Article pubs.acs.org/est
Scaling-Free Electrochemical Production of Caustic and Oxygen for Sulfide Control in Sewers Hui-Wen Lin,† Korneel Rabaey,†,‡ Jürg Keller,† Zhiguo Yuan,† and Ilje Pikaar*,†,§ †
Advanced Water Management Centre (AWMC) and §The School of Civil Engineering, The University of Queensland, Brisbane, QLD 4072, Australia ‡ Laboratory of Microbial Ecology and Technology (LabMET), Ghent University, Coupure Links 653, Ghent 9000, Belgium S Supporting Information *
ABSTRACT: Caustic shock-loading and oxygen injection are commonly used by the water industry for biofilm and sulfide control in sewers. Caustic can be produced onsite from wastewater using a two-compartment electrochemical cell. This avoids the need for import and storage of caustic soda, which typically represents a cost and a hazard. An issue limiting the practical implementation of this approach is the occurrence of membrane scaling due to the almost universal presence of Ca2+ and Mg2+ in wastewater. It results in a rapid increase in the cell voltage, thereby increasing the energy consumption of the system. Here, we propose and experimentally demonstrate an innovative solution for this problem involving the inclusion of a middle compartment between the anode and cathode compartments. Caustic was efficiently produced from wastewater over a period of 12 weeks and had an average Coulombic efficiency (CE) of 84.1 ± 1.1% at practically relevant caustic strengths (∼3 wt %). Neither membrane scaling nor an increase in the cell voltage was observed throughout the experiments. In addition, dissolved oxygen was produced in the anode, resulting in continuously oxygenated wastewater leaving the three-compartment cell. This membrane-scaling control strategy represents a major step forward toward practical implementation of on-site simultaneous electrochemical caustic and oxygen generation for sulfide control in sewers and also has the potential to be applied to other (bio)electrochemical systems receiving wastewater as source for product recovery.
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INTRODUCTION Hydrogen sulfide generation in sewer pipes is a notorious problem that costs wastewater utilities billions of dollars each year globally.1−4 It causes concrete corrosion, results in the release of obnoxious odors, and poses a threat to sewer workers.1−3 Conventional sulfide abatement strategies involve chemical addition to either prevent hydrogen sulfide generation or mitigate its effects after its formation.3 Of these, two of the most commonly used chemical addition strategies by the water industry are oxygen injection and periodic caustic addition to achieve high pH levels (>10.5) for a short period of time (i.e., 2−6 h).5 Caustic shock-loading effectively deactivates the sewer biofilms responsible for hydrogen sulfide generation. In this way, hydrogen sulfide generation can be inhibited for several days up to weeks.6,7 Although oxygen injection and periodic caustic shock-loading have been implemented successfully,3,5 the frequent transport, handling, and storage of oxygen (i.e., pure oxygen) and caustic soda often come with serious safety concerns as well as a negative public image. Hence, on-site production approaches can alleviate many of these concerns. Over the past decade, product recovery using (bio)electrochemical systems has gained significant attention. A variety of studies showed the feasibility of the production of © 2015 American Chemical Society
electricity and valuable chemicals such as hydrogen peroxide, caustic soda, and hydrogen from wastewaters.8−12 Although the feasibility and potential has been shown in several studies, most of these studies involve the use of synthetic solution and pretreated wastewaters with only limited studies using real wastewater.8−11 A novel chemical-free electrochemical method that produces caustic directly from domestic wastewater using a two-compartment electrochemical cell has recently been proposed.13 In brief, wastewater is sent through the anode of the electrochemical cell. Sodium migrates from the wastewater through a cation exchange membrane to the cathode. In the latter, water is reduced to H2 and OH−, which recombines with the Na+ to form NaOH. It was experimentally demonstrated that following this approach caustic could be directly produced from wastewater.13,14 Although the economic and practical potential of on-site caustic generation from wastewater has been demonstrated using a two-compartment electrochemical cell, one key issue to Received: Revised: Accepted: Published: 11395
August 18, 2014 August 18, 2015 September 17, 2015 September 17, 2015 DOI: 10.1021/acs.est.5b02188 Environ. Sci. Technol. 2015, 49, 11395−11402
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Environmental Science & Technology
Figure 1. Schematic diagram of the experimental setup using a three-compartment electrochemical cell.
