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Phosphate Recovery from Excess Sludge by Conventional Electrodialysis (CED) and Electrodialysis with Bipolar Membranes (EDBM) Xiaolin Wang, Yaoming Wang, Xu Zhang, Hongyan Feng, Chuanrun Li, and Tongwen Xu* CAS Key Laboratory of Soft Matter Chemistry, Laboratory of Functional Membranes, School of Chemistry and Materials Science, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ABSTRACT: Phosphate recovery from excess sludge by electrodialysis was investigated on the basis of a reformation of enhanced biological phosphorus removal (EBPR) technology. Electrodialysis operations containing conventional electrodialysis (CED), electrodialysis with bipolar membrane (EDBM), and integration of CED and EDBM were used for phosphate recovery. Phosphate was recovered as a concentrated phosphate solution and phosphoric acid by CED and EDBM, respectively. Both batch and continuous operation models were adopted. Nearly all of the phosphate in the solution could be recovered under batch operation of CED, and the recovery ratio could reach 95.8% under continuous operation. In the integration of electrodialysis, EDBM could realize the in situ conversion of the concentrated phosphate into phosphorus acid effectively, as another recovery form. A net production of phosphorus acid about 0.075 mol/L could be obtained under an operating current of 50 mA/cm2 for EDBM, and the energy consumption was 29.3 kWh/(kg H3PO4) with a current efficiency about 75%.

1. INTRODUCTION Phosphorus (P) is an important element which has wide application in fields of agriculture, industrial, human body growth, and so on. It is an indisputable fact that the phosphorus reserves are being depleted. “Where the P goes” was summarized by Rittmann et al.1 and showed that the largest two flows of lost P are in agricultural runoff and erosion (∼46% of mined P globally) and animal wastes (∼40%). The remaining phosphorus rock is of lower grade and more difficult to access than ever before.2 Elser and White3 have summed up our dilemma: “Establishing a reliable phosphorus supply is essential for assuring long-term, sustainable food security”. Therefore, the existing phosphorus reserves should be cherished and more attention should be paid to enforcing the recovery and reuse from any resources rich in phosphorus. Now, phosphorus levels in the surface water have significantly increased because the use of fertilizer and the discharge of municipal and industrial wastewater.4 Phosphorus is a critical environmental pollutant, despite the extensive efforts that have made to limit the point and nonpoint source discharging to surface waters.5 Too much phosphorus in the surface water will cause eutrophication, which has a bad impact on the water quality and ecosystem balance. Activated sludge processing is a common and effective method employed for treating wastewater containing phosphorus and nitrogen, where the biodegradable organics in it as the growing substance for organisms. Phosphorus removal from wastewater through microbial processes were devised in the late 1950s from the research that found that activated sludge could take up phosphorus in considerable excess to that required for normal biomass growth under certain conditions.6−8 The phosphorus content of the sludge (the aggregative state of PAOs living) could be as high as 5−7%.9 To ensure the normal operation of the enhanced biological phosphorus removal (EBPR), a certain amount of sludge needed to be separated from the return © 2013 American Chemical Society

sludge. The separated sludge, called excess sludge, is troublesome because of the high content of phosphorus. Currently, excess sludge, as a secondary solid waste, is one of the most serious challenges in biological wastewater treatment, and it must be disposed of in a safe and cost-effective way.10,11 Direct application of dewatered biosolids to the soil is one route for managing the excess sludge.12 Direct use as mineral phosphorus has been proved to be an effective method, but the pathogen and metal contamination need to be treated carefully.13 Biological release and thermochemical release, both of which were followed with recovery of phosphorus as an inorganic product, are two other methods for the utilization of P in EBPR sludges.12 However, there exist some disadvantages: for the former is the restricted phosphorus availability in the liquid phase and in the latter is the high cost. Therefore, an effective method for utilizing the phosphorus in excess sludge needs to be developed. In most of the previous studies, there needed to be some pretreatment process for further using the phosphorus in the sludge, like dewatering and incineration, ozonization, anaerobic digestion, and so on. However, a new method was proposed to recover phosphate from excess sludge without these pretreatments in this study, and in which a part of the investment can be saved. The main operating principle is that during the anaerobic operation of EBPR, phosphorus will be released into water body as phosphate that is of a relatively high level compared with that of the normal wastewater. There indeed was little phosphorus in the cell body after the releasing, which was confirmed by Raman microscopy.14 Therefore, the solution of phosphate can be used somewhere or for further treatment. Received: Revised: Accepted: Published: 15896

