Performance of an under-loaded denitrifying bioreactor with biochar amendment

Area of Science:

• Environmental Science
• Agricultural Engineering
• Water Quality Management

Background:

• Denitrifying bioreactors are emerging agricultural best management practices, primarily studied in the US Midwest.
• Limited research exists on their performance in other agriculturally significant regions like the US Mid-Atlantic.
• Nutrient pollution is a key challenge in the Virginia Coastal Plain, necessitating effective management strategies.

Purpose of the Study:

• To evaluate the performance of a denitrifying bioreactor in the Virginia Coastal Plain, a novel geographic region for such practices.
• To assess the bioreactor's effectiveness in removing nitrate-nitrogen (NO3-N) and total phosphorus (TP) from agricultural subsurface drainage.
• To measure greenhouse gas (GHG) emissions from the bioreactor and compare them to existing data.

Main Methods:

• A 25.3 m³ woodchip bioreactor amended with biochar was installed to treat subsurface drainage from 6.5 ha of soybean cultivation.
• Influent and effluent NO3-N and TP concentrations, along with flow rates, were monitored intensively during the second year of operation.
• Surface fluxes of nitrous oxide (N2O), methane (CH4), and carbon dioxide (CO2) were measured using the closed dynamic chamber technique.

Main Results:

• The bioreactor had no significant impact on total phosphorus export.
• Cumulative NO3-N removal efficiency was low (9.5%), with an average removal rate of 0.56 ± 0.25 g N m⁻³ d⁻¹.
• Low NO3-N loading, intermittent flow, short hydraulic residence times, and low pH (mean 5.3) contributed to underperformance; N-limitation was indicated.

Conclusions:

• Denitrifying bioreactor performance in the Virginia Coastal Plain was limited by low nitrate loading and N-limitation, resulting in lower NO3-N removal than observed in Midwest systems.
• Greenhouse gas emissions were comparable to other systems and not environmentally concerning.
• Expectations for bioreactor efficiency should be adjusted for systems experiencing significant N-limitation and low influent nitrate concentrations (<10-20 mg L⁻¹).