Nutrient Recovery by Bio-Electroconcentration is Limited by

Jan 28, 2019 - Nutrient Recovery by Bio-Electroconcentration is Limited by ... Because of low ionic conductivity, the measured current densities did n...
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Article Cite This: ACS Omega 2019, 4, 2152−2159

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Nutrient Recovery by Bio-Electroconcentration is Limited by Wastewater Conductivity Juliette Monetti, Pablo Ledezma, Bernardino Virdis, and Stefano Freguia* Advanced Water Management Centre, The University of Queensland, St. Lucia, Queensland 4072, Australia

ACS Omega 2019.4:2152-2159. Downloaded from pubs.acs.org by 46.161.61.161 on 01/29/19. For personal use only.

S Supporting Information *

ABSTRACT: Removal and recovery of nutrients from waste streams is essential to avoid depletion of finite resources and further disruption of the nutrient cycles. Bioelectrochemical systems (BESs) are gaining interest because of their ability to recover nutrients through ion migration across membranes at a low energy demand. This work assesses the feasibility of the concept of nutrient bio-electroconcentration from domestic wastewater, which is a widely available source of nutrients in ionic form, collected via sewer networks and easily accessible at centralized wastewater treatment plants. Here, we demonstrate the limits of a three-chamber BES for the recovery of nutrients from domestic wastewater. Because of low ionic conductivity, the measured current densities did not exceed 2 A m−2, with corresponding limited nutrient ion recoveries. Moreover, in a 3D electrode, forcing higher current densities through potentiostatic control leads to higher Ohmic losses, resulting in anode potential profiles and runaway currents and potentials, with consequent unwanted water oxidation and disintegration of the graphite electrode. At the current density of 1.9 A m−2, N removal efficiency of 48.1% was obtained at the anode. However, calcium and magnesium salts precipitated on the anion-exchange membrane, putatively lowering its permselectivity and allowing for migration of cations through it. This phenomenon resulted in low N and K recovery efficiencies (12.0 and 11.5%, respectively), whereas P was not recovered because of precipitation of salts in the concentrate chamber.

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recovery of nitrogen from urine using BESs.10−12 More recently, the concept of bio-electroconcentration allowed for a high recovery rate of 7.2 kg NH4−N m−3 d−1.13 Similar to electrodialysis, the up-concentration of nutrients in a bioelectroconcentration cell (BEC) occurs through the implementation of an electrical potential difference between two electrodes, forcing ion migration through ion-selective membranes. In order to reduce the applied voltage, the anode is enriched with electroactive bacteria which catalyze the anoxic oxidation of organic matter. Despite the potential of urine as an electrolyte, sourceseparation of urine can present a significant challenge. Its collection at the source requires the use of urine-diverting toilets and/or waterless urinals and its transport is problematic because of salt precipitation and consequent scaling of pipes. The transition to urine separation and treatment is a long-term goal, as it is not feasible to directly retrofit existing centralized systems for source-separated waste streams.14 Domestic wastewater is another candidate for nutrient recovery using BESs. This resource is widely available and does not require modifications of the current sewerage infrastructure, as it is mostly collected and centrally treated. BESs with the specific purpose of removing nitrogen from domestic wastewater by nitrification and denitrification have been extensively researched.15−20 However, wastewater’s low

INTRODUCTION The world consumption of fertilizers is constantly increasing with population growth and the demand for key macronutrients such as nitrogen (N), phosphorus (P), and potassium (K) is expected to grow yearly by 1.5% until 2020.1 However, current industrial processes for fertilizer production are not sustainable. Nitrogen-based fertilizers are produced through the Haber−Bosch process, which is estimated to account for 1.5% of the world’s energy consumption.2 High-grade ores of K and P will no longer be easily accessible after 100 years.3,4 In addition, global fertilizer consumption leads to the discharge of large amounts of nutrients into waterways. Non-negligible amounts (up to 11−16% for N, 5,6 10% for P 4 ) of anthropogenic nutrient flows are directed through wastewater facilities. Hence, the treatment of different wastewaters has been identified as a crucial step in nutrient recovery and recycling. Of the many technologies in development to recover nutrients from waste streams, bioelectrochemical systems (BES) have shown promise. They work by using microorganisms to catalyze electrochemical reactions, such as the conversion of organic waste into electrical energy. In these systems, ion migration across ion-exchange membranes enables the recovery of nutrients from waste streams including synthetic wastewater, reject water, pig slurry,7 and urine.8 Because of its high ionic conductivity, buffering capacity, chemical oxygen demand (COD) content, nitrogen and phosphorus concentrations, urine has been shown to be an effective electrolyte in BESs.9 Recent work showed successful © 2019 American Chemical Society