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On the basis of the idea about “releasing and recovering”, a reformation about the enhanced biological phosphorus removal (EBPR) was proposed. There needed to be a concentration process for the extensive applications of the phosphate solution. Conventional electrodialysis (CED) was usually used for concentrating and desalting a solution, like a process of dialysis but in which the ions are accelerated by an applied electric field. It can be realized that recovering ionic species from aqueous solutions without chemical consumption or waste generation15 and has already been applied in separation field.16−18 A novel electrodialysis named selectrodialysis was used to improve the efficiency of phosphate recovery from a struvite reactor, a process in which a high availability of the phosphate was achieved and effluent with a high content of phosphate was prevented.19 A bipolar membrane is a composite membrane consisting of a cation-exchange layer (with negative fixed charges) and an anion-exchange layer (with positive fixed charges).20,21 Electrodialysis with bipolar membrane (EDBM) is a kind of technology that integrates solvent and salt dissociation; it can realize salt conversion without second salt pollution and provide H+ and OH− (or CH3O− for alcohol splitting) in situ without the introduction of salts.22,23 Converting sodium phosphate into phosphoric acid and sodium hydroxide by bipolar membranes has been realized.24 Phosphate, present as ions in solution, could be enriched as concentrated phosphate and phosphoric acid by CED and EDBM, respectively. This concentrated solution can be used in multiple aspects compared with the product by other recovery ways. Recent increases in phosphorus pricing, together with the environmental and regulatory need of removing phosphorus from wastewater, make phosphate recovery economically attractive.25 From the above statement, we can know that both the phosphate recovery and the excess sludge disposal are urgent. The excess sludge, as a resource rich in phosphorus, was investigated for phosphate recovery in this study. CED and EDBM, as the major recovery technologies, were adopted to recover the phosphate, and different operation conditions were investigated.

Figure 1. Schematic diagram of the reformed EBPR process. (1) Additional anaerobic phase. (2) Additional settling phase. (3) Electrodialysis unit (electrodialysis with bipolar membranes (EDBM) or/and conventional electrodialysis (CED)). (4) Recovered solution. (5) Treated water. (6) Sludge after the release of phosphate.

(2) tanks (beakers of various capacities) and submersible pumps (AP1000, Guangdong Zhenhua Electrical appliance Co., Ltd., China, with the maximum flow of 400 L/h) to store and circulate the solutions; (3) a peristaltic pump (BT100−1F with a pump head of YZ1515x, Baoding Longer Precision Pump Co., Ltd., China) to regulate the flow rate of feeding; (4) membrane stacks, including the CED stack and the EDBM stack. What is common between these two types of membrane stack are (I) the cathode and anode, which are both made of titanium coated with ruthenium and (II) electrode rinsing solution, which was prepared from sodium sulfate solution (250 mL, 0.3 mol/L), and it was same in each experiment. The CED stack contains (i) plastic partition nets (thickness = 0.7 mm) with the flow channel made by photoetching and (ii) an cation-exchange membrane (JCM-II-07) and an anion-exchange membrane (JAM-II-07), both with an effective membrane area of 99 cm2; their properties are listed in Table 1. For the details of the EDBM stack, it has the following: (a) Plexiglas spacers (thickness = 9 mm) with a round hole in the middle to separate the membranes using Viton gaskets as the seals; (b) a cation-exchange membrane (Neosepta CMX), an anionexchange membrane (Neosepta AMX) and a bipolar membrane (Neosepta BP-1), all with an effective membrane area of 7.07 cm2 whose properties are listed in Table 1; (c) the initial working volume of 0.3 L for the acid tank (0.1 mol/L H3PO4) and the alkali tank (0.1 mol/L NaOH). The configurations (a) and (b) shown in the Figure 2 were adopted for the construction of the CED and EDBM stacks, respectively. The number of repeating units is six for CED, whereas for EDBM, there contains only four pieces of membrane to constitute the structure of three compartments (BP−A−C−BP). Figure 3 shows the flowchart of the integrated operation of electrodialysis (CED and EDBM). During all the electrodialysis experiments, the solution in each tank was circulated for a certain amount of time before the operating current was applied. The aim of the circulation is to eliminate all the visible gas bubbles inside the membrane stack and eliminate the effect of these on the conductivity of membrane stack. The operating current for CED was applied ahead of that for EDBM, and the aim is to increase the concentration of phosphate in the buffer tank and decrease the starting resistance of EDBM stack. This prerunning duration for CED was set as 30 min, which was

