## 3 Results  
### Voltage Generation According to Hydraulic Flow Mode and Direction  
Three MFCs (S-, M-, and L-MFCs) showed different voltage generations because of the different COD loading rates, removed CODs and other factors when all MFCs were operated individually (phase I) (Figures 3 and 4). L-MFC showed the highest voltage generation of 0.76 V, followed by M-MFC (0.52 V), and S-MFC (0.34 V) (Figure 4a). More substrate seemed to be used for electricity generation because of the highest COD loading rate of L-MFC. After a forward hydraulic series connection of three MFCs (phase II), it was expected that the amount of COD removed in three MFCs would become equal and the voltage of M- and L-MFCs decrease to the level of the voltage generated from S-MFC. On the other hand, the average voltage generation, COD loading rate, and removed COD of S- and M-MFCs did not changed much. While the average voltage generation of L-MFC decreased to 0.52 V, which appears to be caused by the decrease in COD removed (Figure 4b). M- and L-MFCs showed similar amounts of COD removed (187 mg/L and 204 mg/L, respectively), and produced the most similar voltage at phase II because of their similar the removed COD. On the other hand, all MFCs showed a different pattern of voltage generation under a reverse hydraulic series connection (phase III). Although M- and L-MFCs showed similar COD loading rates, they produced different voltages and the average voltage generation of L-MFC increased to 0.83 V because of its highest COD  
Figure 2: Diagram of a stacked microbial fuel cell (MFC) system operation according to the hydraulic flow connection mode: (**a**) Individual MFC operation (phase I), (**b**) a stacked MFC system operating in a forward hydraulic flow series connection mode (phase II), and (**c**) stacked MFC system operating in a reverse hydraulic flow series connection mode (phase III); the arrow means the direction of hydraulic flow.  
Electricity generation is generally affected by the organic loading rate, amount of COD removed, carbon source, reactors volumes, and other factors [24]. Two two-chamber MFCs produced similar cell voltage at different substrate concentrations and their stack systems were operated successfully without voltage reversal [25]. Single-chamber MFCs showed similar voltage generation at different organic loading rates because the bacterial activity, internal resistance, and reactor limitation could be different at each OLR [26]. In this study, despite the similar amounts of COD removed, M-MFC at phase III produced a lower voltage than its voltage at phases I and II because of various factors. The Pearson correlation showed that voltage generation was affected by several factors, including the reactor volume, HRT, influent COD concentration, loading rate, removed COD, and internal resistance (Table 2). In particular, voltage generation showed the highest Pearson correlation (0.9445) with the COD removed. In other words, the same the amount of COD removed can produce the same voltage because theoretically the same electrons and hydrogen ions can be generated from the same amount of COD removed. This indicates that equalizing the amount of COD removed can be a way to minimize the voltage imbalance between each MFC in a stack system.