Received: October 10, 2018 Accepted: December 28, 2018 Published: January 28, 2019 2152

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Figure 1. Runaway effect at high potential. (A) BEC chronoamperometry (CA, red) at 0 V vs SHE with recirculation from cathode to anode (R2) and voltage difference between the references in the anode and in the concentrate chamber (green). (B) LSV of the BEC. Plain lines are for calculated values and dashed lines for measured values. Applied potential at RE1 (dashed black), potential difference between the membrane and RE2 ΔVM−RE2 (purple), potential difference because of the membrane ΔVM (green), Ohmic drop between RE1 and the membrane ΔVRE1−M (blue), potential difference between the two REs ΔVRE1−RE2 (dashed orange), and estimated effective anode potential at the membrane EAn (red).

electrical conductivity (EC) and buffering capacity9 are not ideal for treatment in BESs. Because of these key limitations, most studies on N recovery have been performed using highly concentrated wastewater such as pig slurry7 or high ammonia synthetic wastewaters.21 In these investigations, ammonia was primarily recovered via air-stripping after the cathode, reaching maximal concentrations of 350 mg L−1 for synthetic wastewater22 and 800 mg L−1 for reject water.23 In a synthetic wastewater-fed system combining electrodialysis and microbial fuel cell technologies, ammonia was up-concentrated only 1.5 times to a low concentration of 37.8 mg L−1.24 Importantly, in all these technologies, the focus has been on recovering nitrogen alone. However, other macronutrients such as phosphorus and potassium as well as micronutrients (including Fe, Zn, Cu, etc.) are also important to create balanced fertilizers.25 In this article, we present a proof of concept of a BEC for the recovery of N, P, and K from domestic wastewater. In addition to validating the successful up-concentration of N and K, we demonstrate the limitations of using low-conductivity sources which limit the current densities to ∼2 A m−2. If higher currents are forced by the use of a galvanostat/potentiostat, the low conductivity of the electrolyte drives the development of an anodic potential gradient perpendicular to the ion exchange membranes. This gradient is responsible for portions of the 3D electrode (e.g., a bed of graphite granules) to be poised at an effective potential much higher than that applied by the power source in proximity of the reference electrodes (REs). This effect can be so severe that it can cause water oxidation or even oxidation/disintegration of the anode material in the furthermost portions of the electrode.

In order to assess the true performance of the bioanode, samples were taken at the “anode out (AO)” in the case of an R1 configuration. Instead, for an R2 configuration, assuming complete mixing between anode and cathode, the samples were taken at the “cathode out (CO)” sampling port. In an R1 configuration, the CE was measured at 77.2 ± 0.4% at a feed flow rate of 2.54 L day−1 and 87.4 ± 4.0% at 1.72 L day−1. However, in the case of an R2 configuration, the CE was 149.5 ± 6.9%. A CE higher than 100% indicates the presence of a secondary oxidation reaction. Therefore, it can be speculated that not only wastewater-COD was oxidized at the anode, but also some of the cathodically produced H2 that entered the anode because of the recirculation regime, as previously observed.26,27 Applied Potential’s Influence on Operational Stability. Figure 1A shows current versus time traces observed during continuous operation of our system at the applied potential of 0.0 V versus standard hydrogen electrode (SHE). The BEC reached a current density of 20.7 ± 0.2 A m−2 at a feed rate of 2.54 ± 0.10 L day−1 with the R2 recirculation. At that point, the potential difference between RE1 and RE2 ΔVRE1−RE2 was equal to 1.97 ± 0.10 V, and the cell voltage measured with a multimeter was 8.55 V, indicating that the large current densities obtained were not solely because of biocatalysis. Cell voltages for MECs treating domestic wastewater usually range between 0.7 and 1.2 V.28 In a similar threechamber system treating urine, the cell voltage was around 1.5 V for a current density of 29.3 A m−2.13 This attests to the strong influence of the influent conductivity on BESs. With the anodic oxidation of some of the cathodically produced H2, the current produced increased uncontrollably as more H2 was recirculated through the anode chamber. Because of the low conductivity of wastewater, a potential gradient occurred across the anode, resulting in the implementation of a higher potential at the anodic portions further away from the reference electrode (RE1). Because of this gradient, the potential for oxygen evolution (+0.82 V vs SHE) was likely reached, further increasing the current production. This runaway-type effect led to carbon oxidation with consequent destruction of the granular anode, which was confirmed by the observation of black particles in the effluent. Runaway Effect at a Higher Anode Potential. The linear sweep voltammetry (LSV) test provided a profile of anodic current versus measured anode potential. This was combined with calculations to estimate the effective anode potential EAn in areas of the electrode distant from the reference electrode RE1, in particular the part of the anode

