



where EMRC is the energy produced per batch (kJ), ns is the moles of substrate (acetate)

initially fed to the anode (0) and at the end of the batch cycle (f), and ∆Gs is the Gibb’s

free energy of substrate [acetate = –846.6 kJ/mol (35), domestic wastewater = 17.8 kJ/gCOD (36)]. ∆Gmix is the free energy that can be created by mixing of HC and LC

solutions until the two solutions reach equilibrium concentration, calculated as:





Labeled picture of an MRC reactor showing positions of the working (anode and cathode) and

reference (Ag/AgCl) electrodes, the RED membrane stack, as well as high (HC) and low

concentrate (LC) salt solution influent and effluent ports.

Power density curves of the MRC (HC = 0.95 M, SR = 100) at different salt solution flow rates





Roland D. Cusick, Younggy Kim, Bruce E. LoganPublished 1 March 2012 on Science ExpressDOI: 10.1126/science.1219330. The lab scale MRC reactor was constructed as previouslydescribed (13) with minor modifications (Fig. 1). The 4-cm cubic anode chamber (Lexan,30 mL empty bed volume) contained a graphite brush anode (D = 2.7 cm, L = 2.3 cm,0.22 cm2 projected area based on all fibers in the brush; Mill-Rose Labs Inc., OH). Thebrush anode was heat treated (30) before it was inoculated with the effluent from anexisting MFC and enriched in a conventional single chamber MFC prior to MRCoperation. The cathode chamber (2-cm cubic chamber, 18 mL empty bed volume)contained a 7-cm2 (projected surface area) air cathode with a Pt catalyst (0.5 mg Pt/cm2)applied on a carbon cloth as previously described (31), with a Nafion catalyst binder(water side) and four layers of polytetrafluoroethylene diffusion layers (air side).Although Pt was used as a catalyst here in order to benchmark performance againstpreviously tested systems using NaCl solutions (13), nearly identical cathodeperformance has been obtained using activated carbon catalysts instead of Pt catalysts inmicrobial fuel cells (32). The cathode chamber also served as the first flow channel of thehigh concentrate salt stream to prevent the pH rise in the cathode chamberThe RED stack, assembled between the anode and cathode chambers, consists of with6 anion- and 5 cation-exchange membranes (Selemion AMV and CMV, Asahi glass,Japan), creating 5 pairs of alternating HC and LC chambers as previously described (13).Inter-membrane chambers were sealed and separated by silicon gaskets, each with an 8-cm (2 × 4 cm) rectangular cross section cut out. Inter-membrane chamber width (1.3mm) was maintained with a 2 cm2 (0.5 × 4 cm) strip of polyethylene mesh. The total ionexchange membrane area in the RED stack was 88 cm2. The total MRC empty bed 3volume was 58.4 mL (RED stack + Cathode = 28.4 mL; Anode = 30 mL). The HCsolution entered the cathode chamber and flowed serially through the 5 HC cells in thestack, exiting from the cell next to the anode chamber (Fig. 1). The LC stream entered theRED stack near the anode and flowed serially through the 5 LC cells in the stack, exitingfrom the cell next to the cathode chamber. A peristaltic pump (Cole Parmer, IL)continuously fed the HC and LC solutions at a flow rate of 1.6 mL/min, unless specifiedotherwise.After stable performance in the MRC, the working electrodes (anode and cathode)were transferred to a cubic 4-cm (30 mL empty bed volume) single chamber MFCreactors to establish a performance baseline.Peak power, maximum energy recovery, and energy efficiency of the MRC and MFCwere determined in separate experiments. During power density curve experiments freshHC solution was pumped through the RED stack with the effluent collected in separatereservoirs. To maximize energy recovery and energy efficiency, 0.1-L HC and LCsolutions were recycled in airtight flow paths for the duration of anode feeding cyclesover a batch recycle experiment. Before each batch the stack and tubing were flushedwith matching solutions.Solutions. Ammonium bicarbonate HC solutions were prepared by dissolvingammonium bicarbonate salt (Alfa Aesar, MA) into deionized water within an airtightvessel. The initial HC solutions tested were 1.8, 1.1, 0.95, 0.8, and 0.5 M. The LCsolutions were prepared to produce salinity ratios of 50, 100, and 200 by diluting