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Determining whether a certain compound is toxic (or inhibitor) or not to a biomass of a WWTP is crucial in order to avoid WWTP malfunction. Respirometric techniques have been commonly used to assess the toxicity of a certain compound by evaluating the oxygen uptake rate (OUR) profile obtained when a pulse of substrate is added to endogenous biomass and comparing it with the OUR profile obtained when the pulse is a mixture of substrate and the possible toxic or inhibitor (PTI). However, when using this method with biodegradable compounds some difficulties arise because the PTI consumption implies oxygen consumption as well. In this study, a modified procedure for toxicity assessment using respirometry is developed which overcomes the difficulties caused by the toxic biodegradability. This procedure is based on the comparison of different respirograms obtained with pulses of control substrate before and after adding the PTI and with a pulse of PTI and control substrate together. Moreover, some examples are shown with p-phenylenediamine and phenol as an example of toxic and inhibitor for nitrifying biomass. [ABSTRACT FROM AUTHOR]

Achievement of partial nitrification relies on the different characteristics of the microorganisms involved in the two steps of nitrification (AOB, ammonium oxidizing bacteria and NOB, nitrite oxidizing bacteria). Several configurations and conditions have been used to achieve partial nitrification taking into account their different responses in front of some environmental conditions like pH, temperature (T), dissolved oxygen (DO) and the inhibitory compounds free ammonia (FA) and free nitrous acid (FNA). This work is a theoretical study about the utilization of a control strategy based on controlling OUR by manipulating the influent flow-rate together with additional changes in T, pH or DO for achieving stable partial nitrification. Two different configurations were simulated. Configuration A consisted of three continuous stirred-tank reactors in series plus a settler, while configuration B consisted of a single reactor with the same total volume and a settler. The developed control system was shown as a good tool to achieve partial nitrification with both configurations. Nevertheless, configuration A showed better results and partial nitrification was obtained even under theoretically unfavourable conditions as T = 15 degrees Celsius, pH = 8.3 and DO = 3 mg O(2) L(-1).

This work presents the application of expert control strategies for a successful and stable operation of partial nitrification. A high-strength ammonium wastewater was treated in a continuous pilot plant with a configuration of three reactors in series plus a settler. The system was operated at mild temperature (around 25 degrees C) and at high sludge retention time to obtain a significant nitrifying biomass concentration. The implemented control system with expert supervisory rules provided a stable and robust operation of the partial nitrification system even when the pilot plant was subjected to several disturbances as change in influent concentration, failure of aeration in one reactor and operation with variable biomass concentration. The control system was based on local feedback control loops for dissolved oxygen and pH with proper setpoints for nitrite oxidizing bacteria (NOB) inhibition and an inflow control loop based on Oxygen Uptake Rate (OUR) measurements, which allowed working at the current maximum ammonium oxidation capacity. Two additional expert rules were implemented for automatically determine the required OUR setpoint in accordance with the biomass activity and to produce a fast load decrease when a system overload was detected.