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RESULTS AND DISCUSSION Effect of the Recirculation. During the reactor’s operation, two configurations of recirculation were used: anode-to-anode recirculation (R1) and recirculation of the catholyte into the anode chamber (R2). At a feed flow rate of 2.54 L day−1, the observed current was higher on the R2 configuration (3.07 ± 0.01 A m−2) than on the R1 configuration (2.06 ± 0.06 A m−2). Yet, the COD removal was lower on R2 than on R1 (245.5 ± 4.0 mgCOD L−1 d−1 (n = 3) vs 318.9 ± 7.0 mgCOD L−1 d−1 (n = 2), respectively), which might indicate the occurrence of another oxidation reaction at the anode in the R2 configuration. This was also confirmed by the measured Coulombic efficiency (CE), which is an excellent indicator of the electron breakdown in a BES. 2153

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SHE. Hence, these conditions were chosen for the mass balance experiments. Samples were taken when the system was considered to have reached a steady state, identified by a stable current versus time profile and a stable EC of the concentrate. At this feed rate, the current density averaged 1.9 ± 0.1 A m−2 at a cell voltage of 2.9 ± 0.1 V (Figure 2A), which reflects the

which was in direct contact with the cation-exchange membrane (CEM), 1 cm away from RE1 (Figure 1B). During this experiment, the current density reached 20.7 A m−2. The Donnan potential drop was equal to 79.6 mV. The calculated values were compared to the measured potential difference between the two REs ΔVRE1−RE2 at 20.7 A m−2 (limits of the potentiostat) to evaluate a worst-case scenario. At 20.7 A m−2 and an applied anode potential of 0.42 V versus SHE, the measured ΔVRE1−RE2 was 2.8 V. The calculated potential difference because of the electrical resistance of the membrane RM·j was 248.4 mV. The maximal potential difference between the membrane and RE2 ΔVM−RE2 was 207.4 mV. The latter calculated values account for 16.3% of the measured value ΔVRE1−RE2. Therefore, the voltage difference ΔVRE1−RE2 is mainly due to a resistance in the anodic chamber, and any error caused by our theoretical calculation would be minimal. As shown in Figure 1B, the LSV curve experiences an inflection point at a current density of 4.0 A m−2 and an estimated true anode potential at the membrane junction of 0.91 V versus SHE, corresponding to an applied potential of 0.008 V versus SHE at RE1. This inflection may indicate the onset of an additional anodic Faradaic process, in this case likely to be oxygen evolution. Past this inflection, the Ohmic drop ΔVRE1−M between RE and membrane keeps increasing from 0.90 to 2.2 V versus SHE. Oxygen evolution reaction (OER) at pH 7 occurs at the theoretical potential of 0.82 V versus SHE.29 In practice, the OER occurs at a higher potential because of activation overpotentials, but it is well known to occur on carbon anodes above 1.0 V versus SHE.30 The part of the anode closest to the membrane reached potentials as high as +2.7 V versus SHE, thus certainly leading to O2 evolution. This would have further increased the current produced and hence the runaway effect visible in Figure 1B. When the applied potential at the RE was 0.42 V versus SHE, the current density reached 20.7 A m−2 and the granules situated close to the CEM were poised at a potential of +2.7 V versus SHE. At such a high potential, carbon corrosion can also occur, which would explain the black particles of graphite granules observed in the effluent and on the membranes (see Figure S4.1 in the Supporting Information). It can be concluded that in 3D electrodes immersed in lowconductivity media, sharp electrode potential gradients arise in the direction of ion migration. This is because the electrode despite having a uniform solid phase potentialis situated in a volume which is affected by an electrical potential gradient in the electrolyte. The result is that the applied potential is not homogenous across the electrode, and thereby can lead to the occurrence of unwanted reactions such as water electrolysis and corrosion/destruction of the electrode. After this observation, it was decided to operate the reactor at the lower applied potential of −0.123 V versus SHE in order to avoid reaching the potential for OER with a consequent runaway effect. Nutrient Removal and Recovery Rates. During the 12 h period during which the mass balance was established, the influent’s pH was 7.29 ± 0.02 and the EC was 1.38 ± 0.03 mS cm−1. The conditions which allowed for a stable reactor operation were previously identified to be a feed rate of 1.68 L of domestic wastewater per day with the recirculation configuration R2 and an applied potential of −0.123 V versus

