Reactor setup

The MRC reactor was constructed as previously described with minor modifications.²⁰ The reactor consisted of an anode chamber, a cathode chamber, and a small number of the RED stack (Figure 1). The anode and cathode chambers were constructed using poly(methyl methacrylate) with a working volume of 30 mL. A heat-treated graphite fiber brush, 3 cm in diameter and 2.5 cm long, was used as the anode electrode following the method from a previous study and placed vertically in the middle of anode chamber.²¹ The wet-proofed (30%) carbon cloth (1071 HCB; AvCarb, United States) was used as a cathode electrode with a modification of 4 polytetrafluoroethylene diffusion layers on air-side and a Nafion binder mixed with platinum catalyst on solution-side. The Ag/AgCl reference electrodes were assembled on each of the chambers to measure the electrode potentials. The RED stack consisted of 3 cell pairs (except as noted) made of alternation anion and cation exchange membrane (Selemion AMV and CMV, Asahi Glass, Japan). Each membrane had an essential area of 12.25 cm² (3.5 cm × 3.5 cm). These membranes were separated by silicone gaskets and a nylon mesh spacer with a thickness of 0.18 mm, forming flow channels for alternating high-concentration (HC) and low-concentration (LC) solutions.

Figure 1.

Schematic diagram of microbial reverse-electrodialysis cell (MRC). HC indicates high concentration; LC, low concentration; MRC, microbial reverse-electrodialysis cell; RED, reverse-electrodialysis cell, AEM, anion exchange membrane; CEM, cation exchange membrane.

Solutions

The anode was enriched with exoelectrogens in a single chamber of an MFC at fix external resistance 1000 Ω and fed with 1 g/L of sodium acetate (CH₃COONa). The MFC reactor was fed with gradual increased concentrations of NaCl (0.10-0.35 M) to acclimate the exoelectrogens to chloride ions. The adapted anode was transferred to the MRC after reproducible maximum voltages on the MFC were stable at least for 3 consecutive batch cycle.

The anolyte solution was prepared by dissolving the following chemicals in 1 L of deionized water: 1.90 g CH₃COONa except as noted, 0.31 g NH₄Cl, 0.13 g KCl, 6.60 g NaH₂PO₄.2H₂O, 8.19 g Na₂HPO₄, as well as 12.5 g minerals and vitamins. A 1.0 M NaHCO₃ solution was used as catholyte (conductivity ~50.6 mS/cm; pH ~8.03). The HC solution was 35.0 g/L NaCl (~53.6 mS/cm), whereas the LC solution was 0.70 g/L NaCl (~1.39 mS/cm), creating a salinity ratio of 50. Leachate, as a representation of organic-rich wastewater, was used as an electron donor (substrate). The characteristic of leachate is as follows: soluble biochemical oxygen demand (COD) = 2100-2300 mg/L; nitrogen (NH₃) = 130 mg/L, nitrogen (NO₃^-) = 6.5 mg/L, phosphate (orthophosphate) = 86.4 mg/L, conductivity = 13.91 mS/cm, and pH = 7.15.

Reactor operation

Preliminary experiments were conducted to determine the optimum membrane pairs in the RED stack. The MRC reactor was operated using different numbers of the RED stack (1, 3, and 5 cell pairs). The reactor was fed with 1.9 g/L of CH₃COONa and connected to 1000 Ω external resistor. In further experiments, the MRC reactor was operated with different external resistances (150 Ω , 300 Ω , 650 Ω, and 1000 Ω). Food waste leachate was used as a substrate in the anode chamber and was replaced when the generated current decreased to less than 0.5 mA. In the RED stack, HC and LC were used as a feed solution and continuously supplied at a fixed rate of 1.2 mL/min during reactor operation. All of the MRC experiments were done in a constant room temperature (26 ± 1°C).

Experimental analysis and calculations

The potential difference (U) between anode and cathode was monitored and recorded every 5 minutes using a voltage recorder (VR-71; T&D Corporation, Japan) connected to a personal computer. The current was determined as i = U/R, and the current density was normalized by the cathode projected surface area (7 cm²). The maximum power density was obtained from the polarization curve, which was determined using variable external resistance (5-40.000 Ω) at 20-minute intervals. The effluent anolyte, catholyte, HC, and LC solutions were measured for pH and conductivity using a pH meter (Orion 720A+; Thermo Scientific Co.) and a conductivity meter (Orion Three-star, Thermo Scientific Co., United States). The COD of influent and effluent anolyte was measured according to the standard method (DR-2800; Hach Company, Indonesia). Ion flux efficiency (η_flux), CE, energy recovery (r_E), and energy efficiency (η_E) were described as follows.¹⁴

The chemical activity of ion (αi) was determined by multiplying the chemical concentration in molarity by the activity coefficient (γi). The extended Debye-Huckel equation was used to determine the activity coefficient²²:

