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 aseous nitric oxide (NOG) and nitrogen dioxide (N02G) 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/N-tot > 6:1) was treated on site for I 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 mu g g(-1) VSS h(-1). The maximal emission rates of NO2G (0.2 pgg(-1) VSS h(-1)) were reached at the same ORP level and the N02G emissions correlated to the nitrite accumulation in the activated sludge up to 5mg 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. (c) 2006 Elsevier Ltd. All rights reserved.