NITRI-GHG · Exploring novel nitrifier pathways to minimise direct greenhouse gas emissions from WWTPs

Exploring novel nitrifier pathways to minimise direct greenhouse gas emissions from WWTPs.

In wastewater treatment plants, nitrifying bacteria, and in particular, ammonia oxidizing bacteria (AOB) are the main responsible for the nitrous oxide (N2O) emissions detected. N2O is a potent greenhouse gas with a global warming potential 300 times higher than that of CO2 and therefore, little emissions can already be responsible for a big fraction of the carbon footprint of the plant. However, it is still unclear the pathways leading to N2O production in AOB and how to reliable control its emission due to a lack of scientific understanding. A great part of the NITRI-GHG project focused on understanding the mechanisms involved on N2O production pathways in AOB bacteria as well on the factors affecting the production of N2O. To study the factors affecting these emissions, the experiments were conducted with nitrifying mixed cultures from fully controlled laboratory scale reactors and pilot plant installations treating synthetic and real wastewater. It was found a strong correlation between dissolved oxygen concentration and N2O emissions. Also, it was demonstrated that partial nitrification led to higher N2O emissions when compared with full nitrification systems, due to nitrite accumulation in the former ones. Finally, the link between nitric oxide (NO) emissions and N2O emissions was explored also using enriched nitrifying cultures at lab-scale. NO is an important compound involved in the N2O production pathways in nitrifying bacteria. It can cause depletion of the ozone layer and it is toxic for living organisms and therefore its emissions should also be minimized. Through this project it was unravelled a linear correlation between NO emissions and ammonia oxidation rate which differs from the exponential correlation found in the case of N2O. Also, research has been expanded to full-scale nitrifying systems in order to validate the findings of the project and develop effective mitigation strategies useful for wastewater treatment plant operators. The first N2O monitoring campaigns at the national level were conducted in two full-scale wastewater treatment facilities. In both cases N2O emissions were clearly linked to the activity of AOBs and in both cases mitigations strategies were proposed which could significantly reduce these emissions without compromising the effluent quality of the plants. Most of the research outcomes of the project have been published in peer-reviewed scientific journals with a high impact factor within the field of water research. In total 13 scientific manuscript have been published and around 20 conference presentations have been given. Also, dissemination among wastewater treatment plant managers has been promoted with several presentations to different local wastewater authorities. This project presents a good example on how fundamental research can have a direct application to the real world by developing effective minimisation strategies for N2O abatement in full scale nitrifying systems. More detailed information can be found in the project webpage: http://www.icra.cat/projects/exploring-novel-nitrifier-pathways-minimise/48

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