## 4 Conclusions  
The use of hydroxyl radicals as a disinfectant is an environmentally friendly approach to water disinfection. A laboratory-scale flow reactor has been designed to produce hydroxyl radicals and inactivate bacteria. The tubular reactor was filled with a previously synthesized catalyst based on cobalt ferrite. Detailed analyses by XRD, XRF, EDS, and Raman spectroscopy showed that the obtained cobalt ferrite is a single-phase spinel with the formula Co\({}_{1.05}\)Fe\({}_{1.96}\)O\({}_{4}\). The packed-bed catalytic reactor was tested for H\({}_{2}\)O\({}_{2}\) decomposition over a wide range of concentrations, from 3 to 80 mM. At low H\({}_{2}\)O\({}_{2}\) concentrations, the decomposition follows the kinetics of a first-order reaction. The reaction order likely increases with increasing H\({}_{2}\)O\({}_{2}\) concentrations above 10 mM. The synthesized catalyst is stable and remains active for a long time. During the tests, the catalytic reactor operated for over 2000 h without any performance degradation.  
The effectiveness of water disinfection was tested in a wide range of initial bacterial concentrations. The initial bacterial counts for testing were selected based on analysis of water from a nearby medium-sized river. The river flows through rural and urbanized areas, providing a source of water for the municipal water supply system. In water disinfection studies, the effect of contact time on reactor efficiency was investigated. It was found that the kinetics and efficiency of bacterial inactivation depend significantly on the initial bacterial concentration. At low bacterial concentrations (below 13,800 CFU/L), rapid and almost complete inactivation of bacteria was observed. In the case of high bacterial concentrations (\(\geq\)25,000 CFU/L), the bactericidal effectiveness of H\({}_{2}\)O\({}_{2}\) decreased. The limited concentration of bacteria that can be effectively inactivated makes the developed method less favorable compared to other disinfection methods, such as chlorination or ozonation. However, the efficiency of the catalytic reactor can be improved by using a more active catalyst that provides deeper decomposition of H\({}_{2}\)O\({}_{2}\) and thus produces more hydroxyl radicals.  
A regression model was obtained that relates the number of surviving bacteria to their initial number and the time of contact with the catalyst. The conclusion is that the proposed flow reactor can be used for disinfecting water with a moderate bacterial load. Further research is needed to optimize operating parameters and increase disinfection effectiveness.  
Conceptualization, A.S.; methodology, I.D., N.D. and V.H.; software, V.H.; investigation, I.D., N.D., V.B. and V.H.; data curation, N.D.; writing--original draft preparation, A.S., V.B., V.H. and D.Z.; writing--review and editing, A.S. and D.Z.; visualization, D.Z. and V.H.  
funding acquisition, D.Z. and A.S. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the Ministry of Education and Science of Ukraine (project number 0124U000479).  
Not applicable.  
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.  
The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} CFU & Colony-forming unit \\ EDS & Energy-dispersive X-ray spectroscopy \\ SEM & Scanning electron microscopy \\ ROS & Reactive oxygen species \\ TDS & Total dissolved solids \\ WHO & World Health Organization \\ XRD & X-ray diffraction \\ XRF & X-ray fluorescence \\ \end{tabular}