Figure 2. BEC performance. (A) Average current density (blue) and average cell voltage (red) at −0.123 V vs SHE and 1.68 ± 0.05 L of wastewater day−1 over the triplicate 4 h periods. Grey bars show the standard error. (B) Element balances for N, P, K, and Na recovered by the BEC reactor at an average current density of 1.9 ± 0.1 A m−2. The black bars show the standard error.

low influent conductivity and consequent high internal resistance. The concentrate production rate was 9.9 ± 0.4 mL day−1, resulting in a volume reduction factor (ratio of the feed rate to the concentrate production rate) of approximately 170. Macronutrients were up-concentrated in the middle compartment to a plateau as previously demonstrated13the EC averaging at 22.6 ± 0.2 mS cm−1. The key nutrient NH4− N reached a maximum concentration of 1.4 ± 0.1 g L−1 equivalent to an average up-concentration factor of 20.1 ± 1.9 times. Potassium and sodium were also concentrated (respectively 19.6 ± 2.5 and 25.9 ± 1.8 times vs feed concentration), reaching maximum concentrations of 0.61 and 2.56 g L−1. Micronutrients essential to plant growth, such as Fe, Ca, Mg, Zn, and S,31 were also present in the concentrate in small quantities (Table 1). However, phosphorus was not recovered in the liquid concentrate and precipitated as Ca and Mg salts in the concentrate chamber and on the membranes. 2154

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membrane (AEM), as previously described.13 The levels of NH4−N were higher at the “cathode out” (CO) sampling port than at the “anode out” (AO) sampling port. The percentage of NH4−N lost through the AEM, e.g., removed via the CEM but subsequently lost via leaching through the AEM, was 38.9 ± 1.4%. The high concentration gradients between the influent and concentrate can play a role, as well as the diffusion of uncharged species such as NH3.13 In addition, precipitation of crystals on the ion exchange membranes on the sides of the concentrate compartment (see Figure S4.2 in the Supporting Information) might have led to a serious loss of permselectivity. Considering the amount of crystals present on the surface of the AEM, the salt scaling could have caused microcracks in the membrane, further enhancing the unwanted transfer of cations through it. It is also possible that some surface area was unavailable for ion transfer, further affecting the recovery rates. More experiments are warranted to determine the impact of Mg and Ca concentrations on the reactor’s performance. Because of the high cell voltage required and the relatively low current density and N recovery rate, the system consumed 94.4 ± 1.1 kWh per kg of N recovered. Considering N-fertilizer production consumes around 12.5 kWh kgN−1, the BEC’s power consumption makes it currently uncompetitive with conventional technologies.32 In addition to nitrogen, the BEC recovered an average of 11.5 ± 1.4% of the potassium (at a rate of 9.0 ± 0.5 g of K+ m−3 d−1) and 29.6 ± 8.6% of the phosphorus, which were transformed into Ca and Mg salts. 32.2 ± 4.1% of the removed potassium and 14.5 ± 0.7% of the removed sodium migrated through the AEM. However, if the removal at the anode only is taken into consideration, 48.1 ± 1.3% of the NH4−N was removed, resulting in an N removal rate of 96.6 ± 2.4 g of NH4−N m−3 d−1. In a similar fashion, K removal efficiency at the anode was 55.7 ± 4.7% of K. In comparison, the removal efficiency calculated between “anode in” (AI) and CO was 33.0 ± 9.6%.

