## 2 Materials and Methods
### Oxygen Uptake Rate Tests  
The OUR test is used to measure the dissolved oxygen (DO) consumption over time by a microbial community in a given sample. Both endogenous and exogenous OURs were evaluated in this study. Since the endogenous OUR test measures the oxygen consumption rate of microorganisms in the absence of an external substrate, the endogenous OUR only represents the oxygen consumed by biomass respiration. The exogenous OUR test allows us to also quantify the additional fraction of oxygen consumed by microorganisms for the oxidation of the biodegradable organic matter or of the nitrogenous compounds in the presence of an external substrate. The DO concentration was measured using a WTW Multi-parameter portable meter MultiLine(r) Multi 3510 IDS with WTW Optical IDS FDO(r) 925 dissolved oxygen sensors (Xylem Analytics Germany Sales GmbH & Co, Mainz, Germany) (called a DO probe in the next sections). The measured DO concentration was transferred to a PC via a USB connection and the MultiLab(r) Importer; for data acquisition, Excel(r) software (Microsoft Office 2024) was used. In order to assess the actual biological treatability of substrates containing a rather high level of AOXs, in this study, both batch tests and continuous tests were carried out. Five different types of biomasses from different types of plants were tested:  
* Mesophilic biomass from a municipal WWTP (Wastewater Treatment Plant) with a capacity of 50,000 PE (Population Equivalent) that can receive industrial aqueous waste (B1);* Mesophilic biomass from a municipal WWTP with a capacity of 100,000 PE (B2);
* Mesophilic biomass from a municipal WWTP with a capacity of 1500 PE (B3);
* Mesophilic biomass from a municipal WWTP with a capacity of 15,000 PE (B4);
* Thermophilic biomass from a WWTP with a capacity of 50,000 PE that treats industrial aqueous waste (B5). More information about the tested biomasses is reported in Table 1. It was decided to evaluate the exogenous OUR using the substrate as is, and with different dilution factors (DFs): 1:2 and 1:5. Since the results obtained with DFs 1:2 and DF 1:5 were similar, in some cases, the tests were carried out only with the aqueous waste that was diluted 1:5. Temperature and pH were measured using a WTW-IDS Model SenTix(r) 940 probe (Xylem Analytics Germany Sales GmbH & Co, Mainz, Germany). All the OUR tests were conducted at atmospheric pressure and in a temperature range of approximately 12-20 \({}^{\circ}\)C. When the pH of the substrate was too high, it was decided to correct the pH of the solution by dosing it with hydrochloric acid (HCl) or sulfuric acid (H\({}_{2}\)SO\({}_{4}\)). Therefore, in Tables 2 and 3, \"NB\" indicates a non-buffered sample, characterized by a pH of approximately 9 or 10, while \"B\" indicates a buffered sample, characterized by a pH of approximately 8. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Name**} & **Optimal** & **Type of** & **VSS** & **Plant** \\  & **Operating** & **Wastewater Treated** & **Concentration** & **Capacity** \\  & **Temperature** [\({}^{\circ}\)**C] & **Wastewater** & **[g\({}_{\text{VSS}}\) L\({}^{-1}\)]** & **[PE]** \\ \hline B1 & 20 & Municipal wastewater mixed with aqueous waste & 3.2–4.3 & 50,000 \\ \hline B2 & 20 & Municipal wastewater & 2.5–2.8 & 100,000 \\ \hline B3 & 20 & Municipal wastewater & 3.0 & 1500 \\ \hline B4 & 20 & Municipal wastewater & 3.0 & 15,000 \\ \hline B5 & 50 & Aqueous waste & 26.4–27.2 & 50,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the tested aerobic biomasses.