## 3 Results and Discussion
### Evolution of Solids and Soluble Compounds  
The BMP tests resulted in the removal of particulate matter in terms of both TS and VS, as presented in Tables 2 and 3 and illustrated in Figure 2. The removal efficiency of VS is one of the crucial control factors in the AD process [29]. As can be seen in Figure 2A, the percentages of TS removal were 19%, 20% and 24% for the C, N and NO experiments, respectively. In the case of the VS removal, their values were 28%, 28% and 33% for the C, N and NO experiments, respectively. Based on these results, it can be stated that the iron nanoparticles, and especially ozone, modestly contribute to the improvement of AD by achieving slightly higher VS removal percentages. Ozone is an effective method for solid removal and is also used in other scenarios, such as pre-treatment to enhance water disinfection [30]. The percentages of TS and VS removal were less than 50% in all cases, similar to those obtained by [PERSON] et al. [31], where BMP was carried out with digestate from a BHP using sludge, viruses and poultry manure as substrates. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & **MC** & **MN** & **MNO** \\  & **Mean (SD 1 \%)** & **Mean (SD 1 \%)** & **Mean (SD 1 \%)** \\ \hline pH & 8.26 (0.73) & 8.20 (0.92) & 8.25 (1.23) \\ TS (g/kg) & 18.85 (0.22) & 19.88 (0.30) & 18.21 (0.16) \\ VS (g/kg) & 11.55 (0.18) & 12.56 (0.61) & 11.36 (0.62) \\ TCOD (mgO\({}_{2}\)/L) & 22,584.56 (14.97) & 23,381.19 (1.18) & 22,823.55 (0.52) \\ SCOD (mgO\({}_{2}\)/L) & 8882.46 (7.77) & 9599.43 (8.29) & 8762.97 (9.08) \\ DOC (mg/L) & 595.93 (6.24) & 1516.87 (2.70) & 838.00 (1.39) \\ TN (mg/L) & 1977.75 (5.23) & 1726.00 (2.68) & 1552.25 (1.30) \\ \hline \hline \end{tabular} \({}^{1}\) SD: standard deviation (%).  
\end{table} 
Table 3: Results obtained at the end of the Biochemical Methane Potential tests. Control (MC), nanoparticles (MN), and nanoparticles together with ozone pre-treatment (MNO). Figure 2B shows the removal of oxidized compounds in terms of COD. As can be seen in this Figure, the COD removal was enhanced by the addition of iron nanoparticles and the use of ozone as a pre-treatment. A number of studies have claimed that powdered activated carbon (PAC) [32] and iron nanoparticles containing Fe\({}^{0}\)[14] can improve COD removal. Similarly, the same effect has been observed with ozone [33]. In terms of TCOD, the control tests achieved 42% removal, whereas the nanoparticles tests (MN) achieved a removal of 48%, and their combination with ozone (MNO) achieved a removal of 49%. Thus, the increase in TCOD removal was 33% with the addition of the nanoparticles (iron nanoparticles encapsulated in carbon) and 35% when combined with the ozone pre-treatment. Other authors, using powdered activated carbon (PAC) at a concentration of 1.7 g/L in anaerobic membrane bioreactors (AMBRs) and treating simulated domestic wastewater at 35 \({}^{\circ}\)C and pH 7, achieved a 22% COD [34]. In this regard, the results obtained with ozone align with the literature. BMP tests conducted using ozone-pre-treated samples of OMW (olive mill wastewaters) achieve approximately 40% COD removal [35]. Regarding the soluble material (SCOD), its removal was improved by the addition of nanoparticles, reaching a removal percentage of 70%, while without nanoparticles, it was removed by 66%. In other studies [14], a SCOD removal ratio of 82% was achieved by applying 0.5 g Fe\({}^{0}\)/L during mesophilic AD in a batch process involving a mixture of dewatered sludge and secondary sludge from a WWTP. However, this dosage is higher than that used in this study (0.05 g nanoparticles/L), and it is important to note that these nanoparticles also consisted of only 2.5% Fe\({}^{0}\). In the case of the combined treatment with nanoparticles and ozone, the SCOD removal only reached 63%, a value lower than that obtained in the control tests--66%. This could be explained by the previous oxidation of a fraction of the SCOD by the ozone acting negatively on the AD process performance.