Elucidating phosphorus removal dynamics in a denitrifying woodchip bioreactor

Area of Science:

• Environmental Engineering
• Water Quality Management
• Biogeochemistry

Background:

• Denitrifying woodchip bioreactors (DBRs) are effective for nitrate removal.
• Phosphorus (P) removal in DBRs is inconsistent, with unclear mechanisms.
• Iron (Fe) redox cycling is hypothesized to influence P dynamics in DBRs.

Purpose of the Study:

• To investigate the role of Fe redox cycling in P dynamics within DBRs.
• To determine if Fe cycling explains the variable P source-sink behavior observed in field trials.
• To quantify P removal rates and their relationship with Fe cycling and reactor conditions.

Main Methods:

• Monitoring of a pilot-scale DBR in Aotearoa, New Zealand over two drainage seasons.
• In-field measurements of reduced and oxidized solutes (Fe, S, P, nitrate).
• Thermodynamic analysis of redox gradients and calculation of P removal rates.

Main Results:

• Fe redox cycling closely followed O2, nitrate, and MnO2 reduction, preceding sulfate reduction.
• Tight coupling between Fe and P was observed (inlet R² 0.94, outlet R² 0.85).
• DBR acted as a P source in season one (high Fe2+) and a P sink in season two (declining Fe2+).

Conclusions:

• Fe redox cycling is the key mechanism explaining P dynamics and source-sink behavior in DBRs.
• P removal efficiency is dependent on P availability and hydraulic retention efficiency.
• Microbiological P removal in DBRs can be significant, comparable to nitrate removal.