2. EXPERIMENTAL SECTION 2.1. Reformation about the EBPR. The core of reformation is the introduction of an electrodialysis unit into the EBPR, and the schematic diagram is shown in Figure 1, in which the part in the dotted box is the EBPR main process. Another additional anaerobic phase was set to receive excess sludge from the return sludge stream, and a branch from the raw wastewater was introduced into it. The operational approach of this additional anaerobic phase is the same as with the EBPR main process. During this anaerobic phase, the intracellular phosphorus could be released into the liquid phase in the form of phosphate. Subsequently, another settling phase was set to get a clear supernatant that is rich in phosphate. The sludge settled at the bottom should be easier to dispose of than the normal excess sludge, because it contains little phosphorus. The supernatant, which has no visible impurities, can be introduced into the electrodialysis unit for recovering phosphate. 2.2. Apparatus and Operation of the Electrodialysis Unit. The laboratory-scale experimental equipment contained the following sections: (1) the DC stabilizing power supply (N5772A, Agilent Technologies, Co., Ltd. and WYJ-100 V/ 10A, Shanghai Querli Electronic Equipment Co., Ltd., China); 15897

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Table 1. Properties of the Membranes Used in CED and EDBM Stacks membrane type Neosepta CMX Neosepta AMX Neosepta BP-1 JCM-II-07 JAM-II-07 a

thickness (μm)

IEC (meq·g−1)

area resistance (Ω cm2)

220−260

1.5−1.8

2.0−3.5

120−180

1.4−1.7

2.0−3.5

200−350

voltage drop (V)

2.0−2.9

1.0−3.0

160−230

1.8−2.2

4.0−8.0

company Neosepta Tokuyama Soda Inc., Japana

1.2−2.2

160−230

efficiency (%)

>98 Beijing Tingrun Membrane Technology Development Co., Ltd., Chinaa

The data are collected from the product brochure provided by the company.

Figure 2. Schematic of configuration of the CED and EDBM stacks. C, cation exchange membrane; A, anion exchange membrane; BP, bipolar exchange membrane. Pn−, refers to phosphate radical ion (PO43−, HPO42−, H2PO4−, where n = 1, 2, or 3). The number of cation and anion-exchange membrane is six in the CED stack. ① and ⑤ are the anode compartments of CED and EDBM; ④ and ⑨ are the cathode compartments of CED and EDBM; ② is the concentrated compartment; ③ is the diluted compartment; ⑥ is the acid compartment; ⑦ is the feed compartment; ⑧ is the alkali compartment.

Figure 3. Flowchart of the integrated operation of electrodialysis (CED and EDBM). (1) Direct current power supply. (2) CED stack. (3) EDBM stack. (4) Anode solution tank of CED. (5) Cathode solution tank of CED. (6) Anode solution tank of EDBM. (7) Cathode solution tank of EDBM. (8) Diluted solution tank. (9) Peristaltic pump. (10) Simulated wastewater. (11) Buffer tank (Concentrated solution tank for ED and feed solution tank for EDBM). (12) Acid tank. (13) Alkali tank.

in the tanks of CED and of the phosphoric acid and the alkali solution produced by EDBM were analyzed by sampling. 2.3. Wastewater Preparation and Analysis. The solution rich in phosphate was synthetically simulated. This solution feeds into the diluted and concentrated compartments of CED as the initial solution. It contains 100 (mg of P)/L, which is close to the phosphate concentration in the liquor phase at the end of anaerobic phase. The solution was prepared by sodium phosphate tribasic dodecahydrate (Na3PO4·12H2O), disodium hydrogen phosphate dodecahydrate (Na2HPO4· 12H 2 O), and sodium dihydrogen phosphate dihydrate (NaH2PO4·2H2O). Calculated according to the phosphorus element, each of these three salts accounted for one-third of the

referred to in the results of the individual experiments of CED. At the end of the prerunning duration, the current for EDBM was applied, and the solution rich in phosphate began to be fed into the diluted solution tank continuously. Then, CED and EDBM were operated together for 3 hydraulic retention times (HRTs) for the diluted tank at least. Once the integrated operation of electrodialysis began, the phosphate in the feed would be enriched in the buffer tank by CED, and EDBM would take away the phosphate from it to produce phosphoric acid and alkali solution at the same time. The buffer tank, acting as a connection hub, played an important role in achieving the operational compatibility. The concentrations of the phosphate 15898