The Debye-Huckel constants are A = 0.5085 kg^1/2/mol^1/2 and B = 0.3282 kg^1/2/A.mol^1/2. The ion size parameter (α⁰) was 0.78 Â for both sodium (Na⁺) and chloride (Cl⁻), whereas K_Na and K_Cl are 0.105 and −0.009 kg²/mol², respectively. I_s is the ionic strength in molality, and m is the molal concentration. This equation is valid for a NaCl solution up to 1 molality.²³

The ion flux efficiency (η_flux) is a ratio of the current generation in the MRC to the ion flux through the ion-exchange membrane and determined as

where N_MP is the number of cell pairs, i is the current generation in MRC (A), Q is the flow rate of feed stack (L/s), and c is the molar concentration of NaCl (mol/L).

The CE was calculated based on the COD removal and the number of coulombs produced during the reactor operation as previously described²³:

where 8 is the number of electrons for substrate oxidation, i is current produced during the operational time (A), V_An is the volume of the anolyte (L), and ΔCOD is the change of substrate during operational time (g-COD).

The energy recovery r_E is the ratio of power production to the total energy provided in the MRC, and it was defined as

where P is the power produced from the MRC (W), ΔH_C is the heat of combustion of the substrate (kJ/mol), is the mol concentration of substrate at the initial of operational time (mol), and t_B is the operational time of reactor (day). Xⁱⁿ is the theoretical energy (W) provided by the differences in salinity concentration of seawater and river water, and it was determined as²⁴

Energy efficiency η_E is the ratio of the power production to theoretically extractable energy provided by MRC, and it was determined as¹²

where is the mol concentration of substrate at the end of operational time, X^out is the theoretical energy (W) of the salinity gradient energy remaining in seawater and river water effluents.

A Tafel plot test was conducted to investigate bioelectrochemical kinetics of anode biofilm electrode using a potentiostat (DY2300; Digi-Ivy, Inc., Austin, TX, USA) in 3-electrode conventional cells. The anode, saturated calomel electrode, and the cathode served as the working electrode, reference electrode, and counter electrode, respectively.

Performance of MRC using different numbers of membrane cell pairs

The reactor was operated under fed-batch mode with different membrane cell pairs (1 MP, 3 MP, and 5 MP) and fixed external resistance (1000 Ω). As shown in Figure 2, the cell current tended to increase as the number of membrane cell pairs increased. The MRC with 5 MP produced a maximum cell current of 1.12 ± 0.09 mA, which was 1.2- and 1.9-fold higher than that of obtained using 3 MP (0.9 ± 0.02 mA) and 1 MP (0.60 ± 0.03 mA), respectively.

Figure 2.

Current generation in MRC reactor at the indicated number of membrane cell pairs (external resistance of 1000 Ω). MRC indicates microbial reverse-electrodialysis cell.

The polarization curve was measured by varying an external resistance in the MRC reactor to determine the maximum power density. It was observed that the maximum cell potentials produced during the polarization test increased as the number of membrane pairs increased (Figure 3A). Furthermore, adding more membrane pairs increased the maximum current density from 0.51 mA/cm² (1 MP) to 0.85 mA/cm² (5 MP). Meanwhile, consistent with previous research,²⁵ the cell potential decreased with increasing current density due to the activation losses, ohmic losses, and concentration losses during MRC operation. The power density of the MRC reactor was inclined to follow the previous results of maximum cell potentials (Figure 3B). The MRC with 1 MP produced the maximum power density of 1.19 W/m² as normalized by 7 cm² of cathode area or 30 W/m³ as normalized by anolyte volume. Meanwhile, the MRC with 3 MP produced the maximum power density of 2.31 W/m² (58.3 W/m³). Furthermore, the MRC with 5 MP achieved higher power densities, with a maximum power density of 2.75 W/m² (69.4 W/m³). However, the increase in power density as a result of an increase in the number of membrane pairs from 3 to 5 (0.44 W/m²) was smaller than that obtained when increasing membrane pairs from 1 to 3 (1.12 W/m²), suggesting that further increase in membrane pairs did not significantly increase power density.

Figure 3.

(A) Polarization and (B) power densities of MRC at different number of membrane cell pairs. MRC indicates microbial reverse-electrodialysis cell.

These values of power density were relatively higher compared with power density produced by the MFC with the same operational conditions (reactor working volume and electron donor). Based on the previous study,²⁶ 2 chambers of MFC produced 0.45 W/m² of power density. Subsequently, Choi and Ahn²⁷ also reported that the MFC reactor produced 0.77 W/m² of power density with food waste leachate as substrate and 24 mL of working volume reactor. The research from Moharir and Tembhurkar²⁸ also showed relatively the same result, claiming that the MFC reactor produced 0.29 W/m² of power density. These results confirmed that the high power density produced in the MRC reactor is due to the salinity energy created from the small number of membrane pairs (RED stack).

