NH-N versus pH and ORP versus NO-N sensors during online monitoring of an intermittently aerated and fed membrane bioreactor.

Online sensors, which monitor the ammonia oxidation and the dissimilatory nitrate reduction process, can optimize aerobic and anoxic phase duration. The purpose of this study was to comparatively evaluate the effectiveness of online sensors that were in situ-located in an intermittently aerated and fed membrane bioreactor (IAF-MBR) system. Ammonium and nitrate nitrogen sensors equipped with ion-selective electrodes as well as pH and oxidation-reduction potential (ORP) sensors were employed to online monitoring and optimizing of ammonia oxidation and nitrate reduction processes. The "ammonia valley" or pH bending point, which is indicative of ammonia depletion, was effectively and repeatedly detected by measuring the pH profile, while the "nitrate knee" point, which indicates the completion of the denitrification process, was online-detected by obtaining the ORP profile. The "ammonia valley" and "nitrate knee" were detected at pH and ORP values of 6.47 ± 0.02 and - 162 ± 39 mV, respectively. The ORP and pH first derivatives (dORP/dt and dpH/dt) were found to be more suitable than the untransformed ORP and pH values in detecting pH and ORP inflection points and controlling the shift from the anoxic to the aeration phase. Specifically, the ORP and pH bending points were detected at dORP/dt and dpH/dt values of 1.64 ± 0.82 mV min and 0.005 ± 0.001 min, respectively. Moreover, the ORP first derivative has appeared earlier than the ORP bending point.