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

To prevent hypolimnetic anoxia in reservoirs and lake systems, the use of a hypolimnetic oxygenation system (HOS) is increasingly becoming a common management strategy. However, the continuous use of HOS may hinder other important processes crucial for nutrient removal such as denitrification. This study aimed at determining an optimum flow rate to control dissolved oxygen (DO) concentrations while balancing a nitrate addition strategy that is used in water reservoirs to improve water quality. A comparison of the effectiveness of nitrate and oxygen penetration in sediments was also 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 addition is used to improve water quality in the absence of DO. Therefore, it is crucial for this system to operate their HOS at a level that won’t hinder denitrification. Results revealed that a flow rate of 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 flow rates 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 the flow rate of 2.58 g/min and over. These results suggested that the oxygen consumption for this system was continuously adapting with increasing flow rates until it reached a steady-state condition 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 between 4 and 6 mg-N/L. In addition, nitrate maintained the ORP at 300 mV, which was enough to sustain oxidized conditions at the SWI. Finally, the combined use of nitrate and oxygen, provided at the optimum rate, was the best alternative to prevent the release of reduced substances from the sediments