## 2 Materials and Methods
### Oxygen Uptake Rate Tests
#### 2.2.1 Batch Tests  
The immediate performance of the OUR tests in the batch mode, combined with the minimal instrumentation needed and the speed of execution, is a significant advantage [27; 28]. The experimental apparatus employed in this study was similar to that used by [PERSON] et al. [29] and [PERSON] et al. [30]. The components included a flask, a magnetic stirrer with a magnetic bar, a DO probe, a personal computer, a mechanical aerator, a porous stone and sealing film. The procedure is described below:  
1. The volatile Suspended Solid (VSS) concentration (g\({}_{\text{VSS}}\) L\({}^{-1}\)) of the tested biomass was analyzed;
2. The biomass volume was put into a flask placed on a magnetic stirrer; the magnetic bar spinning at the bottom of the flask allowed the biomass (or the biomass-substrate mixture in the case of an exogenous test) to remain well mixed during the test;
3. The porous stone connected to the mechanical aerator was immersed in the biomass volume to increase the DO value, together with the probe to measure the DO value;
4. The biomass was aerated until saturation: when the DO value was stable, the aeration was interrupted;
5. Only in the case of an exogenous test, after turning off the aeration, the volume of substrate to be tested was poured into the flask together with the biomass;
6. Immediately after turning off the aeration, the flask was sealed with parafilm in order to isolate the volume and to avoid air exchange with the external environment;
7. Lastly, data transfer from the probe to the computer was started. The data were recorded in an Excel spreadsheet at 5 s intervals. At the end of the test, the trend in the DO value over time was plotted in a scatter chart using the calculation program Excel. The slope of the linear regression line, which interpolates the measured data, was extrapolated from the scatter chart. Only the central section of the curve was considered at this stage. Then, the specific OUR (sOUR) value, expressed in mg\({}_{\text{DO}}\) g\({}_{\text{VSS}}\)\({}^{-1}\) h\({}^{-1}\), was calculated by dividing the slope of the DO utilization curve (mg\({}_{\text{DO}}\) L\({}^{-1}\) h\({}^{-1}\)) by the VSS concentration in the batch reactor (g\({}_{\text{VSS}}\) L\({}^{-1}\)). The sOUR value indicates the oxygen consumption rate of the biomass with respect to the degradation of the organic matter contained in the substrate alone. In other terms, sOUR is the OUR value of endogenous aliquot. If it was necessary, the non-specific OUR value was normalized for a temperature (T) of 20 \({}^{\circ}\)C using the following formula (valid for T values between 15 \({}^{\circ}\)C and 25 \({}^{\circ}\)C):  
\[\text{OUR}_{(20\ ^{\circ}\text{C})}=\text{OUR}\cdot\theta^{(20\ -\text{T})} \tag{1}\]  
where \(\theta\) is equal to 1.05 if T < 20 \({}^{\circ}\)C and to 1.07 if T > 20 \({}^{\circ}\)C.  
The endogenous OUR tests were first conducted with 300 mL of biomass to study the endogenous respiration alone. In the exogenous OUR tests, 300 mL of biomass was aerated up to a DO concentration of 7.5-8.0 mg L\({}^{-1}\), and then mixed with 300 mL of a substrate with a certain AOX concentration. At this point, the aeration was stopped and the laboratory scale batch reactor with a 600 mL mixture was hermetically closed to avoid \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Substrate** & **AOX [mg L\({}^{-1}\)]** & **pH (NB) [-]** & **pH (B) [-]** & **Tested Biomass(es)** \\ \hline Sub1 (real) & 2.00 \(\pm\) 0.01 & 10.20 & 8.10 & B1/B2 \\ \hline Sub1 (real) diluted 1:5 & 0.40 \(\pm\) 0.01 & - & 7.80 & B1 \\ \hline Sub1 (real) diluted 1:10 & 0.20 \(\pm\) 0.01 & - & 7.80 & B1 \\ \hline Sub6 (real) & 30.80 \(\pm\) 0.01 & 8.00 & - & B1/B5 \\ \hline Sub7 (synthetic) & 80.00 \(\pm\) 0.01 & 8.00 & - & B1 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Biomasses and characteristics of substrates used in carrying out continuous tests. Regarding pH values, “NB” indicates a non-buffered sample, while “B” indicates a buffered sample; “-” means “not measured”.