Environ. Sci. Technol. 2010, 44, 4315–4321
High Current Generation Coupled to Caustic Production Using a Lamellar Bioelectrochemical System ¨ TZER, KORNEEL RABAEY,* SIMONE BU ¨ RG KELLER, AND SHELLEY BROWN, JU ´ A. ROZENDAL RENE Advanced Water Management Centre, Gehrmann Building, The University of Queensland, Brisbane, Queensland 4072, Australia
Received December 15, 2009. Revised manuscript received March 23, 2010. Accepted April 22, 2010.
Recently, bioelectrochemical systems (BESs) have emerged as a promising technology for energy and product recovery from wastewaters. To become economically viable, BESs need to (i) reach sufficient turnover rates at scale and (ii) generate a product that offsets the investment costs within a reasonable time frame. Here we used a liter scale, lamellar BES to produce a caustic solution at the cathode. The reactor was operated as a three-electrode system, in which the anode potential was fixed and power was supplied over the reactor to allow spontaneous anodic current generation. In laboratory conditions, with acetate as electron donor in the anode, the system generated up to 1.05 A (at 1.77 V applied cell voltage, 1015 A m-3 anode volume), and allowed for the production of caustic to 3.4 wt %, at an acetate to caustic efficiency of 61%. The reactor was subsequently operated on a brewery site, directly using effluent from the brewing process. Currents of up to 0.38 A were achieved within a six-week time frame. Considerable fluctuations over weekly periods were observed, due to operational parameter changes. This study is the first to demonstrate effective production of caustic at liter scale, using BESs both in laboratory and field conditions. It also shows that input of power can easily be justified by product value.
Introduction Microbial fuel cells (MFCs) have generated considerable interest in the past few years (1, 2). Briefly, MFCs use whole microorganisms as biocatalysts for the oxidation of (in)organic electron donors at an anode. From the anode, electrons gained from the oxidation are conveyed toward a cathode, the latter has a higher potential. As electrons flow from a low to a high potential, a power output is generated. While attractive at first glance, MFCs battle complexity and low value of the output product, electricity (3). By adding power to the cell, at the cathode hydrogen can be generated in a so-called microbial electrolysis cell (MEC) (4, 5). In the slipstream of MFCs and MECs, a whole range of other applications have emerged, focusing on (bio)production, nutrient removal (6), pollutant removal (7), and remote power generation for sensors (8). To encapsulate the broader goal of these systems, they are nowadays generally referred to as bioelectrochemical systems (BESs) (9). Key products inves* Corresponding author phone: +61 7 3365 7519; fax +61 7 3365 4726; e-mail:
[email protected]. 10.1021/es9037963
2010 American Chemical Society
Published on Web 05/06/2010
tigated thus far were hydrogen gas (4, 5), methane gas (10), and hydrogen peroxide (11). One particularly complex issue BESs face is caused by the presence of cations, such as sodium and potassium, in wastewater or other feedstocks supplied to the anode. As the concentration of these cations is generally more than 2 orders of magnitude higher than the proton concentration, they are typically transported to a high extent through the cation exchange membrane to restore the charge balance between anode and cathode. As a result, the anode tends to acidify due to proton generation in the anode reaction, while the cathode tends to become more alkaline due to proton consumption in the cathode reaction (12). Diverse strategies have been developed to avoid this issue. Liu et al. (13) omitted the membrane as a whole, thus decreasing the system’s ohmic resistance and pH gradient build-up. However, such an approach may cause cross over of anode fuel or cathodic oxygen, causing a decrease of Coulombic efficiency. Torres et al. (14) provided carbon dioxide to the cathode of the BES, which functioned as a partial pH neutralizing agent by reacting with hydroxyl ions to bicarbonate. This in conjunction with an anion exchange membrane between anode and cathode allowed for better balancing of the anode pH. More complete neutralization using a similar method was achieved recently by Fornero and co-workers (15). A third strategy by Freguia et al (16) involved directing the anode effluent to the cathode and vice versa, leading to a reuse of alkalinity, and salts. While attractive for MFCs and nitrogen removing