Table 1. Composition of the Real Domestic Wastewater during the Entire Operation Period and the Concentrate Produced during the Mass Balance Experimenta parameter pH EC COD NH4−N PO4−P K Na Ca Mg S Fe Cu Mn Zn

real domestic wastewater

concentrate

7.1 ± 0.1 8.0 ± 0.2b 1.3 ± 0.1 22.5 ± 0.4 (mS cm−1) (mS cm−1) −1 0.3 ± 0.1 (g L ) 2.5 ± 0.6 (g L−1) −1 63.4 ± 4.5 (mg L ) 1.4 ± 0.1 (g L−1) 9.3 ± 1.1 (mg L−1) 2.5 ± 0.5 (mg L−1) 23.3 ± 2.6 (mg L−1) 0.55 ± 0.04 (g L−1) 84.8 ± 3.0 (mg L−1) 2.3 ± 0.1 (g L−1) 28.3 ± 1.3 (mg L−1) 100.1 ± 31.0 (mg L−1) 17.3 ± 0.3 (mg L−1) 886.9 ± 46.2 (mg L−1) 11.1 ± 1.7 (mg L−1) 31.3 ± 5.4 (mg L−1) 0.2 ± 0.1 (mg L−1) 1.9 ± 4.4 (mg L−1) 0.07 ± 0.04 0.7 ± 0.1 (mg L−1) (mg L−1) 0.8 ± 0.03 (mg L−1) 0.02 ± 0.01 (mg L−1) 0.03 ± 0.03 0.05 ± 0.05 (mg L−1) (mg L−1)

up-concentration factor

8.6 ± 1.3 22.8 ± 0.5 0.3 ± 0.02 23.5 ± 1.0 27.4 ± 0.9 3.5 ± 0.9 51.2 ± 1.7 2.8 ± 0.1 10.4 ± 17.3 9.2 ± 2.8 36.9 ± 15.4 1.8 ± 0.1

a

Error values are 95% confidence intervals for the wastewater and standard error (N = 4) for the concentrate and standard error (N = 4) for the concentrate and up-concentration factor. b95% C.I. During the mass balance experiment, pH was 7.2 ± 0.2, because of the addition of acid in the burette to prevent ammonia volatilization.

Under steady-state conditions, the BEC only removed an average of 15.0 ± 2.7% of the nitrogen (as NH4−N). This is equivalent to an N removal rate of 29.9 ± 5.0 g of NH4−N m−3 d−1. The corresponding N recovery efficiency was 12.0 ± 1.4%, equivalent to an N recovery rate of 24.0 ± 3.0 g of NH4−N m−3 d−1 (Figure 2B). This relatively low rate was due to an important loss of N through the anion-exchange

Figure 3. Operating principle of the BEC. AI stands for “anode in”, AO for “anode out”, CO for “cathode out”, and M for “middle chamber”. Two recirculation configurations were enabled: R1 is the recirculation of the anodic effluent into the anodic compartment and R2 is the recirculation of the cathodic effluent into the anodic compartment. 2155

