Inhibition of Biological Denitrification by Nitric Oxide, Its Early Detection and Prevention of Negative Impact on the Denitrification of Industrial Wastewater

Biological denitrification is used to treat industrial wastewater containing high concentrations of nitrate and sulfate from nitrocellulose production, which contains high concentrations of nitrates and sulfates. The reactor emits brown nitrogen dioxide gas suggesting significant Nitric Oxide (NO) formation and reaction with air. NO may significantly inhibit the denitrification process, reduce treatment efficiency and cause potentially serious issues in nitrocellulose production. An NO-sensitive oxygen luminescence probe demonstrated an accumulation of exceptionally high NO levels. The presence of excess NO under conditions that replicate the industrial denitrification process have further revealed that NO formation is triggered by an oversupply of ethanol, as the reducing substrate, under high nitrate loading. This overdosing is ultimately caused by the failure of an automatic ethanol dosage control, which relies on the signal from a nitrate- and nitrite-sensitive Nitratax probe. When the reduction rate of nitrate and nitrites appears to slows, nitrites continue to accumulates. Thus, the Nitratax probe signals for additional ethanol delivery even when it is unnecessary. Nitrite accumulation and nitrite reduction increase NO concentrations. Excess NO, or nitrite, apparently disrupts the next step in microbial denitrification, the reduction of NO by NO-reductases, and the denitrification pathway that ultimately yields N2. An NO or nitrite-specific probe may better serve as an early warning system by providing the timely feedback required to prevent inhibitory conditions. Additionally, a correlation was observed between inhibition events and lower pH levels in the denitrification reactor, particularly within the range of 6.1-6.3. However, no connection was found between NO evolution and temperature within the range of 16-35°C. Elevated industrial reactor NO may significantly inhibit the denitrification process, reduce treatment efficiency and cause potentially serious issues in nitrocellulose production.