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Perera GN, Rojas DT, Rivas A, Barkle G, Moorhead B, Schipper LA, Craggs R, Hartland A. Elucidating phosphorus removal dynamics in a denitrifying woodchip bioreactor. THE SCIENCE OF THE TOTAL ENVIRONMENT 2024; 917:170478. [PMID: 38301780 DOI: 10.1016/j.scitotenv.2024.170478] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/03/2023] [Revised: 01/23/2024] [Accepted: 01/24/2024] [Indexed: 02/03/2024]
Abstract
Denitrifying woodchip bioreactors (DBRs) are an established nitrate mitigation technology, but uncertainty remains on their viability for phosphorus (P) removal due to inconsistent source-sink behaviour in field trials. We investigated whether iron (Fe) redox cycling could be the missing link needed to explain P dynamics in these systems. A pilot-scale DBR (Aotearoa New Zealand) was monitored for the first two drainage seasons (2017-2018), with supplemental in-field measurements of reduced solutes (Fe2+, HS-/H2S) and their conjugate oxidised species (Fe3+/SO42-) made in 2021 to constrain within-reactor redox gradients. Consistent with thermodynamics, the dissolution of Fe3+(s) to Fe2+(aq) within the DBR sequentially followed O2, NO3- and MnO2(s) reduction, but occurred before SO42- reduction. Monitoring of inlet and outlet chemistry revealed tight coupling between Fe and P (inlet R2 0.94, outlet R2 0.85), but distinct dynamics between drainage seasons. In season one, outlet P exceeded inlet P (net P source), and coincided with elevated outlet Fe2+, but at ⁓50 % lower P concentrations relative to inlet Fe:P ratios. In season 2 the reactor became a net P sink, coinciding with declining outlet Fe2+ concentrations (indicating exhaustion of Fe3+(s) hydroxides and associated P). In order to characterize P removal under varying source dynamics (i.e. inflows vs in-situ P releases), we used the inlet Fe vs P relationship to estimate P binding to colloidal Fe (hydr)oxide surfaces under oxic conditions, and the outlet Fe2+ concentration to estimate in-situ P releases associated with Fe (hydr)oxide reduction. Inferred P-removal rates were highest early in season 1 (k = 0.60 g P m3 d-1; 75-100 % removal), declining significantly thereafter (k = 0.01 ± 0.02 g P m3 d-1; ca. 3-67 % removal). These calculations suggest that microbiological P removal in DBRs can occur at comparable magnitudes to nitrate removal by denitrification, depending mainly on P availability and hydraulic retention efficiency.
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Affiliation(s)
- Gimhani N Perera
- Environmental Research Institute, School of Science, Faculty of Science and Engineering, University of Waikato, Kirikirioa Hamilton, New Zealand; National Institute of Water and Atmospheric Research Ltd (NIWA), PO Box 11115, Kirikirioa Hamilton 3251, New Zealand
| | - Dorisel Torres Rojas
- Environmental Research Institute, School of Science, Faculty of Science and Engineering, University of Waikato, Kirikirioa Hamilton, New Zealand
| | - Aldrin Rivas
- Lincoln Agritech Ltd, Ruakura, Kirikirioa Hamilton 3214, New Zealand
| | - Greg Barkle
- Land and Water Research Ltd, Kirikirioa Hamilton 3217, New Zealand
| | - Brian Moorhead
- Lincoln Agritech Ltd, Ruakura, Kirikirioa Hamilton 3214, New Zealand
| | - Louis A Schipper
- Environmental Research Institute, School of Science, Faculty of Science and Engineering, University of Waikato, Kirikirioa Hamilton, New Zealand
| | - Rupert Craggs
- National Institute of Water and Atmospheric Research Ltd (NIWA), PO Box 11115, Kirikirioa Hamilton 3251, New Zealand
| | - Adam Hartland
- Environmental Research Institute, School of Science, Faculty of Science and Engineering, University of Waikato, Kirikirioa Hamilton, New Zealand; Lincoln Agritech Ltd, Ruakura, Kirikirioa Hamilton 3214, New Zealand.
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