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avoid membrane deformation. The effective hydraulic volumes of the anode and the cathode were, respectively, 62 and 66 mL (tubing not included); the middle chamber volume was 104 mL. An Ag/AgCl (in saturated KCl solution) reference electrode (RE1) (−0.197 V vs SHE) was inserted in the anode compartment. Two peristaltic pumps were used in the system: one to continuously feed real wastewater to the anode compartment (Watson Marlow SCI-Q 323S, UK) and a second one for mixing/recirculation. The anode effluent was fed into the cathode compartment, where protons were reduced to hydrogen, as in microbial electrolysis cells. The produced concentrate was constantly overflowing out of the reactor (its volume vs time quantified using a 50 mL burette) because of the osmotic/electroosmotic flow accompanying the migration of ions through the exchange membranes.13,34 Media Composition and Inoculation. Real domestic wastewater was collected weekly from a local pumping station in Brisbane (Australia). After each collection, a 15 L reservoir was filled up with the fresh wastewater and left at room temperature to feed the reactor. Wastewater COD could be maintained for up to 3 days before natural biological degradation significantly altered wastewater composition. The medium was changed weekly and replaced by newly collected wastewater. The characteristics of the feed can be seen in Table 1. The initial solution in the concentrate chamber of the BEC was a synthetic electrolyte solution, consisting of 0.14 g L−1 CaCl2·H2O, 1.13 g L−1 KCl, 0.11 g L−1 Na2SO4, 0.06 g L−1 K2HPO4, 0.02 g L−1 MgCl2·6H2O, and 1 mL L−1 of a trace element solution adapted from Wolfe’s mineral solution (Table S1.1).35 The reactor was inoculated with 10 mL of anaerobic sludge from a local wastewater treatment plant and fed with real domestic wastewater at a rate of 1.12 L day−1 [hydraulic retention time (HRT) of the bioanode 80 min] until a steady current was observed. Within a week, once the current started increasing, recirculation of the anode effluent into the anodic compartment (R1, see Figure 3) was switched on to enhance mixing and minimize mass transfer limitations. Sampling and Chemical Analyses. The reactors had four sampling ports, as shown in Figure 3“anode in” (AI), “anode out” (AO), “cathode out” (CO), and “middle chamber” (M). 1−3 mL of liquid were sampled regularly using syringes, with compact meters used to determine the pH and EC at each compartment’s inlet and outlet (LAQUAtwin pH22 and EC22; HORIBA, Japan). To measure the reactor’s CE, the COD of each sample was measured using standard potassium dichromate kits (range 25−1500 mg L−1; Merck-Millipore, Australia) and the CE was evaluated according to eq 136

This suggests that much higher recovery rates (and energy efficiency) could be achieved once a method to avoid the unwanted migration of cations through the AEM is developed. In the scenario where no loss of cations through the AEM occurs, the energy consumption of the BEC would be lowered to 23.5 ± 2.0 kWh per kg of N removed. This energy consumption would be in the same order of magnitude as the energy required to produce fertilizer via Haber−Bosch combined with the energy required to treat the wastewater through nitrification−denitrification (each consuming around 12.5 kWh kgN−132). Further reduction of voltage losses may be achieved through improved reactor designs (e.g., thinner compartments), which could ultimately make this technology relatively competitive to the traditional linear approach comprising N-fixation from the atmosphere for fertilizer production and biological denitrification of sewage at the end of the pipe.

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CONCLUSIONS

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EXPERIMENTAL SECTION

In this work, we demonstrated the limits of a bio-electroconcentration system for the recovery of nutrients from regular domestic wastewater. The ammonia recovery in a BEC is highly reliant on a high current density to drive the migration of ions. However, in a system with a poorly conductive electrolyte, a higher current results in high power consumptions and higher internal resistance. When a 3D electrode is used, this high resistance leads to runaway current and potential, with consequent unwanted Faradaic processes. The current density of the BEC system needs to be closely monitored in order to prevent a runaway-effect leading to destruction of the granular anode. A high nutrient removal was obtained at the anode. However, the cations migrated through the anion exchange membrane, resulting in low recovery rates. Investigations are still needed to understand the role played by Ca and Mg salts on the high cation migration. We hypothesize that the recovery rates could be improved if Ca and Mg were precipitated in a pretreatment step, rather than on the membranes. In addition to N and K, micronutrients were also up-concentrated in the product. However, the current density of the reactor will likely not be higher than ∼2 A m−2 because of the low influent conductivity. The recovery rates would have to be high in order to make the BEC competitive with conventional technologies in terms of energy consumption.

The BEC, shown in Figure 3, was a flat plate/frame type reactor made up of three identical 200 cm3 chambers [20 cm (height) × 5 cm (width) × 2 cm (depth)]. A 100 cm2 CEM (CMI-7000, Membranes International, USA) separated the anodic compartment from the concentrate compartment, itself separated from the cathodic compartment by an AEM of the same dimensions (AMI-7001, Membranes International, USA). The granular anode consisted of 240 g of synthetic graphite granules (El Carb-100, Graphite Sales, USA) with two plain graphite rods (Ø 5 mm, Morgan Advanced Materials, UK) used as current collectors. The granules were washed beforehand in 1 M HCl then 1 M NaOH to remove trace metals and contaminants.33 The cathode was made of 180 g of these granules with a 150 cm2 titanium mesh (Advent Research Materials, UK) as current collector. Glass beads were placed into the concentrate chamber as separators and to