be addressed is the occurrence of membrane scaling.13 A deposit of calcium and magnesium hydroxide was formed on the cation exchange membrane (CEM) due to membrane transport of bivalent cations such as magnesium and calcium through the CEM and subsequent reaction with hydroxide ions.15 The latter resulted in an increase in membrane resistance, thereby increasing the cell voltage and energy requirements. It also causes membrane disintegration due to internal crystal formation, which requires frequent cleaning.13−15 In addition, scaling may become more severe when a concentrated caustic solution (higher pH value) is produced.15 Many studies have observed the formation of calcium and magnesium hydroxide at membrane surfaces in various electromembrane processes, and thus, scaling is considered to be a major issue that hinders the practical implementation of electromembrane processes.16−18 Our previous studies investigated scaling mitigation using periodic polarity switching (the anode becomes the cathode and vice versa) in electrochemical caustic generation from wastewater.13,15 Periodic polarity switching could temporarily overcome membrane scaling during short-term laboratory scale experiments.13 However, it was found to be less successful over longer periods of time during field trials.15 Hence, there is a need to find more efficient solutions to either prevent membrane scaling or to mitigate the scaling after its formation to enable the practical implementation of onsite caustic generation from wastewater. The relatively low Coulombic efficiency (CE) for caustic generation is another problem associated with electrochemical caustic generation from wastewater.13 The relatively low CE for caustic generation is mainly caused by unwanted transport of protons, magnesium, calcium, and ammonium present in wastewater from the anode to the cathode compartment and a reverse backflux of hydroxide ions from cathode to anode.13,14 This transport lowers process efficiency.
It was recently shown that with increased sodium concentrations in wastewater, the membrane transport of sodium can be enhanced, and the unwanted membrane transport of calcium and magnesium was significantly reduced, leading to less membrane scaling and higher process efficiency for electrochemical caustic generation from wastewater.14 If conditions are created in which sodium is the only or main cation present in the wastewater, both high caustic CE and reduced membrane scaling could be achieved. While the continuous addition of sodium chloride to the wastewater can achieve this goal, it is not practically or economically feasible. Here, we propose and test an alternative approach that provides the sodium required by combining membrane electrolysis with electrodialysis. The electrochemical cell consists of three compartments. The middle compartment is fed with a concentrated NaCl solution and is separated from the anode by an anion exchange membrane (AEM) and from the cathode by a cation exchange membrane (CEM). This would create a chloride flux from the middle compartment to the anode and a sodium flux from the middle compartment to the cathode. Because the cations (e.g., magnesium, calcium, and ammonium) in the wastewater are unlikely to migrate to the middle compartment through the AEM, a scaling-free operation of the electrochemical cell, achieving high CE for caustic generation, would be achieved. Importantly, oxygen will be continuously generated in the anode compartment, resulting in continuously oxygenated wastewater effluent benefiting sewer pipes downstream. Both short- and long-term experiments were performed in this study to prove the concept and to assess the performance of the three-compartment cell both in terms of CE for caustic generation and of membrane scaling prevention.
