University of Oulu

Büngener, L., Postila, H., Kujala, K., Kinnunen, J., Prieto Fernandez, F., Ronkanen, A.-K., & Heiderscheidt, E. (2023). The effects of cold frozen conditions on nitrogen removal and transformation in wetlands purifying wastewater: Insights from combined full- and pilot-scale observations. Ecological Engineering, 191, 106951.

The effects of cold frozen conditions on nitrogen removal and transformation in wetlands purifying wastewater : insights from combined full- and pilot-scale observations

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Author: Büngener, Lina1; Postila, Heini1; Kujala, Katharina1;
Organizations: 1Water, Energy and Environmental Engineering, Faculty of Technology, 90014 University of Oulu, Finland
2Finnish Environment Institute, Marine and freshwater solutions, Paavo Havaksen Tie 3, P. O. Box 413, FI-90014 Oulu, Finland
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 3.9 MB)
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Language: English
Published: Elsevier, 2023
Publish Date: 2023-08-31


In wastewater treatment, wetlands are regarded as cost-efficient and sustainable purification systems. Currently, different types of wetland are used for year-round treatment of municipal wastewater (the polishing step after the secondary or tertiary units) in cold climate regions. However, there is a lack of understanding regarding the effect of freezing cold conditions on pollutant removal processes, in particular regarding those linked to nitrogen cycling. This study evaluated the effect of cold winter conditions on contaminant removal, with a focus on nitrogen transformation and removal in a pond-type surface flow wetland with reed (Reed-SF) and a peat-based horizontal subsurface flow (Peat-HSSF) wetland. Year-round full-scale wetland monitoring was complemented with pilot-scale laboratory experiments, which allowed to follow the cold-climate induced effects on contaminant removal and nitrogen processes, along with other water quality, environmental, and microbial parameters. Overall, it was observed that the effect of cold climate conditions on nitrogen removal was dependent on the wetland type in combination with the quality of the inflow water. When treating ammonium (NH4) rich wastewater, removal of total nitrogen (Ntot), as well as NH4, was higher in the Peat-HSSF than in the Reed-SF wetland. Under frozen conditions, NH4 removal decreased slightly but remained positive in the pilot Peat-HSSF, whereas it declined and even turned into leaching in the pilot Reed-SF wetland. Conditions encountered in the Peat-HSSF wetlands (high abundances of active nitrifying bacteria, high levels of dissolved oxygen (DO) and redox potential) supported nitrification, which continued under cold conditions. Whereas in the Reed-SF, a low abundance of active nitrifying bacteria was found, especially in the water column under frozen conditions. DO here was low and decreased in cold conditions with ice cover.

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Series: Ecological engineering
ISSN: 0925-8574
ISSN-E: 1872-6992
ISSN-L: 0925-8574
Volume: 191
Article number: 106951
DOI: 10.1016/j.ecoleng.2023.106951
Type of Publication: A1 Journal article – refereed
Field of Science: 218 Environmental engineering
1172 Environmental sciences
Funding: The study was funded by the European Regional Development Fund (A74546, RaMiKo project 2019-2022).
Copyright information: © 2023 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (