Integration of titrimetric measurement, off-gas analysis and NO_x⁻ biosensors to investigate the complexity of denitrification processes

The denitrification process, namely the reduction of nitrate (NO₃⁻) to nitrogen gas (N₂), often cannot be simply modelled as a single step process. For a more complete and comprehensive model the intermediates, particularly nitrite (NO₂⁻) and nitrous oxide (N₂O), need to be investigated. This paper demonstrates the integration of titrimetric measurements and off-gas analysis with on-line nitrite plus nitrate (NO_x⁻) biosensors, highlighting the necessity of measuring process intermediates with high time-scale resolution to study and understand the kinetics of denitrification. Investigation of activated sludge from a full-scale treatment plant showed a significant accumulation of NO₂⁻, which appeared to impact on the overall denitrification rate measured as NO_x⁻ reduction or N₂ production. A different sludge obtained from a lab-scale bioreactor produced N₂O instead of N₂ as the end product of denitrification. The two examples both illustrate the complexity of denitrification and stress the need for the more versatile and detailed measurement procedures, as presented in this paper.