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MATERIALS AND METHODS Experiments were performed using a three-compartment electrochemical cell (Figure 1) that consisted of three parallel 11396
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3 L/h using a peristaltic pump (Masterflex pump). Experiments were operated at a fixed current density of 10 mA/cm2. Anode influent and effluent samples were taken for the measurement of pH, conductivity, temperature, total chemical oxygen demand (COD), ammonium, chloride, total chlorine concentration, and total organic halogen (TOX). The dissolved oxygen (DO) concentrations in the wastewater were measured and recorded every 5 min using a DO meter (YSI Pro Plus model 1020 field kits, YSI Inc.). Anion and cation concentrations of solutions in all three compartments were analyzed at the beginning and at the end of each experiment. The second set of experiments (n = 9), each lasting for 4 h, was conducted to determine the impact of the current-towastewater flow rate on the DO levels in the wastewater. A total of three different anode flow rates of 9, 4.5, and 2.4 L/h were applied, while the current and all other experimental parameters were kept identical to those used in the first set of experiments. The dissolved oxygen concentrations and pH in the wastewater were measured and recorded every 5 min. The third set of experiments, each lasting for 4 h, was conducted to determine the impact of NaCl concentration and current densities on the cell voltage. First, experiments were carried out with NaCl concentrations of 20 and 40 g/L in the middle compartment under the same experimental conditions of the first set of experiments (n = 6). On the basis of these results, experiments were conducted with NaCl concentrations of 40 g/L in the middle compartment at current densities of 7.5, 10, and 12.5 mA/cm2, respectively (n = 9). The fourth set of experiments (n = 6), each lasting for 4 h, was conducted to determine the reduction in caustic CE caused by hydroxide back-diffusion from the cathode to the middle compartment. To assess the latter, we controlled the pH of the middle compartment at a constant level of 6.7 (similar to the pH in the wastewater; hence, the transport of protons and hydroxide across the AEM would be negligible) by dosing a hydrochloric acid solution. The hydroxide ions migrated from the cathode to the middle compartment were thus neutralized with the acid dosed into the middle compartment. Consequently, the amount of hydroxide ions diffused back from the cathode to the middle compartment was accurately determined by quantifying the amount of acid dosed. Besides the pH control in the middle compartment, all other experimental parameters were identical to those used in the first set of experiments. The fifth set of experiments (n = 6) were carried out to determine the amount of caustic trapped within the cation exchange membrane, which cannot be quantified using the pH control in the middle compartment. Experimental conditions were whose used in the first set of experiments with some modifications. In the first three tests, wastewater was substituted by a 20 g/L NaHCO3 solution (2 L), whereas the cathode electrolyte was increased from 150 to 900 mL, and the duration of the experimental run was prolonged to 8 h. The pH of the middle compartment was controlled at a constant pH of 6.7 by dosing a hydrochloric acid solution. The experiments were then repeated, with the NaHCO3 solution being replaced by continuous wastewater flow without controlling the middle compartment pH to demonstrate that a similar CE for caustic generation could be achieved (n = 3). In addition to the determination of whether the occurrence of scaling is independent of the type of CEMs used in the operation of a two-compartment cell, two types of monovalent cation exchange membranes were also tested in a two-
Perspex frames. The internal dimensions of the anode and cathode compartments were either 20 cm × 5 cm × 1 cm (short-term experiments) or 20 cm × 5 cm × 0.5 cm (longterm experiments), while the internal dimensions of the middle compartments in both short-term and long-term experiments were 20 cm × 5 cm × 1 cm. The anode and middle compartments were separated by an anion-exchange membrane (Ultrex AMI-7001, Membranes International Inc.), whereas a cation-exchange membrane (Ultrex CMI-7000, Membranes International) was used to separate the middle and the cathode compartments. A mesh shaped Ru/Ir coated titanium electrode (thickness: 1 mm; specific surface area: 1.0 cm2/cm2, Magneto Special Anodes BV, Schiedam, The Netherlands) was used in the anode chamber with a projected surface area of 24 cm2 for short-term experiments and 4 cm2 for the long-term experiments, respectively. Stainless-steel mesh (6 mm mesh size, 0.8 mm wire) with a projected surface area of either 24 cm2 (shortterm experiments) or 4 cm2 (long-term experiments) was used as the cathode electrode. The use of the smaller anode electrode and anode volume in the long-term experiment allowed the reduction of anode flow and, hence, the amount of wastewater required. The membrane surface areas exposed to the liquid in the short-term and long-term experiments were 24 and 4 cm2, respectively, which was equal to the surface areas of the electrodes used. Fresh domestic wastewater was collected weekly from a local pumping station in Brisbane and immediately stored at 4 °C. The wastewater was heated to ambient temperatures (22 ± 2 °C) using a water bath before being fed to the electrochemical cell. An Ag/AgCl reference electrode (RE-5B, Bio Analytical) was placed in the anode compartment (assumed +197 mV versus standard hydrogen electrode (SHE)). All electrode potentials reported here were adjusted versus SHE. All experiments were galvanostatically operated using a Wenking potentiostat and galvanostat (KP07, Bank Elektronik, Germany). The short-term experiments were operated at an applied current of 180, 240, and 300 mA, which was equal to current densities of 7.5, 10, and 12.5 mA/cm2, respectively. The long-term experiments were operated at an applied current of 40 mA, which equaled to a current density of 10 mA/cm2. The anode and cathode potentials and cell voltage were recorded every 2 min using a data acquisition unit (34970A Data Acquisition Unit, Agilent Technologies). A water-lock was used to prevent pressure build-up in the cathode compartment due to the formation of hydrogen gas.