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concentration. All the chemicals used in this study were of analytical reagent grade, and deionized water was used throughout. Samples were collected with a predetermined time interval and analyzed for the phosphate concentration. The phosphates in the diluted tank and concentrated tank were measured in accordance with the Standard Methods (4500-P E. Ascorbic Acid Method).26 The main principle is: ammonium molybdate and potassium antimonyl tartrate react in acid medium with orthophosphate to form a heteropoly acid, phosphomolybdic acid, which is reduced to intensely colored molybdenum blue by ascorbic acid. Phosphoric acid and sodium hydroxide produced by EDBM were determined by titration with calibrated NaOH and HCl solution, and phenolphthalein and methyl orange, respectively, were used as indicators. 2.4. Calculation of Current Efficiency and Energy Consumption. The current efficiency (η) of producing H3PO4 by EDBM was calculated by the following equation:27 η=

z(C t − C0)VF NIt

Where Ct and C0 (mol/L) are the concentrations of phosphoric acid at times t and 0; V (L) is the volume of the solution in acid tank (V = 0.3 L; there is almost no change in volume); F is the Faraday constant (96 485 C/mol); N is the number of repeating units; I (A) is the operating current; t (s) is the running time of EDBM; z is the absolute valence (z = 3 for PO43−, z = 2 for HPO42−, z = 1 for H2PO4−; each of these three salts accounted for one-third in the concentration, so z is equal to 2, theoretically.). The energy consumption E [kWh/(kg H3PO4)] was calculated by extrapolating the results for production of 1 kg of H3PO4 by EDBM based on the following equation:27 E=

∫0

t

Figure 4. Treatment of phosphate solution by CED under constant operating voltage (62.3 V). (a) Phosphate concentration in the diluted compartment. (b) Phosphate concentration in the concentrated compartment.

UIdt C tVM

Where U (V) is the voltage drop across the EDBM stack; M is the molar weight of H3PO4 (M = 98.0 g/mol).

Figure 4b illustrates the concentrations of phosphate in the concentrated compartment. What can be imagined is that the phosphate concentration increases as time elapses. When the diluted compartment contained little or no phosphate, the P concentration in the concentrated compartment rose to the maximum and began to be steady. For each condition, the same amount of time will be taken for diluted and concentrated compartments to reach their respective end stages because the phosphate removed from diluted compartment will be enriched into the concentrated compartment. 3.2. Effect of Operating Voltage on the Performance of Individual CED. In this part, the volumes of diluted and concentrated compartments were constant at 1.5 and 0.3 L, respectively. Figure 5a demonstrates the change in the phosphate concentration in the diluted compartment under different operating voltage conditions. We can see that the higher the operating voltage is, the faster the concentration dropped because changing the operating voltage will make a difference on the processing capacity of CED stack. With regard to a certain membrane stack, operating voltage and the effective membrane area are the two key points related to the processing capacity. Increasing the voltage could shorten the working time obviously when the membrane area is fixed. Under a higher voltage, the removal rate of phosphate in the diluted compartment will be faster. The time is shortened from 75 to 20 min when the operating voltage is increased from 24.5 to 75

3. RESULTS AND DISCUSSION 3.1. Treatment of Phosphate Solution by Individual CED. Figure 4 shows the results of treating phosphate solution of different volumes by CED under a fixed operating voltage of 62.3 V. The experiment was shut down at the time when the current dropped to a minimum and began to be steady. The initial volume of the solution in the diluted tank varied from 0.9 to 6.0 L, whereas that in concentrated tank remained 0.3 L constant. Figure 4a shows the concentrations of phosphate (calculation with phosphorus element, following the same) in the diluted compartment, which decreased as time elapsed. The decreasing rate slowed down with an increasing volume of solution in diluted tank. It is easy to understand that the smaller the volume is, less phosphate will be contained. The performance of CED still remains the same, nearly, at a fixed operating voltage, so the phosphate concentration will drop faster and less time will be taken when treating a smaller volume. The final concentration in each experiment could fall almost to zero. However, the operating time will be longer for treating a larger volume of P-rich solution, which is logically true. Just about 20 min is enough for removing the phosphate in a solution of 0.9 L, whereas the time for 6.0 L needs around 180 min. 15899

