UNRAVELING THE SOURCE OF NITRIC OXIDE EMISSION DURING NITRIFICATION
Abstract:Nitric oxide (NO) is produced during nitrification in a laboratory scale bioreactor, operated at conditions relevant to municipal nitrifying wastewater treatment plants (WWTPs). This study aims to determine which type of micro-organism and which metabolic pathway is responsible for NO emission during nitrification. Simulation studies were used to identify which pathway is the main source of NO emission based on three hypothetical pathways for NO emission. A) nitrification, B) denitrification by ammonia oxidizing bacteria with ammonium as electron donor, C) heterotrophic denitrification. The results of the study suggest that in a nitrifying reactor treating wastewater containing solely ammonium and nutrients, denitrification by ammonia oxidizing bacteria is the main NO producing pathway. At the present aeration rate (500 m3air m−3reactor d−1) 0.025% of the treated ammonium is emitted as NO, leading to 0.2 to 1 ppm NO in the off-gas. The level of emission appears to be dependent on the biological conversion rates, not on the concentration of NO in the medium. As a consequence the NO concentration in the off-gas depends on the aeration rate. In full-scale WWTPs the volumetric aeration rate is significantly lower, implicating potentially higher NO concentrations in the off-gas. Nitrite presence and oxygen limitation increase the NO emission. The presented modeling approach can be used as an extension of the Activated Sludge Models to predict gaseous emissions from WWTPs.
Document Type: Research Article
Publication date: January 1, 2007
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