BESs, such an approach may impede the formation of valuable chemicals at the cathode due to crossover of organics, oxygen consumption, and pollution of the end product. A similar approach involves temporal potential switching of the anode and cathode (17). Rather than battling the pH increase of the cathode, one could use this as an advantage to harvest an alkaline solution. Indeed, proton consumption in the cathode reaction in combination with the transport of sodium and/or potassium to the cathode generates a caustic solution, mainly comprising of sodium and/or potassium hydroxides. When a small clean water stream is introduced as the influent cathode, the caustic solution can be harvested. Caustic soda is one of the most widely used chemicals on earth. One of the largest industrial sectors using caustic soda is the pulp and paper industry, which requires this chemical mainly during the pulping and bleaching stage. It is estimated that the annual global demand for caustic soda is 63.4 million tonnes per annum (Global Industry Analysts, August 1, 2006, Pub ID: GJOB1365250). Besides the pulp and paper industry other industries such as breweries and dairy plants make extensive use of caustic for cleaning in place of process equipment. All of the aforementioned industries generally have abundant and biodegradable wastewater with a high organic load available, which would allow for the anodic fuel supply. Considering the above, we aimed to investigate the potential of BES to produce caustic during wastewater treatment. For this, we developed a novel, liter scale reactor with a lamellar layout. We operated this BES at high anode throughput to supply high amounts of electron donor, the reactor was operated as a three electrode system in electrolysis mode. This implies here that the anode potential was set at a potential where the microorganisms spontaneously generated current, the potentiostat adjusted the current level to maintain the required anode potential, and power was added to the system to drive the cathodic reaction. At the same time, we supplied a limited cathode fluid flow to obtain a relatively concentrated caustic flow, as used in industry. VOL. 44, NO. 11, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Schematic overview of the lamellar concept used in the BES reactor. Left panel: top view of cross-section; right panel: side view of cross-section. 1: membrane; 2: anode; 3: current collector; 4: cathode; 5: collector rod; 6: cathode collector plate; 7: anode collector plate. Parameters of interest were the attainable current, the energy requirement, and the organics removal, as well as the effect of using real wastewater rather than synthetic laboratory feeds.
Materials and Methods Microbial Fuel Cell Construction. Figure 1 provides a schematic overview of the used reactor. A lamellar type reactor was constructed by creating 2 welded cation exchange membrane (CMI-7000, Membranes International Inc.) envelopes (170 × 200 mm) of 1 cm thickness. Two anodic chambers were located inside the envelopes while an additional single sheet membrane was used as a third anode chamber as depicted in the left panel of Figure 1. The membranes were clamped and glued (Bostix, Australia) in a bottom and top groove, surrounding an 8 × 200 mm opening. Inside the membrane envelope, on both sides a graphite felt anode was inserted (Alfa Aesar) (164 × 200 mm), clamped to the sides by inserting a corrugated stainless steel mesh (316SS, 6 mm mesh, 0.6 mm wire) (Locker, Australia) (a picture of the mesh is provided in the Supporting Information). As a cathode, either only a corrugated stainless steel mesh (316SS, 5 mm mesh, 0.6 mm wire) or this mesh plus two finely woven stainless steel meshes (316 SS, size 300, Locker, Australia) were inserted in the cathode sleeves (164 × 200 mm). This leads to a cathode thickness of 8 mm. As such, the total projected surface area of the anodes and cathodes was 1640 cm2 and 984 cm2 respectively. All corrugated meshes were welded on the side of the compartment to perpendicular stainless steel rods (316SS, 5 mm 4316