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remains in the membrane channels, whereas the diluted compartment is empty of phosphate. This part of the phosphate could be driven out and enriched in the concentrated compartment at a higher operating voltage. 3.3. Effect of Flow Rate of Feeding on the Individual Continuous CED. In the two parts before, batch operations for phosphate recovery from a solution rich in phosphate were conducted. The phosphate could be recovered effectively and enriched as concentrated phosphate solution. However, as far as we know, a batch operation is labor intensive and time consuming compared with a continuous operation. In this part, a continuous operation of CED was studied using the solution rich in phosphate as feed solution to be treated under the fixed operating voltage of 62.3 V. The initial volume of solution in the diluted tank of CED is 1.5 L and that of the concentrated tank is 0.3 L. The feed solution was pumped into the diluted tank continuously by peristaltic pump, and the flow rate ranged from 10 to 50 mL/min. The experiment under each condition lasted for 3 HRTs for the diluted tank at least, the reason for that is to investigate the operational stability of this continuous process. Figure 6a shows the phosphate concentrations in the diluted compartment under different flow rates. In the first half hour, it is a static CED process without feeding. Therefore, a uniform and overlapping result was obtained, which is a decrease in the

Figure 5. Effect of operating voltage on the performance of CED; the volumes of the diluted and concentrated compartment are 1.5 and 0.3 L. (a) Phosphate concentration in the diluted compartment. (b) Phosphate concentration in the concentrated compartment.

V. Nearly all phosphate in the diluted compartment could be recovered under each condition. However, higher operating voltage will damage the membranes and lead to a short working life of membrane, which will be uneconomical. A slight burning phenomenon of the ion exchange membrane had been discovered during electrodialysis.28 A proportional relationship between the electrodialysis yield and the total electricity applied was found in converting NaNO3 into HNO3 and NaOH, regardless of the constant current or constant voltage operation mode.29 In the study of ref 29, the minimum energy consumption was achieved under the minimum operating current and voltage when treating the same amount of NaNO3, which could also reveal that the current efficiency may be the maximum. Therefore, selection of a suitable operating voltage is important for an electrodialysis process. Figure 5b shows the effect of operating voltage on the phosphate concentration in concentrated compartment, which is almost a mirror image of the Figure 5a. A high operating voltage will achieve a fast and efficient change in concentration. That is also because that the processing capacity of the CED stack will increase by increasing the operating voltage. The maximal concentrations at the end are slightly different from each other under different voltages, and the final measured concentration in diluted compartment is almost zero. The final concentrated phosphate concentration will be a little higher at a higher voltage. The reason may be that some phosphate still

Figure 6. Effect of flow rate of feeding on the performance of CED. The operating voltage is 62.3 V, and the volumes of diluted and concentrated compartments are 1.5 and 0.3 L. (a) Phosphate concentration in the diluted compartment. (b) Phosphate concentration in the concentrated compartment. 15900