According to the above results, 3 MP was selected as the optimum operational condition as the increase in the number of membrane pairs to 5 MP did not substantially increase the reactor performance. Therefore, in further experiments, the reactor was operated using 3 MP with different external resistors (150 Ω, 300 Ω, 600 Ω, and 1000 Ω).

Effect of external resistance on power generation

In this section, the MRC reactors were operated for several cycles with food waste leachate as a donor electron and were connected to different external resistors (150 Ω, 300 Ω, 600 Ω, and 1000 Ω). Figure 4A shows that the power density increased from 0.82 ± 0.038 W/m² to 1.53 ± 0.198 W/m² as the external resistance decreased from 1000 Ω to 300 Ω. Furthermore, the power density significantly decreased to 0.34 ± 0.067 W/m² as the external resistance decreased to 150 Ω. These results indicated that when external resistance dropped, power density would increase as a result. However, a fall in power density occurred when the external resistance exceeded its optimum point. The differences in power density produced were likely due to the changes in the internal resistance of the MRC reactor. Based on the slope of the polarization curve, the total internal resistance of the reactor was 544.9 Ω, 507.7 Ω, 372.0 Ω, and 401.1 Ω for the MRC with the external resistance of 1000 Ω, 600 Ω, 300 Ω, and 150 Ω, respectively. The MRC reactor with the external resistance of 300 Ω achieved higher power production, compared with the other reactors, as the total internal resistance in this reactor was close to the value of its external resistance. This result was consistent with the previous study²⁹; it is stated that the reactor’s power density was maximized when the external resistance connected to it was equal or close to the total internal resistance.

Figure 4.

(A) Power density and (B) ion flux efficiency of MRC reactor at the indicated external resistance. MRC indicates microbial reverse-electrodialysis cell.

To determine the influence of external resistance to the salinity energy created by the RED stack, the ion flux efficiency was measured. Ion flux efficiency is a ratio of the current generation in the MRC reactor to the ion flux through the ion-exchange membrane. Based on the concentration change in the LC effluent, the ion flux efficiency was 82 ± 2%, 88 ± 3%, 94 ± 6%, and 90 ± 5% with the average currents produced were 0.74 ± 0.038 mA, 1.04 ± 0.098 mA, 1.90 ± 0.198 mA, and 1.13 ± 0.067 mA for the MRC reactor with the external resistance of 1000 Ω, 600 Ω, 300 Ω, and 150 Ω, respectively (Figure 4B). These results showed that the ion flux efficiency increased (from 82 ± 2% to 94 ± 6%) with a decrease in the external resistance (from 1000 Ω to 300 Ω). This increase led to a rise in the cell current produced in the MRC reactor (0.74 ± 0.038 mA to 1.90 ± 0.198 mA). The ion flux efficiency dropped to 90 ± 5% as the external resistance fell to 150 Ω; thus, the cell current produced decreased to 1.13 ± 0.067 mA. When the reactor was operated with low external resistance, the reactor generated a higher current due to the more frequent electron transfer from anode to an electrode, which then would support faster cathode reaction and high electrogenic activity.²⁹ According to the above results, it can be concluded that the external resistance affected the ion flux efficiency through the magnitude of current produced in the reactor.

Effect of external resistance on CE and COD removal

Coulombic efficiency is the ratio of the total recovered coulombs to the theoretical amount of coulombs provided by the substrate. Generally, the CE value is influenced by substrate removal (as the soluble COD removal) in the reactor. Based on the calculation of initial and effluent of soluble COD in the MRC reactor, the substrate removal reached up to 73 ± 2.8%, 62 ± 2.1%, 52 ± 2.3%, and 41 ± 2.6% for the reactor with external resistance of 1000 Ω, 650 Ω, 300 Ω, and 150 Ω, respectively (Figure 5A). It was observed that the COD removal increased as the external resistance increased. The higher COD removal was likely a result of the longer batch cycle time of the MRC reactor with 1000 Ω than those of other reactors. The longer the cycle time of the reactor, the more substrate was consumed by the bacteria; thus, high-soluble COD removal in the reactor was achieved.

Figure 5.

(A) COD removal and CE and (B) Tafel plots of MRC reactor at the indicated external resistance. CE indicates Coulombic efficiency; MRC, microbial reverse-electrodialysis cell.

The CE value of the MRC reactor at different external resistance can be seen in Figure 5A. The CE obtained from the MRC reactor with 300 Ω (70 ± 2.6%) and 150 Ω (70 ± 2.9%) was higher than that of the reactor with 1000 Ω (34 ± 2.7%) and 650 Ω (60 ± 2.3%). The higher CE value on the MRC reactor with 300 Ω and 150 Ω was considered to be a result of the shorter batch cycle time. Low external resistance would bring a high electron transfer rate in the reactor; thus, it probably resulted in a shorter batch cycle time. The batch cycle time would affect the CE value as the reactors were started with a mixed culture inoculum. The substrate removal by the non-exoelectrogen caused substrate loss to non-current-generating processes, which resulted in high COD removal and low CE.