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diameter), which electrically connected them to either an anode or a cathode collector plate (316SS, 3 mm thickness). The reactor was then connected to recirculation and feed circuits as shown in Figure 2. The total anode liquid volume was 1.02 L, the total cathode volume was 0.61 L. These volumes do not include the sumps and a small space next to the cathodes, including wall thickness the total reactor volume was ultimately 3.313 L. Reactor Operation and Medium. The inoculum for the initial start up of the reactor was obtained from a lab scale microbial fuel cell, fed with wastewater from the mixing tank of a brewery wastewater treatment plant as well as from a pilot scale microbial fuel cell fed with brewery wastewater. During the lab based runs (see below) the anode was fed with a mixture of two media. The basic medium (initially 6.9 L d-1, increased up to 30 L d-1) contained per liter: 0.1 g NH4Cl, 0.1 g KH2PO4, 0.1 g MgSO4.7H2O, 0.02 g CaCl2.2H2O and 1 mL of nutrient solution as described previously (18). To this medium, a concentrate containing sodium acetate (as appropriate for increasing current, starting at 3.93 g acetate L-1) and NaHCO3 (variable quantity to ensure pH neutrality of the incoming concentrate) was added as required to achieve a target current density depending on the status of the reactor. The flow of this concentrate was varied to achieve increasing loading rates (starting rate was 0.7 L d-1), to a maximum loading of 9.89 g acetate d-1 (10.27 kg COD m-3 anode d-1). The anode was recirculated at 7 L d-1, which roughly represents a 1/1 recirculation. The increasing concentrate addition caused increasing conductivities of the anode medium over time, that is, for run 2 the conductivity
FIGURE 2. Schematic overview of the overall reactor connections and feed streams. increased from 4.29 to 9.02 mS cm-1. The cathode was continuously fed with a salt solution (1 g NaCl L-1), at a rate of 0.7 L d-1, and recirculated at a rate of 7 L d-1. The operational period can be divided in three runs: (i) first lab based run (ii) second lab based run, and (iii) brewery based run. During the first run, the cathode only contained the corrugated mesh as cathode and current collector. The system was operated for 64 days, during which the anode feed was progressively increased by increasing both concentrate concentration and flow. The experiment was terminated shortly after a failure due to gas production (see further). Imperfect sealing between anode and cathode was observed, therefore the reactor was dismantled and rebuilt. At this stage (second lab based run) the finer meshes were inserted into the cathodes to serve as electrode, next to the corrugated mesh as current collector. The system was operated likewise to the first run, for 46 days. After this period, the reactor was moved to Fosters brewery (Yatala, Australia) where “mixing tank” wastewater was fed to the reactor. The composition of the incoming wastewater can be seen in Table 1. The influent was mixed in (1/1) with anaerobic digester effluent to achieve a higher influent pH and gain more alkalinity (composition also in Table 1). The cathode flow was 0.71 L d-1, the anode influent flow was varied between 51 and 702 L d-1. Electrochemical Control, Monitoring, and Data Representation. Measurements and calculations were performed according to previous reports (1, 19). Potentiostatic measurements and controls were performed using a PAR VMP-3 Potentiostat (Princeton Applied Research, U.S.) in the laboratory, and with a Bank-IC KP307 potentiostat (BankIC, Pohlheim, Germany) in the field. The potential setting for the anode, between -0.12 and -0.3 V vs Ag/AgCl was chosen based on existing observations that effective current can be generated in that range (20). The ohmic resistance of the reactor was measured (in laboratory conditions) using a frequency response analyzer installed on the VMP3 system, the scan range was from 100 kHz to 5 mHz, at a set cell voltage of 1.2 V with a 10 mV amplitude. Chemical Analyses. Immediately after sampling, the samples obtained from the anode and cathode compartments were filtered through 0.22 µm sterile filters. The volatile fatty acid (VFA) content was determined by adding 0.9 mL of samples to 0.1 mL of 10% formic acid and subsequently analyze with a gas chromatography method using a polar capillary column (DB-FFAP) at 140 °C and a flame ionization detector at 250 °C. The COD measurements were done
TABLE 1. Representative Composition of the Mixing Tank Wastewater and Anaerobic Digester Effluent Obtained at the Brewery. All Concentration Values Are Given in mg L-1
pH alkalinity (as HCO3-) volatile fatty acids acetic acid Propionic acid i-butyric acid n-butyric acid i-valeric acid n-valeric acid hexanoic acid ammonia-N cations calcium sodium potassium anions chloride sulfide-S sulfite-S sulfate-S thiosulfate-S soluble COD total COD
mixing tank
anaerobic digester
6.1 242
6.8 856
226 307 3 125 3 120 6 101
29 26 1 2 1 2