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concentration from 100 to 5 mg/L. Then, solution rich in phosphate began to be pumped into the diluted tank continuously. The phosphate concentration in the outlet began increasing in varying degrees, but all went into a steady state in the later period. As the flow rate increased, the steady concentration level got higher. Because the phosphate pumped into the tank went beyond the limit of the processing capacity of CED stack, more phosphate will be overflowed under a higher flow rate. It is an optimistic sign that little or no increase was found for flow rates under 10 mL/min, under which the removal ratio of phosphorus could reach 95.8%. The solution turned into low content of phosphate after the CED processing; that is to say, this continuous integrated operation did perform. However, the processing capacity of the feed solution is small under a low flow rate, and the phosphate concentration in the effluent remains high under the high flow rate. Therefore, a trade-off condition needs to be adopted for this process. The variation of phosphate concentration in the concentrated compartment was shown in Figure 6b. The results indicate that there is also a uniform and overlapping part in the first half hour. There appears an inflection point at 30 min, after which the increase rate of the concentration gives different trends. Another conclusion obtained from the data graph is that at a fixed time, the larger the flow rate is, the larger the concentration will be. The reason can be summed up in two points: (1) the thickness of diffusion layer between the membrane-liquid interfaces was thinned by increasing the flow rate, and (2) the increasing flow rate could result in a higher phosphate concentration in the diluted compartment, which causes a higher concentration gradation between the membrane−liquid interfaces. Therefore, more phosphate will be enriched into the concentrated compartment. From the data graph, we can also get that the final concentrations under each flow rate are different. A greater final concentration actually resulted under the operating condition of a smaller flow rate. The main reason may be that the smaller flow rate will result a longer HRT, and then the actual running time of CED will be longer. Furthermore, the operating voltage for electrodialysis is same, so that the processing capacity of the membrane stack is nearly consistent. So the longer the operating time is, the higher the final phosphate concentration that can be obtained. 3.4. Effect of Operating Current Density of EDBM on the Performance of CED. In the treatment of solution rich in phosphate by individual CED, the phosphate concentration in the concentrated compartment was growing along the running time. In order to reduce the concentration effect and test another way to recover phosphate, an EDBM stack was introduced into the individual CED treatment. The integrated operation of electrodialysis (CED and EDBM) is shown in the Figure 3, in which the concentrated compartment of CED and the salt compartment of EDBM merged into one as buffer tank. The operating of CED was same as that in section 3.3 except that the flow rate of the feed is fixed at 20 mL/min. The only operating variable in this section is the operating current density for the EDBM. Figure 7a shows the phosphate concentration in the diluted compartment of CED under integrated conditions and the comparison with the result under individual CED operation. It can be observed that there exist obvious differences between the two operating modes after a running time of 30 min. A decrease about 5 (mg of P)/L in the effluent (about 31.6% of the concentration under “CED only”) can be the result from

Figure 7. Effect of operating current density of EDBM on the performance of CED. The operating voltage is 62.3 V, the volumes of diluted and concentrated compartments are 1.5 and 0.3 L, and the flow rate of feeding for CED is 20 mL/min. (a) Phosphate concentration in the diluted compartment. (b) Phosphate concentration in the buffer tank.

the integrated operation of CED and EDBM. It can thus be concluded that the new introduced EDBM indeed had an effect on the performance of CED, and the effect was a positive one. The main reason is possibly due to taking away phosphate from the buffer tank by EDBM, which can decrease the concentration gradient of the two adjacent compartments of CED. Then, there will be less back-diffusion of phosphate from the concentrated compartment to the diluted one and less resistance for transporting phosphate by electric field from the diluted compartment to the concentrated one. Therefore, more phosphate will be removed compared with the single CED operation. The phosphate concentrations in the effluent of the diluted tank under different operating current densities are nearly same. This may be due to the fact that the processing capacity of CED depends mainly on operating voltage, although the concentration effect was alleviated, as said above. Figure 7b shows the phosphate concentration in the buffer tank and the comparison with the result under individual CED. We can see that all the concentrations under integrated conditions are lower than those under individual CED, except the one under the lowest operating current density of 10 mA/ cm2. Under a low operating current density, a little effect will be produced on the CED because the processing capacity of EDBM is also low. After the running of EDBM, the increase 15901