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EXPERIMENTAL PROCEDURES Short-Term Experiments. A total of five sets of experimental runs were conducted. The first set of experiments (n = 10), each with a duration of 4 h, was conducted to determine the process efficiency in terms of CE for caustic and oxygen generation and to prove the concept of the scaling-free operation. Wastewater was continuously fed to the anode compartment at a flow rate of 9 L/h using a peristaltic pump (Watson Marlow) resulting in an HRT of ∼1 min. The rationale for using such a short HRT in the anode compartment is to prevent a significant pH drop in the anode.13 NaCl solution (1 L, 20 g/L) was used as the electrolyte in the middle compartment. A 2 wt % NaOH with a total volume of 150 mL was used as the catholyte. An initial concentration of 2 wt % NaOH was used to reduce the duration of each short-term experimental run.14 The NaCl and NaOH solutions were continuously recirculated at a flow rate of 11397
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compartment cell similar to that in our previous study.13 A detailed description of the experimental protocol used and results obtained can be found in the Supporting Information. Long-Term Experiment. A 40 g/L NaCl solution (1 L) was used for the middle compartment, which was replaced weekly to maintain sufficient sodium chloride levels. We aimed to produce a moderate-strength caustic soda solution of ∼3 wt % after 1 week of operation, which would be suitable for practice.14 To achieve this, we used a 340 mL NaOH solution (initially 0.01 wt % NaOH at t = 0 to convey initial conductivity) as the catholyte, which was replaced weekly. The NaCl and NaOH solutions were recirculated at a flow rate of 1 L/h using a peristaltic pump (Masterflex pump, Extech Equipment Pty. Ltd.) to provide mixing. The wastewater was continuously fed into the anode at a flow rate of 1 L/h using a peristaltic pump (Masterflex pump, Extech) resulting in an HRT of ∼3 min. Because the anode flow rate was only 1 L/h, an external recirculation flow rate of 10 L/h was applied to the anode compartment for sufficient mixing using a peristaltic pump (Masterflex pump, Extech Equipment Pty. Ltd.). The electrochemical cell was operated over a period of 12 weeks to determine the long-term performance of the system in terms of CE for caustic generation and scaling-free operation (i.e., constant cell voltage). Anode samples were taken on a daily basis for the measurement of pH, conductivity, and temperature. After each week, the solutions in the middle and cathode compartments were replaced. In a practical situation, the produced caustic would be dosed to the sewer on a weekly basis to deactivate the sewer biofilm. Chemical Analyses. The pH, temperature, and conductivity of the wastewater were measured using a hand-held pH meter (Cyberscan pH 110, Eutech Instruments) and a handheld conductivity meter (TPS), respectively. The alkalinity of the cathode (and the middle compartment) was determined by alkalinity titration using a 1 M hydrochloric acid solution at the end of each batch. This allows for the accurate measurement of the hydroxide production (or hydroxide back-diffusion), as described elsewhere.14 The dissolved oxygen concentration in the wastewater was measured using a DO sensor (YSI Pro Plus model 1020 field kits, YSI Inc.). The calculations of Coulombic efficiency for caustic and oxygen generation are given in the Supporting Information and were calculated according to Panizza et al.19 Total chlorine was measured using the chlorine test kit (Hach Lange). Chloride concentration was measured by ion chromatography (IC) equipped with a Dionex 2010i system. Total organic halogen (TOX) measurement was performed using an Automatic Quick Furnace (AQF-2100H, Mitsubishi) and a ion chromatograph (ICS-2100) according to Li et al.20 Wastewater samples taken were immediately filtered using a 0.22 μm filter (Millipore) and preserved in a freezer prior to the TOX measurement. Ammonium concentration was analyzed using a flow-injection analyzer (Lachat QuikChem8000, Lachat Instruments). COD concentration was measured using a cuvette test (range 25−1500 mg/L, Merck). Anion and cation concentrations were measured using ion chromatography and inductively coupled plasma (ICP) (ICP− OES Optima 7300DV). The membrane surfaces were examined using a variable-pressure scanning electron microscope (SEM) (Hitachi). Membrane samples were imaged after degassing overnight and without metal or carbon coating.
