Solar-powered pumping at a remote denitrifying bioreactor

Pumping surface water from a ditch into a denitrifying woodchip bioreactor could improve nitrate-nitrogen (N) removal by minimizing flow variabilities such as early flow cessation at a given subsurface drainage outlet and flashy drainage hydrographs. Few field-scale subsurface drainage bioreactors with pumping configurations have been assessed. Such evaluations would help better bound reasonable expectations of the benefits and drawbacks at these more advanced bioreactors. An underloaded "ditch diversion" bioreactor constructed in 2018 in Illinois, USA (LW: 4.6 × 9.1 m), was retrofitted with a solar-powered pumping system in 2021 and was then monitored for a 29-d period in 2022 and a 93-d period in 2023. The pumped bioreactor achieved N removal rates averaging 7.5 and 5.2 g N/m³-d and N removal efficiencies of 50 and 61% for the monitoring periods in 2022 and 2023, respectively. Pumping generally improved the bioreactor's performance compared to the same monitoring windows from the historic (non-pumped) 2019-2021 periods. Regression analysis indicated the addition of a pump slightly improved N load removal compared to what might be expected for a conventional bioreactor. The somewhat unintended diurnal batch mode operation resulting from the solar-powered pumping system boosted water temperature in the overnight batches. Bioreactor performance can be improved with a pump, especially at underloaded sites, but the additional complexity and cost need to be carefully weighed.