## 4 Conclusions  
In this study, the effect of magnetic nanoparticles (magnetite) along with hydraulic retention time and organic loading rate were studied in the degradation of municipal wastewater into bioenergy. This was carried out in the UASB reactor using the OFAT system at constant mesophilic temperature and solid-to-liquid ratio. The conventional AD of the municipal wastewater was monitored over 30 days to allow adaptation and the stimulation of bacterial activity. With such a low concentration of solids and organics in municipal wastewater, the maximum degradation and biogas production was achieved between 18-21 days. This is due to the use of the UASB reactor that allows the settleability of solids  
Figure 8: Effect of organic loading rate on the removal of color and turbidity.  
Figure 7: The effect of organic loading rate on (**a**) cumulative biogas production; (**b**) degradation of chemical oxygen demand (COD).  
and contact between organics and microorganisms. A shorter HRT would be economically favorable, since it reduces the cost of operation in terms of labor and energy consumption. To further enhance the performance, the effect of magnetite addition was studied by varying magnetite load from 0.25-1 g/L and compared to the control system. The addition of magnetite reduces the lag phase by stimulating early bacterial activity and increases methane content in biogas by 40%. Furthermore, a significant increase in the removal of contaminants is achieved with the addition of 0.25 g/L (4 g) magnetite with COD reduction, color, and turbidity removals reaching 91%, 95%, and 98.9%, respectively. It is evident that the iron ions in magnetite play a significant role in the activity of microorganisms. The 4 g dose (0.5 g/L load) and 21 days HRT were further investigated in the digestion of highly concentrated wastewater effluent for biogas production. The production of biogas increases by over 300% with the increase in organic loading rate, however, there is a decrease in the removal of color, turbidity, and COD reduction compared to the other digesters, which, therefore, warrants a post-treatment step for this stream for the production of cleaner and reusable effluent. Conclusively, the addition of magnetite has the potential to improve the anaerobic digestion of municipal wastewater and increase the production of biogas. These findings form the baseline of upscaling a lab-scale UASB setup into a pilot plant.  
Conceptualization, E.K.T. and L.N.; methodology, L.N. and D.S.; investigation, L.N.; resources, E.K.T. and S.R.; writing--original draft preparation, L.N.; writing--review and editing, E.K.T. and S.R.; supervision, E.K.T. and S.R.; project administration, S.R.; funding acquisition, E.K.T. and S.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the South African Water Research Commission (WRC Project: C2021/2022-00958).  
Not applicable.  
The authors would like to thank the Green Engineering Research group, under the Department of Chemical Engineering at the Durban University of Technology, South Africa, for their support.  
The authors declare no conflict of interest.