Article
RESULT AND DISCUSSION Caustic and Oxygen Generation from Wastewater Using a Three-Compartment Electrochemical Cell. In the first phase, short-term experiments were performed (Figure 2).
Figure 2. Typical profiles of anode and cathode potentials and the cell voltage during a 4 h experiment for the production of caustic from wastewater using a three-compartment cell at a current density of 10 mA/cm2.
The anode and cathode potentials and cell voltage were stable, contrarily to our previous studies, in which a significant increase in the cell voltage due to membrane scaling was observed within a 4 h period.13,14 On average, the anode potential and cell voltage were 1.6 ± 0.1 and 5.9 ± 0.7 V, respectively, showing the reproducibility of the experiments. Both the anode and cathode potentials were measured relative to the reference electrode equipped in the anode compartment; thus, the cathode potentials observed also included the losses caused by the ohmic resistance of membranes and electrolyte. In addition to the stable cell voltage, no scaling was visually observed on either the electrode or the membrane surface. The latter was confirmed by the SEM analysis of the membranes, which showed that there was no scaling on both the AEM and CEM (Figure S2). Furthermore, ICP results showed that the Ca2+ and Mg2+ concentrations in the solutions of the middle and cathode compartments were below their detection limits and the Ca2+ and Mg2+ concentrations in the anode influent and effluent samples remained constant at 40 and 25 mg/L, respectively. At an anode pH of ∼6.8, we determined that these Ca2+ and Mg2+ concentrations were several orders of magnitude lower than their respective threshold levels, causing the precipitation of Ca(OH)2 and Mg(OH)2 on the basis of the solubility products of Ca(OH)2 and Mg(OH)2.21 The constant Ca2+ and Mg2+ concentrations in the anode influent and effluent indicate that these cations in the wastewater did not migrate to the middle compartment through the AEM to any significant extent, as was further confirmed by the undetectable Ca2+ and Mg2+ concentrations in the middle compartment. The undetectable Ca2+ and Mg2+ concentrations in the middle and cathode compartments also imply that the precipitation of Ca(OH)2 and Mg(OH)2 was unlikely to have occurred in these compartments. The observed CEs for caustic generation were on average 76.2 ± 1.5% (n = 10) at a final caustic strength of 2.7 ± 0.0 wt 11398
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concentrations in our previous study. In these studies, it was found that at these concentrations, the formation of chlorine was negligible.23,24 Indeed, an analysis of the total chlorine concentrations revealed that similar to our previous studies, chlorine formation was negligible. This was further supported by the TOX (total organic halogen) measurements, which also showed a negligible production in TOX concentrations (data not shown). Impact of the Anode Flow Rate on the Wastewater Dissolved Oxygen Levels. Figure 3A,B shows the typical DO and pH profiles in the wastewater after electrochemical treatment. It can be seen that when a lower anode flow rate is applied, the system showed a higher DO concentration that ranged from 3.76 ± 0.47 to 12.62 ± 1.44 mg/L O2. Importantly, the CEs for DO generation remained fairly stable and were 46.7 ± 5.8%, 50.0 ± 3.0%, and 42.3 ± 4.8% at flow rates of 9, 4.5, and 2.4 L/h, respectively. In addition, the DO concentrations in the wastewater effluent remained constant over the course of the experiment, which indicates that oxygen is being generated at a constant rate and that the system is capable of maintaining wastewater effluent aerated without any oxygen addition. A complete overview of the