow groundwater and a shallow aquitard such as clay[3].

Bioreactors may produce nitrous oxide (a greenhouse gas) if not managed properly. Bioreactors generally produce only small amounts of N2O[1].

The cost-effectiveness of a bioreactor in removing the target pollutant/s needs to be considered relative to other treatment structures. Refer to cost considerations for more information.

Pollutant Size/Type Treatment Performance Description of Treatment Process
Coarse pollutants Deposition of any coarse pollutants in a sediment trap upstream of bioreactor.
Fine sediments and particulate nutrients Fine sediments and particle-bound nutrients may be removed by a sediment trap. Or pass through the bioreactor.
Dissolved nutrients and pesticides Dissolved nitrates reduced via denitrification by microbiological processes.


In addition to the standard disclaimer located at the bottom of the page, please note the content presented is based on published knowledge of treatment systems. Many of the treatment systems described have not been trialled in different regions or land uses in Queensland. The information will be updated as new trials are conducted and monitored. If you have any additional information on treatment systems or suggestions for additional technologies please contact us using the feedback link at the bottom of this page.


  1. ^ a b Christianson, LE & Schipper, LA (2016), 'Moving Denitrifying Bioreactors beyond Proof of Concept: Introduction to the Special Section', Journal of Environmental Quality, vol. 45, pp. 757-761.
  2. ^ Queensland Department of Agriculture and Fisheries (2018), Bioreactors: key aspects for effective design, operation and monitoring - Interim guideline for bioreactor trials July 2018. [online], Queensland Department of Agriculture and Fisheries, Queensland. Available at:
  3. ^ a b c d Schipper, L, Robertson, WD, Gold, AJ, Jaynes, DB & Cameron, SC (2010), 'Denitrifying bioreactors—An approach for reducing nitrate loads to receiving waters', Ecological Engineering, vol. 36, pp. 1532-1543.

Last updated: 11 September 2018

This page should be cited as:

Department of Environment and Science, Queensland (2018) Bioreactors — Key considerations , WetlandInfo website, accessed 13 May 2021. Available at:

Queensland Government
WetlandInfo   —   Department of Environment and Science