CE =

MI FbqΔCOD

(1) −1

where M is the molecular weight of oxygen (32 g mol ), I [mA] is the steady-state current, F is the Faraday constant (96 485 C/mol e−), b is the number of electrons released by oxidation of COD (4 electrons), q [L s−1] is the volumetric influent flow rate, and ΔCOD [mg L−1] is the difference between the COD measured in the anode influent and effluent. For the mass balance analyses, sampling was undertaken on the reactor operating at a steady state and after completion of three HRTs. The reactor was considered to be at a steady state when the current and concentrate EC remained stable for at least three HRTs. After an initial sample was taken, three 2156

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the concentrate chamber was 8.0 ± 0.2, occasionally reaching high values up to 9.6. The speciation software Visual MINTEQ 3.140 was used to calculate the saturation index of potential precipitates for the conditions observed in the middle chamber (Table S2.1). Under these conditions, the physicochemical modeling confirmed that calcium, magnesium, and phosphate do precipitate as hydroxyapatite Ca10(PO4)6(OH)2, calcium phosphate Ca3(PO4)2, and struvite MgNH4PO4·6H2O. Some of the salts on the AEM were dissolved in 1 M HCl and a qualitative ICP-OES analysis confirmed the presence of the ions NH4+, PO43−, Mg2+, and Ca2+. The measurements did not allow for determination of the fraction of each Ca and PO4 precipitate, and as it was not possible to recover all the solid PO4−P precipitates, the calculations were simplified considering all the calcium, magnesium, and phosphate salts as a sole precipitate. The equimolar amount of magnesium that was removed but not recovered was used to calculate the theoretical amount of struvite produced (see Figure S3.1 in the Supporting Information). Determination of the Effective Anode Potential. The aim of this subsidiary experiment was to estimate the electrode potential effectively applied to the portions of the granular anode closest to the CEM, which were the furthermost from the RE, at a 1 cm distance from it. This was accomplished by calculating the potential difference in the anode chamber between RE1 and the membrane. This Ohmic drop ΔVRE1−M was calculated as

replicate samples were collected every 4 h (HRT of 110 min), then filtered through 0.22 μm filters, and acidified with 100 μL of 1 M HCl to prevent NH3 volatilization. Flow-injection analysis was used to determine total NH4−N and PO4−P concentrations using a Lachat QuickChem8500 (Lachat Instruments, USA). Inductively coupled plasma optical emission spectrophotometry (ICP-OES) was performed after nitric acid digestion to determine soluble cation concentrations (Na+, Ca2+, Mg2+, and K+) across the cell for the four samples with an Optima 7300DV detector (PerkinElmer, USA). Reactor Operation. Because of the natural variation of the influent’s composition (Table 1) and supply rate, steady state could not be obtained for more than 1 day. The electrolyte was recirculated continuously at a rate of 16.8 mL min−1 either in configuration R1 to provide good mixing in the anode chamber or in configuration R2, where the catholyte was mixed with the anolyte to ensure a sufficient supply of buffer capacity and equalize the pH across the cell37 (see Figure 3). With a potentiostat (Bio-Logic VMP-3, France), a potential (first 0.0 V vs SHE, then −0.123 V vs SHE) was applied to the working electrode (WE), the anode. The cathode was set as the counter electrode. The current versus time was recorded in CA mode. For all CA tests, the current density j [A m−2] is expressed relative to the projected membrane surface area (100 cm2). All experiments were conducted at room temperature (22.0 ± 2.5 °C). All values are averaged and presented with the standard error (±, n = 4), unless stated otherwise. LSV and Polarization Curve. An experiment was designed to determine the effective potential applied at different parts of the anode at high current densities. A reference electrode (RE2) was placed in the middle compartment. An LSV experiment was performed by varying the anode potential relative to RE1 from −0.2 to 0.5 V versus SHE at the rate of 1 mV s−1. The current at the WE and the potential difference between RE1 and RE2 (ΔVRE1−RE2) were both recorded and were used to reconstruct the polarization profile across the WE (see below for details). Nutrient Removal and Recovery Estimations. The nutrient removal and recovery rates for NH4−N, PO4−P, Na+, and K+ were averaged over three replicate periods of 4 h and normalized with respect to the total reactor volume (600 cm3). The recovery efficiencies (%) were calculated by comparing total effluent and influent concentrations. The fraction called “lost through AEM” refers to the fraction of cations that had originally been removed from the anodic chamber by migrating across the CEM toward the concentrate chamber, but subsequently migrated through the AEM toward the cathodic chamber. After operation, crystals were observed in the concentrate chamber. Raman spectra acquired on the crystals were consistent with those of struvite MgNH4PO4·6H2O and hydroxyapatite Ca10(PO4)6(OH)2 (data not shown). Precipitation of struvite on ion exchange membranes was previously observed12 and high concentrations of Ca and Mg were shown to result in membrane scaling.38 Furthermore, it is usually recommended to operate ion-exchange membrane systems at concentrations of Ca and Mg lower than 5 mg L−1 to prevent scaling of membranes, particularly because of struvite precipitation, which occurs at high pH.39 Here, the concentrate had 100.1 mg of Ca L−1 and 886.9 mg of Mg L−1 over the course of the mass balance (Table 1). In addition, the pH in

