Orthophosphate uptake onto woodchips in denitrifying bioreactors is enhanced by anoxic-(anaerobic-)oxic cycling

Woodchips are widely used as a low-cost and renewable organic carbon source for denitrifying biofilms in passive nutrient removal systems. One limitation of wood-based biofiltration systems is their relatively poor removal of phosphorus (P) from subsurface drainage and stormwaters, necessitating the use of additional filter media when co-treatment of nitrogen (N) and P is required. Here, we show that anoxic-oxic cycling of woodchip media, which enhances nitrate (NO₃^-) removal by increasing the mobilization of organic carbon from wood, also improves orthophosphate (P_i) uptake onto woodchips. Orthophosphate removal rates in flow-through woodchip columns ranged from 0 to 34.9 μg PO₄^3- L^-1 h^-1 under continuously-saturated (anoxic) conditions, and increased to 17.5 to 71.9 μg PO₄^3- L^-1 h^-1 in columns undergoing drying-rewetting (oxic-anoxic) cycles. The highest P_i removal efficiencies were observed in the first 20 h after reactors were re-flooded, and were concurrent with maxima in polyphosphate kinase (ppk) gene expression by the polyphosphate accumulating organisms (PAOs) Accumulibacter spp. and Pseudomonas spp. Batch experiments confirmed that anoxic-anaerobic-oxic pre-incubation conditions led to orthophosphate uptake onto woodchips as high as 74.9 ± 0.8 mg PO₄^3-/kg woodchip, and batch tests with autoclaved woodchips demonstrated that P_i uptake was due to biological processes and not adsorption. NO₃^- removal in batch tests was also greatest under oxic incubation conditions, attributed to greater carbon availability in hypoxic to anoxic zones in woodchip biofilms. While further research is needed to elucidate the mechanisms controlling enhanced P_i uptake by woodchip biofilms under anoxic-(anaerobic-)oxic cycling, these results suggest a role for enhanced P_i uptake by PAOs in a nature-based system for treatment of nonpoint source nutrients.