analiquot of the HC solution. The anode solutions contained 1 g/L of sodium acetate(organic substrate for exoelectrogenic bacteria growing on anode), in 50 mM carbonate.buffer (4.2 g/L NaHCO3-) containing 0.231 g/L NH4H2PO4 and trace vitamins andminerals (33). Domestic wastewater was collected from the primary clarifier of the PennState University wastewater treatment plant. The cathode contained ammoniniumbicarbonate HC solution, therefore protons for oxygen reduction at the cathode wereprovided by ammonium and bicarbonate ions as well as water dissociation.A second order relationship between ammonium bicarbonate solution concentrationand solution conductivity (determined by conducting a stepwise dilution series) was usedto estimate initial and final concentrations of HC and LC streams. Conductivity and pHof the HC and LC streams were measured (Mettler-Toledo, OH) before and after eachbatch recycle experiment.Analysis. Power production in batch recycle experiments was determined bymeasuring the potential drop across a fixed external resistance (300 Ω) for both MRC andsingle chamber MFC operations. Voltage drop was recorded every 20 minutes by adigital multimeter (Keithley Instruments, OH). Electrical current (i) was determined byOhm’s law. Power was calculated by multiplying the electrical current and total cellvoltage. Reported power densities were based on the cathode projected area (7 cm2). Todetermine the maximum MRC power (PMRC) production at each condition the reactor washeld at open circuit voltage for one hour and then the external resistance was decreasedfrom 1,000 to 50 Ω every 20 minutes with the voltage recorded at each resistance. Powercontribution by the electrode reactions (PMFC) was determined by measuring the anodepotential (Ean) and cathode potential (Ecat) against Ag/AgCl reference electrodes (BASi,IN): PMFC = (Ecat – Ean)× i. The RED stack power contribution was calculated by finding stack voltage (Vstk) with two reference electrodes located on both ends of the stack as:PRED = Vstk × i.The MRC anode was transferred to a single chamber MFC to determine baselinepower production in fed-batch experiments. In the single chamber MFC, same substratesolutions (sodium acetate in carbonate buffer solution and domestic wastewater) wereprovided to determine peak power production.Coulombic efficiency was determined as previously described (34). Energy recovery(rE) is defined by the ratio of energy produced by the MRC reactor and the energy inputas substrate and salinity gradient as written in Eq. (1). Energy efficiency (ηE) wascalculated as the ratio of energy produced to the energy consumed based on the substrateused and the salinity gradient, according to (13):where R is the ideal gas constant (8.314 J/mol-K), T is solution temperature, V is thevolume of solution, c is the molar concentration of ionic species i in the solution, and a is the activity of species i in the solution.At a neutral pH, concentrated ammonium bicarbonate is dominated by ammonium (NH4+) and bicarbonate (HCO3-) ions, but significant amounts of carbamate (NH4CO3-) and carbonate (CO32-) also contribute to ionic strength. Species specific concentrations and activities were estimated with OLI Stream Analysis software (OLI Systems, Inc., Morris Plains, NJ) at a pH of 7 and temperature of 25 °C.To determine ammonia transport into the anode, total ammonia nitrogen (TAN = NH3+ NH4+) concentration in the substrate was estimated before and after each fed-batch cycle (HACH, Loveland, CO) (31). Based on observed pH, corresponding free ammoniaconcentration (NH3) was calculated by:Polarization curves of the MRC using different HC salt solutions, compared to that of an MFCMRC energy input (acetate and salinity energy) and output at different HC concentrations.a) Peak power density and b) anode (A) and cathode potentials (C) of MRC and single chamberMFC fed domestic wastewater. Notice that the anode and cathode potentials remained relativelyconstant over the range of current densities. The relatively constant potential indicates that thepower performance is stable, suggesting that the system could easily sustain higher powerdensities with higher organic matter concentrations in the wastewater.