ΔVRE1− M = ΔVRE1− RE2 − ΔVM − RE2 − ΔVM

(2)

where ΔVRE1−RE2 (V) was the measured voltage difference between RE1 and RE2 (Figure 4).

Figure 4. Schematic of the voltage difference between RE1 and RE2.

The voltage difference between the concentrate side of the CEM and RE2 in the concentrate chamber ΔVM−RE2 (V) was calculated as a result of Ohm’s law and Pouillet’s law ΔVM − RE2 =

1 L · ·I ECC A

(3)

where ECC [S cm−1] was the average EC measured in the concentrate chamber over the course of the reactor’s operation, L (1 cm) the distance between the membrane and RE2, A (52 cm2) the CEM projected surface area available for ion transfer, and J [A] the current measured during the LSV. The voltage difference through the membrane ΔVM (V) was estimated as follows ΔVM = RM·j + |ΔVDonnan| 2157

(4) DOI: 10.1021/acsomega.8b02737 ACS Omega 2019, 4, 2152−2159

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where RM (120 Ω cm2) was the maximal electrical resistance of the membrane, j [A cm−2] was the current density measured during the LSV, and ΔVDonnan was the Donnan potential (V). The Donnan potential, assumed to be a constant, is a result of a thermodynamic potential because of the concentration gradient between the two electrolytes on either side of the membrane. It was measured as the electrical potential difference between the two REs situated in the two chambers at open circuit. The effective anode potential EAn could then be calculated as the sum of the applied voltage E during the LSV and the Ohmic drop ΔVRE1−M. The effective anode potential calculated is an estimation, as the second RE was not placed in the anode chamber but across the membrane in a much higher concentrated solution. However, the potential differences because of the membrane electrical resistance RM·j and between the membrane and RE2 ΔVM−RE2 were calculated using worst-case scenario hypotheses to simulate a maximal value. The maximal value of 120 Ω cm2 (vs 30 Ω cm2 according to the manufacturer) was taken as the electrical resistance of the membrane. This increased value took into account the presence of a salt concentration difference across the membrane, as described by Geise et al.41 A value of 19.2 mS cm−1 was used for the concentrate EC, which was the average value for the 9 months of operation. Due to the presence of glass beads, the liquid volume of the concentrate occupied 52 % of the compartment’s total volume. Hence, the membrane surface area available for ion transfer was estimated at 52 cm2.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02737. Trace element solution; saturation index; mass balance; and membrane pictures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61 7 3346 3221. Fax: +61 7 336 54726 (S.F.). ORCID

Pablo Ledezma: 0000-0003-1366-639X Bernardino Virdis: 0000-0001-8036-8937 Stefano Freguia: 0000-0002-2294-9036 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Australian Research Council project LP 150100402 in partnership with Queensland Urban Utilities (QUU) and ABR Process Development. P.L. furthermore acknowledges an ECR Development Fellowship from The University of Queensland.

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DOI: 10.1021/acsomega.8b02737 ACS Omega 2019, 4, 2152−2159