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rate of the phosphate concentration becomes low with the increasing operating current density. Because a higher operating current density will cause a larger processing capacity of EDBM, the quantity of phosphate taken away from the buffer tank by EDBM will increase. Therefore, the EDBM will suppress the increasing of concentration in the buffer tank. On the basis of the above analysis, it can be included that the removal of phosphate by EDBM will indeed decrease the phosphate concentration in the buffer tank of CED directly and then affect the performance of CED. Integrated operation of electrodialysis is also a feasible way besides recovering phosphate by individual CED. 3.5. Effect of Operating Current Density on the Production of Phosphoric Acid and Alkali Solution by EDBM. As discussed in section 3.4, a positive effect can be exerted on single CED operation due to the decrease in the concentration gradient of the adjacent compartments by EDBM. The quantity of phosphate reduced in the buffer tank was the part taken away by EDBM. A classic configuration for EDBM (shown in the Figure 3b) was adopted. Under this condition, the phosphate in the salt compartment will be divided into phosphoric acid and alkali solution, which were enriched in the acid and alkali compartments, respectively. The technical feasibility of converting phosphate to obtain the useful phosphoric acid and sodium hydroxide as byproduct has been demonstrated.24 Therefore, phosphate could also be recovered in the form of phosphoric acid, besides the concentrated phosphate solution achieved by CED. Figure 8a shows the concentrations of phosphoric acid and alkali solution produced by EDBM respectively. The results that appear on the data graph are pure production of the acid and alkali solution. We can see that both the acid and alkali concentrations increase along with the time. Also, the higher the operating current density is, the higher the concentration is. Because when the current density increases, the capacity of producing acid and alkali solution by the bipolar membrane becomes larger. However, it is not advisable to improve the capacity of the bipolar membrane through blindly increasing the operating current. It is a trade-off between the efficiency of water splitting and the lifetime of bipolar membranes,30,31 so a suitable current should be adopted. As seen from the data provided by the graph, the molar concentration of alkali solution is nearly three times larger than that of the phosphoric acid. The reason is that a phosphoric acid molecule has three hydrogen ions. Figure 8b,c shows the voltage drop across the stack, the current efficiency, and energy consumption under different operating current densities. As the current density increases, the voltage drop and the energy consumption increase. However, for the current efficiency, a phenomenon of decreasing first and then increasing is found. The ion selectivity of membranes will decrease as the operating current density increases, which will lead to a decrease in current efficiency at low current.32 When the current density increases to a higher level, a short transition time or less relative salt ion transport will be the result.33 Therefore, there exists an increase in current efficiency in the higher region of current density. In this study, the highest current efficiency (80.3%) was achieved at 10 mA/cm2, in which condition the energy consumption [5.3 kWh/(kg of H3PO4)] is the minimum. Although the highest concentration of H3PO4 was achieved under 50 mA/cm2, the energy consumption for producing is high, and it is 29.3 kWh/(kg of H3PO4). This suggests that low current density is proper for

Figure 8. Effect of operating current density on the production of phosphoric acid and alkali solution. (a) Effect of current density on the yield of phosphoric acid alkali solution. (b) Effect of current density on the voltage drop across the stack. (c) Effect of current density on current efficiency and energy consumption.

treating a solution of low concentration (less than 0.3 mol/L in this study) from an economic viewpoint. Therefore, we can conclude that the integrated operation of CED and EDBM is another feasible way to recover phosphate as phosphoric acid from the solution rich in phosphate. Because a classical configuration for EDBM (shown in the Figure 3) was adopted in this study, the alkali solution was produced as a byproduct, but it can be used in some other right way.

4. CONCLUSIONS An experimental study was carried out on the phosphate recovery from excess sludge by using electrodialysis, including CED and EDBM, on the base of a reformation of EBPR. Batch 15902

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and continuous operation models, single CED operation, and integrated operation of electrodialysis (CED and EDBM) were all investigated. Concentrated phosphate solution and phosphoric acid were obtained in the operation of individual CED and integrated electrodialysis operation, respectively. The phosphate recovery was investigated through the factors including the initial volume of solution rich in phosphate, operating voltage for CED in the batch CED operation, feeding rate, and operating current density for EDBM in the continuous integrated electrodialysis operation. The results indicate the following: (I) in the batch operation, the solution rich in phosphate contains nearly no phosphate after the treatment of CED; (II) in the continuous operation, low phosphate concentration in the effluent can be achieved under a low feed rate; for example, a phosphate removal ratio of 95.8% can be achieved under 10 mL/min; (III) both single CED operation and integrated electrodialysis operation are feasible ways to recover phosphate from a solution rich in phosphate. The current efficiency for producing H3PO4 by EDBM can reach 80.3% with an energy consumption about 5.3 kWh/kg under the operating current density 10 mA/cm2. The effective recovery of phosphate suggests that there is a possibility for the reformation of EBPR process. In that case, this study could bring us two benefits: the phosphorus content in the bothersome excess sludge becomes low and scarce phosphorus can be recovered effectively.

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AUTHOR INFORMATION

Corresponding Author

*T. W. Xu. Phone: +86-551-6360-1587. E-mail: twxu@ustc. edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported in part by National High Technology Research and Development Program 863 (No. 2012AA03A608), the National Natural Science Foundation of China (Nos. 21025626, 21206154), and the Programs of Anhui Province for Science and Technology (No.11010202157).

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