Optimum Oxygen Flux Determination to Regulate Denitrification in Freshwater Sediments Using Micro-Profiling Techniques

MD KAWSER BIN ZAMAN

Georgia Southern University

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To prevent hypolimnetic anoxia in reservoirs and lakes, the use of a hypolimnetic oxygenation system (HOS) is becoming a common management strategy. However, continuous use of HOSs may hinder important processes crucial for nutrient removal such as denitrification. This study aimed at determining an optimum flowrate to control dissolved oxygen (DO) concentrations while balancing nitrate concentrations in a water supply reservoir. A comparison of the effectiveness of nitrate and oxygen penetration in sediments was studied using DO and redox profiles at the sediment-water interface (SWI) of microcosms seeded with sediments from the Occoquan Reservoir. The Occoquan reservoir is an indirect potable reuse system where nitrate and a HOS are used to improve water quality. Results revealed that 2.58 g/min was the optimum oxygen flowrate because increasing the flowrate to higher values (e.g., 3.24 and 3.89 g/min) did not increase oxygen penetration at the SWI. Maximum oxygen penetration depths (OPD) for flowrates ranging between 2.5 and 4 g/min were not significantly different (p-value = 0.815). For the studied flows, total oxygen uptake at the SWI ranged from 95.5 mmol m-2 d-1 to 116 mmol m-2 d-1, averaging 104.5 mmol m-2 d-1, and diffusive oxygen flux ranged from 6.0 mmol m-2 d-1 to 16.4 mmol m-2 d-1, averaging 11.3 mmol m-2 d-1 for a flow of 2.58 g/min and over. These results suggested that oxygen consumption at the SWI was continuously adapting to the increased flowrates in a nonlinear way until reaching steady-state conditions at 2.58 g/min. Results also revealed that nitrate penetration (~8 mm) was higher than oxygen (6 mm) when the nitrate concentration above the SWI remained at 4-6 mg-N/L. Further, nitrate maintained the ORP at 300 mV at a depth of 8mm, which was enough to sustain oxidized conditions at the SWI. Finally, the combined used of nitrate and oxygen was proven to be the best alternative to prevent the release of reduced substances from the sediments.

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