results can be found in the Figures S3−5. Impact of the NaCl Concentration in the Middle Compartment and Applied Current Density on the Cell Voltage. Figure 4A,B shows the impact of the NaCl concentration in the middle compartment as well as the impact of the applied current density on the overall cell voltage. Figure 4A shows that there was a 2 V difference in cell voltage between two experiments (operated using NaCl concentrations of 20 and 40 g/L), clearly showing the importance of a high NaCl concentration (i.e., 40 g/L) in the middle compartment to reduce the ohmic resistance (both electrolyte and membranes)25 over the reactor and, as such, the power consumption of the system. The pH-related membrane potentials were calculated according to Sleutels et al.26 and estimated to be at 0.27 V. Considering the overall cell voltage in our system, the pH-related membrane potential only contributed to 5.9% (20g/ L NaCl) and 11.2% (40g/L NaCl) of the ohmic resistance. In addition, Figure 4B shows the impact of the applied current density on the overall cell voltage. In all experimental runs, the
%. The pH in the middle compartment increased over time to 11.3 ± 0.2 at the end of the experimental run, indicating that there was some back-diffusion of hydroxide ions from the cathode to the middle compartment through the CEM. The latter accounted for 5.4 ± 2.2% of the total charge supplied to the system. Apart from the charge used for caustic generation and the loss of hydroxide ion back-diffusion, 18.7 ± 1.9% of charge losses could not be explained using the setup used. Therefore, an additional set of experiments was conducted to determine these charge losses, which is discussed in detail further below in the section of determination of Coulombic efficiency losses. The average DO concentrations in wastewater before and after electrochemical treatment were 0.04 ± 0.01 and 3.76 ± 0.47 mg/L, respectively (see Table 1). This equals a CE for Table 1. Wastewater Characteristics of the Anode Compartment during 4 h Experiments short-term experiments (4 h) parameters
unit
anode influent
anode effluent
chloride ammonium (as N) COD dissolved oxygen pH temperature conductivity
mg/L mg/L mg/L mg/L − °C mS/cm
165 ± 23 (n = 5) 45 ± 2 (n = 4) 422 ± 9 (n = 4) 0.04 ± 0.01 (n = 4) 7.2 ± 0.1 (n = 10) 22 ± 2 (n = 10) 1.28 ± 0.03 (n = 3)
212 ± 23 (n = 5) 46 ± 2 (n = 9) 417 ± 6 (n = 12) 3.76 ± 0.47 (n = 4) 6.8 ± 0.2 (n = 32) 23 ± 2 (n = 32) 1.42 ± 0.08 (n = 9)
dissolved oxygen generation of 46.7 ± 5.8%. Most likely, some of the produced oxygen was vented off from the wastewater because the formation of air was visually observed but could not be quantified using the present setup. This production of dissolved oxygen in wastewater is certainly beneficial for sulfide oxidation in the downstream sewer section.15 Importantly, this efficiency is significantly higher than the efficiencies for conventional oxygen injection used by the water industry, which is normally below 30%.22 The increase in chloride concentration in wastewater was limited from 165 ± 23 mg/L (n = 5) to 212 ± 23 mg/L (n = 5) (see Table 1), which is well within the range of chloride
Figure 3. Typical profiles of (A) dissolved oxygen concentrations and (B) pH values in the anode effluent during 4 h experiments operated at a fixed current density of 10 mA/cm2 with different anode flow rates of 9, 4.5, and 2.4 L/h. 11399
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Figure 4. Typical profiles of the cell voltage during the 4 h experiments operated at (A) a fixed current density of 10 mA/cm2 with NaCl concentrations of 20 and 40 g/L in the middle compartment and (B) current densities of 7.5, 10, and 12.5 mA/cm2 with a NaCl concentration of 40 g/L in the middle compartment.
