Denitrification in a low-temperature bioreactor system at two different hydraulic residence times: laboratory column studies

ABSTRACT

Nitrate removal rates in a mixture of pine woodchips and sewage sludge were determined in laboratory column studies at 5°C, 12°C, and 22°C, and at two different hydraulic residence times (HRTs; 58.2–64.0 hours and 18.7–20.6 hours). Baffles installed in the flow path were tested as a measure to reduce preferential flow behavior, and to increase the nitrate removal in the columns. The nitrate removal in the columns was simulated at 5°C and 12°C using a combined Arrhenius-Monod equation controlling the removal rate, and a first-order exchange model for incorporation of stagnant zones. Denitrification in the mixture of pine woodchips and sewage sludge reduced nitrate concentrations of 30 mg N L⁻¹ at 5°C to below detection limits at a HRT of 58.2–64.0 hours. At a HRT of 18.7–20.6 hours, nitrate removal was incomplete. The Arrhenius frequency factor and activation energy retrieved from the low HRT data supported a biochemically controlled reaction rate; the same parameters, however, could not be used to simulate the nitrate removal at high HRT. The results show an inversely proportional relationship between the advection velocity and the nitrate removal rate, suggesting that bioreactor performance could be enhanced by promoting low advection velocities.

KEYWORDS:

• Woodchip bioreactor
• temperature
• denitrification
• hydraulic residence time
• dual-porosity

Acknowledgements

Thanks are extended to Carmen Vega Riquelme (Department of Earth Sciences, Uppsala University) for assisting with the chemical analyses, and to two anonymous reviewers for their valuable comments.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCiD

Albin Nordström http://orcid.org/0000-0002-0311-8368

Roger B. Herbert http://orcid.org/0000-0002-7561-757X

Additional information

Funding

This work was supported by VINNOVA, The Swedish Innovation Agency [grant number P31054-1], as part of the larger project ‘miNing – Reduction of nitrogen discharges in mining processes and mitigating its environmental impact’, in collaboration with the Luossavaara-Kiirunavaara Aktiebolag (LKAB) and Boliden AB.