**14.7 The effect of hydraulic retention time (HRT)**

Hydraulic retention time (HRT) is a critical parameter in the treatment of wastewater and regulates the residual substrate concentration and the amount of dissolved oxygen (DO) in the reactor. When HRT decreases, the concentration of the substrate increases, leading to the utilization of the entire substrate with an improved voltage and power density. On the other hand, higher concentrations of DO in the influent wastewater lead to an increase in the potential for oxidation-reduction (ORP), resulting in a reduction in the voltage and power density generated in the MFC. To understand the impact of HRT on bioelectricity generation, the MFC was run continuously with dairy wastewater at seven different HRTs (2, 3, 4, 5, 6, 7, and 8 days) [96].

The explanation for the rise in voltage as HRT rises may be usually due to the long interaction time between biofilm and organic matter, which may demonstrate the benefit of biofilm, degradation of a substrate, electron output, and transition to the surface of the anode. Despite this, the voltage decreases slightly when HRT increases (8 days). These observations are compatible with the conclusions of single-chamber MFC energy production with the aerial cathode in the existence and exclusion of proton exchange membrane, and also the results of electricity generation and the wastewater treatment utilizing single-chamber MFC [104].