anode potentials were ∼1.6 ± 0.1 V versus SHE independent of the current density applied, while the average cell voltage increased from 3.7 to 4.9 V depending on the current density applied. The increase in cell voltage was caused by the increased in ohmic resistance (both electrolyte and membranes). A more detailed representation of the results is given in Figures S6−9. Determination of Coulombic Efficiency Losses. The first set of experiments showed that 18.7 ± 1.9% of the total charge supplied to the system could not be explained by the caustic production and the charge lost through hydroxide ion back-diffusion from the cathode to the middle compartment. In the first set of experiments the pH in the middle compartment increased over time to 11.3 ± 0.2. Hence, because the pH of the wastewater (i.e., ≈7) is lower than the pH of the middle NaCl solution, and because of the fact that an anion exchange membrane is highly permeable for hydroxide ions, membrane transport of hydroxide ions from the middle compartment to the anode compartment cannot be ruled out. This could (partly) explain the observed charge losses. To verify this, we performed experiments by dosing hydrochloric acid in the middle compartment to maintain a constant pH of 6.7 (approximately the pH in wastewater). In this way, membrane transport of hydroxide ions from the middle compartment to the anode is avoided, thereby allowing an approximation of the total hydroxide back-diffusion. However, similar results were obtained with an average CE for caustic generation of 76.4 ± 1.8% (n = 6), with hydroxide ion backdiffusion only accounting for 7.0 ± 0.8% of the total charge. The loss of charge at 16.5 ± 1.3% is similar to that obtained in the first set of tests, suggesting that: (1) the back-diffusion of hydroxide from the cathode to the middle compartment was not significantly enhanced by the pH control in the middle compartment; and (2) the possible transport of hydroxide from the middle to anode compartment cannot explain the charge loss observed in the both types of tests. We therefore further hypothesized that the charge loss was caused by sorption of a certain amount of hydroxide ions within the cation-exchange membrane (ion-exchange membranes have a certain ion-exchange capacity). By increasing the total
cathode volume, the total amount of caustic increases (i.e., a 2.0 wt % NaOH solution is used as starting solution), while the amount of caustic trapped within the membrane would remain constant. To verify this, we performed experiments using a larger cathode volume (i.e., 900 mL instead of 150 mL). A NaHCO3 solution (20 g/L) was used to substitute wastewater to maintain anodic pH > 7, thereby ruling out the possibility of caustic CE reduction by proton crossover from the anode to the middle compartment. A CE for caustic generation of 87.9 ± 0.0% was obtained (n = 3) with hydroxide back-diffusion accounting for 9.4 ± 0.4% of the charge losses. On the basis of the ion exchange capacity of the CEM (total ion capacity: 1.6 meq/g; the weight of the dry membrane contacted with solution: 1.64 g) used in the experiments, we estimated the CE loss for OH− calculated at 3.7% for the CEM. Hence, the overall charge balance of the system was estimated at 101.0 ± 0.4% (n = 3), strongly supporting our hypothesis. This was further confirmed by the experiments in which we replaced the NaHCO3 solution by real wastewater while obtaining very similar CE for caustic generation (i.e., 85.8 ± 3.6%) (n = 3). Also, this was further supported by the long-term experiment, which showed similar results (as discussed below). Long-Term Performance of Caustic Generation from Wastewater. In the previous section, we showed the scalingfree caustic generation from wastewater at a high CE using a three-compartment electrochemical cell during 4 h experimental runs. For practical implementation, it is essential to achieve efficient and scaling-free caustic generation over a much longer period of time. Therefore, long-term (12 weeks) experiments were conducted to determine the long-term cell performance. Figure 5 shows the profile of anode and cathode potentials and the cell voltage over the course of 12 weeks. The anode and cathode potentials and cell voltage were all stable during the course of the experiment, which conclusively demonstrated the three-compartment cell is able to continuously produce caustic without the membrane scaling observed in previous studies.13,14 In addition, the scaling of membranes (both AEM and CEM) in a three-compartment cell was not observed during the 12 week operation. Overall, caustic was produced at an average CE of 84.1 ± 1.1% at a caustic strength of 2.5 ± 0.0 wt % NaOH, 11400
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without the need for frequent cleaning of the system. It could be implemented in various electromembrane processes, such as (bio)electrochemical systems for wastewater remediation and product recovery from wastewater. In addition to cathodic caustic generation, oxygen is continuously being generated in the anode compartment (wastewater-based). The two chemicals can be applied to sewer pipes alternatively with periodic caustic cleaning of sewer biofilms (e.g., weekly caustic shock-loading) and oxygensuppressing biofilm development between caustic shocks, possibly leading to prolonged biofilm recovery and thus reducing the required frequency for caustic shock-loading. As an oxidant, oxygen could also be used for oxidation of any residual sulfide. In real applications, both the hydrogen and undissolved oxygen produced in the system would be vented to the atmosphere by forced ventilation due to the amount of produced hydrogen and do not warrant beneficial reuse. An economic assessment based on the results obtained in this study (Table S2) clearly showed the economic potential of the three-compartment configuration with an estimated cost of $11.9/ML of wastewater. The latter is well below the chemical costs for conventional oxygen injection and caustic dosing of $12.8−74.0 /ML and $39.6−99.1/ML of wastewater, respectively.5 In this study, we designed the short-term and long-term experiments to have similar caustic end concentrations of ∼3 wt %; thus, the caustic end concentrations of all of our experiments were ∼3 wt %. The rationale for using a 3 wt % caustic solution in a proposed sewer context and the dosing strategy has been discussed in detailed in Table S2. Here, we have clearly demonstrated these results over a period of 12 weeks without any scaling, as well as without any deterioration of the performance in terms of caustic production and power consumption, using the three-compartment configuration. Longer-term tests are recommended, preferably through a pilot-plant study. Although further studies are needed to investigate the potential synergistic effect of caustic and oxygen on sewer biofilms, the demonstration of simultaneous caustic and oxygen production from wastewater is an important step forward.