To determine the electron transfer rate in the MRC reactor, the bioelectrokinetic of the anode was then analyzed using Tafel plots in terms of exchange current density (i_o) and anodic Tafel slope (b_a) as shown in Figure 5B. Exchange current density is a parameter related to the bioelectrochemical activity of the anode at the equilibrium condition. Based on the Tafel plots shown in Figure 5B, the exchange current density of biofilm at 300 Ω MRC reached 23.2 mA/m², which was higher than those of other reactors (18.6 mA/m² and 12.0 mA/m² for the MRC reactors with 650 Ω and 1000 Ω, respectively). In terms of anodic Tafel slope (b_a), the biofilm at the MRC reactor with 300 Ω revealed a lower value of 0.73 V/decade than the reactors with 1000 Ω and 650 Ω (0.93 V/decade and 0.84 V/decade, respectively). These results indicated that the reactor with lower external resistance could easily detach the electron from the biofilm into the anode electrode compared with the reactor with higher external resistance.

According to the results above, it can be concluded that external resistance affected the CE value of the MRC reactor through the length of batch cycle time and bioelectrokinetic anode value (exchange current density and Tafel slope). When the MRC reactor was connected to a high external resistance, the reactor would have a longer batch cycle as the exchange current density was low; thus, large COD removal and low CE value were achieved (and vice versa).

Effect of external resistance on energy recovery and energy efficiency

The energy recovery (r_E) is the ratio of power production to the total energy provided in the MRC. Meanwhile, energy efficiency (η_E) is the ratio of the power production to theoretically extractable energy provided by MRC. The energy efficiency and energy recovery of the MRC reactor at different external resistance are shown in Figure 6. It was observed that the energy recovery increased from 1.7 ± 0.1% to 3.0 ± 0.4% as the external resistance decreased from 1000 Ω to 300 Ω. The energy recovery then significantly decreased to 0.6 ± 0.1% when the external resistance decreased to 150 Ω. Compared with another study with 3-membrane pairs of MRC conducted by Liu et al,¹⁷ energy recovery results from this study were significantly lower. This low energy recovery was likely due to the relatively high feed stack flow rate used here (1.2 mL/min compared with 0.80 mL/min) compared with the lower power produced in the reactor (1.03 mW compared with 2.4 mW). The high feed stack flow rate resulted in higher energy input creating from the RED stack (34 mW compared with 24 mW). Moreover, Kang et al³⁰ reported that the MRC reactor with 5 mL/min of feed stack flow rate and 2.4 mW of power produced achieved 2.0% energy recovery, which was lower than the result of the recent study. In this case, this result proves that the feed stack flow rate significantly affected the energy recovery of the MRC reactor compared with the energy produced in the reactor. In this study, the value of energy efficiency mainly depended on the power production in the MRC reactor (P varied from 0.22 to 1.03 mW) as the summation of energy created from salinity gradient (Xⁱⁿ) and substrate loading (∆H_c x n_sⁱⁿ/t_b) was relatively constant (0.034 W). These calculations suggested that the MRC reactor with high power production would achieve high energy recovery.

Figure 6.

Energy efficiency and energy recovery (inserted picture) of MRC reactor at the indicated external resistance. MRC indicates microbial reverse-electrodialysis cell.

Based on the calculation of energy recovered in the system compared with the energy available on the substrate, the energy efficiency of the MRC reactor tended to follow the trend of the energy recovery result. The energy efficiency increased from 35 ± 1.6% to 53 ± 7.1% as the external resistance decreased from 1000 Ω to 300 Ω. The energy efficiency then significantly decreased to 11 ± 2.4% when the external resistance decreased to 150 Ω. The higher energy efficiency was observed in the MRC reactor with the external resistance of 300 Ω, which was 1.5, 1.3, and 4.8 times larger than the MRC reactors with the external resistance of 1000 Ω, 650 Ω, and 300 Ω, respectively. Similar to energy recovery, the value of energy efficiency in the reactor mainly depended on the power production compared with the energy input (based on the entering minus leaving) to the reactor. The power production in the reactor varied from 0.22 mW to 1.03 mW, while the energy input was relatively constant (1.30 W). Thus, the relatively higher power production of the reactor with external resistance of 300 Ω resulted in larger energy efficiency compared with other reactors.

According to the above results, it can be concluded that the external resistance affected the value of energy efficiency and energy recovery through the magnitude of power production in the reactor. When the reactor was connected to the proper external resistance, the reactor created high power production; thus, the high energy efficiency and energy recovery were achieved.