Nitrification and denitrification as a source for NO and NO2 production in high-strength wastewater

Laboratory and half-technical scale experiments were performed to evaluate nitric oxide (NO) and nitrogen dioxide (NO2) production during biological N-elimination from wastewater with high ammonium concentration (about 700 mg N L-1). In a laboratory scale bioreactor with biomass retention, the ammonia oxidizer Nitrosomonas europaea and the denitrifier Paracoccus denitrificans were grown as reference organisms in co-culture in order to simulate the nitrifying and denitrifying community of wastewater treatment plants. Synthetic wastewater and sludge liquor from the municipal wastewater treatment plant in Lueneburg (Germany) were used. In the laboratory scale reactor, during the treatment of synthetic wastewater, 0.28% of the oxidized ammonium-N was released as NO-N by a pure culture of Nitrosomonas. A simultaneously nitrifying and denitrifying co-culture only released 0.04 to 0.2%. NO2 formation was not observed. NO production was much higher in sludge liquor. A pure culture of Nitrosomonas produced 0.52% NO + NO2-N (= NOx-N), a co-culture of Nitrosomonas and Paracoccus even 1.64% NOx-N. The production rate strongly depended on the media and the organisms used. In a co-culture of N. europaea and P denitrificans, Nitrosomonas was shown to be the most efficient NO producer. NO production increased with ammonium oxidation rate and with nitrite concentration of the medium. In synthetic wastewater, NO production was not influenced by reduced oxygen content. However, in sludge liquor NO production rate increased with decreasing O2 concentration. Here, for the first time, the formation of significant amounts of NO2 during simultaneous nitrification/denitrification could be demonstrated. In half-technical scale experiments, only 0.07% of the oxidized ammonium-N was released as NO-N from the nitrification stage. NO2 was not detectable. Release of nitric oxide from the denitrification stage was mainly diffusion limited and the amount produced did not exceed 0.0001%. A calculation on the basis of the results presented, revealed that biological treatment of nitrogen-rich wastewater is not a significant source for pollution of the atmosphere with NOx in industrial areas.