Bioreactors

Bioreactors

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• Overview
• Key considerations
• Planning and design
• Construction and operation
• Links and references

Other name/s

Woodchip bioreactor, denitrifying bioreactor, denitrifying wall, bioreactor wall, denitrifying bed, bioreactor bed

Description

Denitrifying bioreactors are systems filled with organic-matter (e.g. woodchip) designed to remove nitrate from shallow groundwater or surface water, through the process of denitrification. Bioreactors work by passing stormwater runoff, irrigation tailwater, wastewater or shallow groundwater containing nitrates through a carbon source (generally woodchips). As water fills and slowly passes through the woodchip, the dissolved oxygen levels decrease to create an anaerobic or anoxic environment suitable for the naturally-occurring denitrifying microbes to convert the nitrate into inert dinitrogen gas (N2), thereby permanently removing nitrogen from the water[6]. Under conditions with low dissolved oxygen levels, warm temperature, non-nitrate limited conditions and suitable pH (5.5-8.0), full denitrification to dinitrogen gas can occur[1][2][7][4].

Nitrate limitation in a bioreactor occurs when the nitrate is completely consumed and there is enough carbon to support additional denitrification, which can lead to undesirable by-products.

There are two main types of bioreactors, bioreactor walls and bioreactor beds, suited to soils and hydrology of the site, specifically how nitrate is leaving the production area and where nitrate-rich water can be intercepted[7].

Bioreactor walls are used to treat shallow groundwater. They are designed to intercept and remove nitrate that has entered the soil profile and leached into the groundwater. They consist of a trench filled with woodchips, located perpendicular to the groundwater flow.

Bioreactor beds consist of a bed of woodchips that receive and treat either surface water, via an open drain, or sub-surface drainage from pipes (e.g. tile drains, ag pipe) or wastewater (e.g. from aquaculture or protected cropping system)[8]. Water enters the bioreactor via an inlet structure and exits via an outlet structure, with excess flow bypassing the bioreactor. A sediment trap may be required upstream to prevent clogging of the woodchip[1][3][7]. There are three main types of bioreactor bed:

• Located within a drainage line (in-line bioreactors)
• Receive water diverted from a drainage line (off-line bioreactors)
• Connected to sub-surface drainage, including ag-pipe or tile-drains (ag-line bioreactors).

Effectiveness as a treatment system

Denitrifying bioreactors have proven effective at reducing nitrate in surface water and groundwater on sugarcane and horticulture farms in Queensland, with nitrate removal rates between 0.08 – 7.1 g N/m³/day[8][4].

The effectiveness of a bioreactor for removing nitrate and converting it to dinitrogen gas is strongly influenced by[8][4][5]:

• nitrate concentration of the inflow (recommend at least 5mg N/L for bioreactor walls and 3mg N/L for bioreactor beds, for most of the time)
• water movement into, through and within the bioreactor, including timing (hydraulic residence time) and duration of inflow, and even flow through the woodchip
• dissolved oxygen levels in the bioreactor (<2.0mg/L)
• suitable pH (between 5.5-8.0).

A well-located and designed bioreactor, meeting these conditions, can remove nitrate at a cost of around $100 per kilogram of nitrate removed (based on an average nitrate removal rate of 3.4g N/m³/day in a 100m³ bioreactor bed receiving run-off for 250 days a year taking into account all planning, construction and maintenance costs over a 10 year lifespan[8]).  The actual cost-effectiveness of a bioreactor will depend on the specific site conditions and project objectives and needs to be considered relative to other treatment systems or management intervention options. Refer to cost considerations for more information.

Services and benefits

The main service that bioreactors provide includes:

• Water treatment (nitrogen reduction)

Case study: bioreactor bed on a cane farm near Ayr.

References

1. ^ a b Addy, K, Gold, AJ, Christianson, LE, David, MB, Schipper, LA & Ratigan, NA (2016), 'Denitrifying Bioreactors for Nitrate Removal: A Meta-Analysis', Journal of Environmental Quality, vol. 45, pp. 873-881.
2. ^ Christianson, L, Tyndall, J & Helmers, M (2013), 'Financial comparison of seven nitrate reduction strategies for Midwestern agricultural drainage', Water Resources and Economics, vol. 2-3, pp. 30-56.
3. ^ 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.
4. ^ a b c Manca, F, Wegscheidl, C, Robinson, R, Argent, S, Algar, C, De Rosa, D, Griffiths, M, George, F, Rowlings, D, Schipper, L & Grace, P (15 December 2021), 'Nitrate Removal Performance of Denitrifying Woodchip Bioreactors in Tropical Climates', Water. [online], vol. 13, no. 24, p. 3608. Available at: https://www.mdpi.com/2073-4441/13/24/3608 [Accessed 23 May 2022].
5. ^ Rivett, MO, Buss, SR, Morgan, P, Smith, JWN & Bemment, CD (2008), 'Nitrate attenuation in groundwater: A review of biogeochemical controlling processes', Water research. [online], vol. 42, no. 16, pp. 4215-4232. Available at: Scopus.
6. ^ Robertson, WD (November 2010), 'Nitrate removal rates in woodchip media of varying age', Ecological Engineering. [online], vol. 36, no. 11, pp. 1581-1587. Available at: https://linkinghub.elsevier.com/retrieve/pii/S0925857410000340 [Accessed 23 May 2022].
7. ^ a b c 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.
8. ^ a b c d Wegscheidl, C, Robinson, R & Manca, F (2021), Using denitrifying bioreactors to improve water quality on Queensland farms. [online], Queensland Department of Agriculture and Fisheries, Townsville, Queensland. Available at: https://www.publications.qld.gov.au/dataset/treatment-system-technologies-to-improve-water-quality/resource/5a663c1c-9734-4367-9348-bf6e3f24b493.

Last updated: 24 May 2022