NOx monitoring of a simultaneous nitrifying–denitrifying (SND) activated sludge plant at different oxidation reduction potentials

Simultaneous nitrification–denitrification (SND) allows biological nitrogen removal in a single reactor without separation of the two processes in time or space but requires adapted control strategies (anoxic/aerobic conditions). In this study, the formation of gaseous nitric oxide (NOG) and nitrogen dioxide (NO2G) was monitored for SND in relation to the oxidation-reduction potential (ORP) and nitrogen removal in a lab batch reactor and a pilot membrane bio-reactor (MBR). In addition hospital wastewater (COD/Ntot>6:1) was treated on site for 1 year. The highest total nitrogen removal rates of max 90% were reached at 220–240 mV ORP (given as Eh) with corresponding maximal NOG emissions rates of 0.9 μg g−1 VSS h−1. The maximal emission rates of NO2G (0.2 μg g−1 VSS h−1) were reached at the same ORP level and the NO2G emissions correlated to the nitrite accumulation in the activated sludge up to 5 mg l−1 NO2L-N. It was shown that this correlation was due to biological production and not due to pH-dependent chemical conversion. Therefore, NO2G can be used as additional control loop for ORP-controlled SND systems to avoid the inhibition of denitrification and high nitrite concentrations in the plant effluent.