Design parameters for nitrogen removal by constructed wetlands treating mine waters and municipal wastewater under Nordic conditions

Katharina Kujala; Teemu Karlsson; Soile Nieminen; Anna-Kaisa Ronkanen

doi:10.1016/j.scitotenv.2019.01.124

Design parameters for nitrogen removal by constructed wetlands treating mine waters and municipal wastewater under Nordic conditions

Sci Total Environ. 2019 Apr 20:662:559-570. doi: 10.1016/j.scitotenv.2019.01.124. Epub 2019 Jan 14.

Authors

Katharina Kujala¹, Teemu Karlsson², Soile Nieminen³, Anna-Kaisa Ronkanen⁴

Affiliations

¹ Water Resources and Environmental Engineering Research Unit, Faculty of Technology, University of Oulu, P.O. Box 4300, FI-90014, Finland.
² Industrial Environments and Recycling Unit, Geological Survey of Finland, P.O. Box 1237, FI-70211 Kuopio, Finland.
³ Centre for Economic Development, Transport and the Environment, P.O. Box 115, FI-87101 Kajaani, Finland.
⁴ Water Resources and Environmental Engineering Research Unit, Faculty of Technology, University of Oulu, P.O. Box 4300, FI-90014, Finland. Electronic address: anna-kaisa.ronkanen@oulu.fi.

PMID: 30699376
DOI: 10.1016/j.scitotenv.2019.01.124

Abstract

Nitrogen (N) loads from municipal and mine wastewater discharges typically increase N concentrations in recipient water bodies which should get more attention especially in cold-climate regions. This study compared N removal efficiency of six constructed wetlands (CWs) treating mine waters and three CWs polishing municipal wastewater. There were clear impacts of point source N loading to recipient water bodies in all cases studied and >300-fold increase in N was seen in some cases. First-order N removal coefficient was determined for seven of these CWs. All CWs studied were observed to remove N efficiently during the warm growing season but the amount of N released increased significantly during the cold season. Although some year-round purification was achieved by both peat-based and pond-type CWs, removal of nitrate + nitrite-N ((NO₃^- + NO₂^-)-N) was low during winter. The first-order N removal coefficient varied from 4.9 · 10^-6 to 1.9 · 10^-3 d^-1 and showed that peat-based CWs were slightly more efficient in N removal than pond-type CWs. However, purification efficiency was steadier and higher for pond-type CWs, as lower hydraulic load or longer water residence time compensated for purification performance. Pond-type CWs showed mean removal efficiency of 59% and 46% for ammonium-N (NH₄⁺-N) and (NO₃^- + NO₂^-)-N, respectively, whereas peatland-type CWs had lower removal efficiency for NH₄⁺-N (mean of 26%) and in many cases negative removal for (NO₃^- + NO₂^-)-N. Correlation analysis revealed no clear, systematic relationship between temperature and N removal. However, in some CWs the highest correlation was between temperature and (NO₃^- + NO₂^-)-N, reflecting lower denitrification rate at lower temperature. More than 50% removal was found to require a hydraulic load below 10 mm d^-1. In order to achieve 70% of NH₄⁺-N removal, N_tot load lower than 75 g m^-2 year^-1 and a residence time longer than 80 d are needed in CWs in cold-climate regions.

Keywords: Mine emissions; Nitrogen load; Passive treatment; Recipient water body; Removal kinetics.