Elucidating phosphorus removal dynamics in a denitrifying woodchip bioreactor

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⁺/SO₄^2-) 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 MnO_2(s) reduction, but occurred before SO₄^2- 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.