## 3 Results
### Assessment of Process Efficiency: Linking Physico-Chemical Indicators with Process Performance
#### 3.1.2 Comparative COD Decrease  
Figure 3 presents the results of chemical oxygen demand (COD), expressed as a percentage decrease after the ECZ and ECZ-MAG processes. The highest COD decrease was achieved in the ECZ-MAG process at a mixing speed of 250 rpm and a contact time of 30 min, amounting to 89.87%. The lowest COD decrease was recorded for the ECZ-MAG process at 350 rpm over 10 min, with a reduction of 79.88%. In all other experiments, COD decreases exceeded 80%.  
The presence of the magnetic field proved to be effective only in the processes lasting 30 min, where higher COD decrease percentages were recorded compared to the non-magnetic experiments. This indicates that the magnetic field's positive impact on pollutant removal is time-dependent. For shorter contact times (10 and 20 min), the magnetic  
Figure 3: COD decrease after ECZ and ECZ-MAG processes at different mixing speeds of 250 rpm and 350 rpm for (**a**) 10 min, (**b**) 20 min, and (**c**) 30 min. Note: COD removal is expressed as percentages of mean COD values; standard deviation from the average value for COD is in the range \(\pm\)(11.50–30.90) mg O\({}_{2}\)/L.  
field does not significantly enhance COD decrease, likely because these durations are insufficient for the magnetic field to induce notable improvements in micro-mixing, ion transport, or floc formation. In contrast, during longer treatment times (30 min), the magnetic field promotes better particle destabilization and aggregation through enhanced ionic movement and micro-convection, leading to improved COD decrease. Therefore, the benefits of applying a magnetic field become more apparent when the process duration allows sufficient time for these mechanisms to develop fully.  
This finding contrasts with the results obtained in our previous investigation on the influence of magnetic assistance in EC processes employing Fe electrodes [29], where no notable enhancement in COD removal was observed with the application of a magnetic field. These differences suggest that the type of electrode material plays a critical role in determining the synergistic effect of magnetic fields. Specifically, it appears that in the case of Fe electrodes, the electrochemical behavior and magnetic influence may counteract each other, limiting process efficiency. Conversely, with Al electrodes, the magnetic field contributes positively to COD reduction, indicating improved synergy and enhanced pollutant removal performance.  
Despite the satisfied COD decrease percentages, it is important to compare the final COD values in the treated compost-derived wastewater with the regulatory limits prescribed by the Ordinance (Official Gazette NN 26/20), especially given the initially high COD concentration (COD\({}_{0}\) = 1642 \(\pm\) 105). The results show that the final COD values range from 166 \(\pm\) 11.5 to 330 \(\pm\) 22.56 mg O\({}_{2}\)/L, which exceeds the maximum permissible limit for discharge into surface waters (125 mg O\({}_{2}\)/L) but is below the allowed limit for discharge into the public sever system (700 mg O\({}_{2}\)/L).