Figure 5. Profiles of anode and cathode potentials and the cell voltage in the long-term test over a period of 12 weeks.
which is very similar to the CE obtained in the short-term experiments (i.e., 85.8 ± 3.6%) (n = 3). Because the electrode used in long-term experiments was very small (4 cm2) and centered in the compartment, the reference electrode could not be placed adjacent to the anode electrode and was placed 0.5 cm below the anode electrode. As a result, the measured anode potential was 2.1 ± 0.1 V versus SHE, which is 0.5 V higher than in short-term experiments (1.6 ± 0.1 V) operated with 40 g/L NaCl in the middle compartment at a current density of 10 mA/cm2. However, the latter did not affect the overall cell voltage of the systems (3.6 ± 0.1 V versus 4.0 ± 0.1 V for the short-term experiments). Implications for Practice. The use of a three-compartment configuration allows for efficient and scaling-free caustic and oxygen production from wastewater at practically relevant caustic concentrations. The only inputs to the system are electricity and sodium chloride. Sodium chloride is a readily available, nontoxic, noncorrosive, and low-cost commodity chemical, its storage and use does not come with serious safety concerns. Due to the low resulting chloride concentrations in the anode (i.e 212 ± 23 mg/L), chlorine production was below detection. We demonstrated the long-term operation over a period of 12 weeks of the three-compartment configuration without the occurrence of biofouling. The latter might be related to the strong oxidative environment (i.e., the anode is placed directly at the AEM surface). In practical situations, biofouling might still occur over a longer period of time. Acid and caustic additions are two commonly used methods to mitigate biofouling. These chemicals can be in-situ generated using the three-compartment configuration. While caustic is produced at the cathode, acid can be produced at the anode by stopping the anode flow, which allows for the regular cleaning of the system to handle any biofouling that occurs.15 The three-compartment configuration can be installed for virtually any type of wastewater, and its performance is independent of the sodium, calcium, and magnesium concentrations in wastewater, contrary to conventional setups.14 The results obtained here represent a major step forward toward the practical implementation of this electrochemical technology because it allows for continuous operation
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02188. Additional information on the occurrence of scaling on monovalent cation exchange membranes in a twocompartment electrochemical cell and calculations of coloumbic efficiency for caustic and oxygen generation. Figures showing SEM images of AEM and CEM in a three-compartment electrochemical cell, profiles of DO concentration and pH in anode effluent, and profiles of anode and cathode potentials and the cell voltage during 4-hour experiments. Tables showing a summary of comparisons between experimental results, and estimated capital, operational, and total costs for electrochemical caustic and oxygen production from wastewater for sulfide control in sewers. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Phone: +61 7 3345 1389; e-mail:
[email protected]. 11401
DOI: 10.1021/acs.est.5b02188 Environ. Sci. Technol. 2015, 49, 11395−11402
Article
Environmental Science & Technology Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS H.-W.L. thanks the University of Queensland for scholarship support. This work was funded by the Australian Research Council, District of Columbia Water and Sewer Authority, ACTEW Corporation Limited, The City of Gold Coast, Queensland Urban Utilities, and Yarra Valley Water through the ARC Linkage project LP0882016, “In-situ electrochemical generation of caustic and oxygen from sewage for emission control in sewers”. K.R. is supported by the Multidisciplinary Research Partnership “Ghent Bioeconomy” and by EU FP7 project “Kill-Spill”. The authors acknowledge the facilities and the scientific and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy and Microanalysis (The University of Queensland) and also acknowledge Dr. Bogdan Donose, Dr. Beatrice Keller-Lehmann, Marion Revalor and Nathan Clayton for their helpful assistance with the chemical analyses.
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DOI: 10.1021/acs.est.5b02188 Environ. Sci. Technol. 2015, 49, 11395−11402
