## 4 Conclusions  
This study was the first attempt to minimize the voltage loss through a simple hydraulic connection of MFCs. The S-, M-, and L-MFCs showed different voltage productions at phase I (individual operation) because of a range of factors. When they were connected hydraulically in series (phase II), some MFCs produced a similar voltage by the same amount of COD removed. Moreover, the energy loss (<30%) could be minimized, even in the electric series and parallel connection. On the other hand, there was still high energy loss. Therefore, it will be necessary to find a solution through studies on the effects of the loading rates, actual wastewater, and long-term operations on the stack performance. This finding shows that connecting MFCs with different volumes can be a new option that can minimize the energy loss in an MFC stack system.  
**Funding:** This study was supported financially by the National Research Foundation of Korea (NRF) grant funded by Korea Government (MIST) (No. 2015R1C1A1A01054204 and 2018R1D1A1B07046741) and by Korea Electric Power Corporation through Korea Electrical Engineering and Science Research Institute (No. R15 XA03-01).  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.  
**Conflicts of Interest:** The authors declare no conflict of interest and 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.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{\
\begin{tabular}{c} **Electrode** \\ **connection** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **MFC** \\ **Stacks a** \\ \end{tabular} } & \multicolumn{3}{c}{**Theoretical Voltage b (V)**} & \multicolumn{3}{c}{**Actual Voltage c (V)**} & \multicolumn{3}{c}{**Voltage Loss (\%)**} \\ \cline{3-10}  & & & & & & & & & & \\ \cline{3-10}  & & & & & & & & & & \\ \hline \multirow{4}{*}{Series} & S-M & 0.863 & 0.865 & 0.409 & 0.242 & 0.211 & 0.102 & 72.1 & 75.6 & 75.1 \\  & S-L & 1.103 & 0.844 & 0.975 & 0.116 & 0.264 & 0.095 & 89.5 & 68.7 & 90.3 \\  & M-L & 1.282 & 1.045 & 1.096 & 0.321 & 0.715 & 0.146 & 75.1 & 31.6 & 86.7 \\  & S-M-L & 1.624 & 1.377 & 1.240 & 0.101 & 0.143 & 0.094 & 93.8 & 89.6 & 92.4 \\ \hline \multirow{4}{*}{Parallel} & S-M & 0.432 & 0.433 & 0.205 & 0.211 & 0.223 & 0.104 & 51.2 & 48.5 & 49.3 \\  & S-L & 0.552 & 0.422 & 0.488 & 0.312 & 0.209 & 0.101 & 43.5 & 50.5 & 79.3 \\ \cline{1-1}  & M-L & 0.641 & 0.523 & 0.548 & 0.351 & 0.377 & 0.211 & 45.2 & 27.9 & 61.5 \\ \cline{1-1}  & S-M-L & 0.541 & 0.459 & 0.413 & 0.212 & 0.221 & 0.109 & 60.8 & 51.9 & 73.6 \\ \hline \hline \end{tabular}  
* S-M; S- and M-MFCs stack, S-L; S- and L-MFCs stack, M-L; M- and L-MFCs stack, and S-M-L; S-, M-, and L-MFCs stack. b  Theoretical voltage is calculated based on the actual voltage generated by the individual cells when individual cells are connected. c  The actual voltage is actually measured voltage when the individual cells are connected.  
\end{table}
Table 3: Theoretical and actual measured voltage productions and voltage loss in a stacked system according to the electrode connection methods at phases I, II, and III.