A Ten Liter Stacked Microbial Desalination Cell Packed With Mixed

Jul 30, 2014 - ... Rongjun Yan , Bart Van der Bruggen , Arcadio Sotto , Congjie Gao , Jiangnan Shen ... Xi Chen , Peng Liang , Xiaoyuan Zhang , Xia Hu...
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A Ten Liter Stacked Microbial Desalination Cell Packed With Mixed Ion-Exchange Resins for Secondary Effluent Desalination Kuichang Zuo, Jiaxiang Cai, Shuai Liang, Shijia Wu, Changyong Zhang, Peng Liang,* and Xia Huang* State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing, 100084, P.R. China S Supporting Information *

ABSTRACT: The architecture and performance of microbial desalination cell (MDC) have been significantly improved in the past few years. However, the application of MDC is still limited in a scope of small-scale (milliliter) reactors and high-salinity-water desalination. In this study, a large-scale (>10 L) stacked MDC packed with mixed ionexchange resins was fabricated and operated in the batch mode with a salt concentration of 0.5 g/L NaCl, a typical level of domestic wastewater. With circulation flow rate of 80 mL/min, the stacked resinpacked MDC (SR-MDC) achieved a desalination efficiency of 95.8% and a final effluent concentration of 0.02 g/L in 12 h, which is comparable with the effluent quality of reverse osmosis in terms of salinity. Moreover, the SR-MDC kept a stable desalination performance (>93%) when concentrate volume decreased from 2.4 to 0.1 L (diluate/concentrate volume ratio increased from 1:1 to 1:0.04), where only 0.875 L of nonfresh water was consumed to desalinate 1 L of saline water. In addition, the SR-MDC achieved a considerable desalination rate (95.4 mg/h), suggesting a promising application for secondary effluent desalination through deriving biochemical electricity from wastewater.

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matter in wastewater. After invented in 2009,8 the MDC has received a significant improvement in structure and performance. Chen et al.,9 for the first time, established a stacked MDC and investigated the influence of stack number on desalination performance. Morel et al.10 packed mixed cation-/anionexchange resins in the desalination chamber, decreasing internal resistance by 63%. Other extended works, including air cathode MDC,11 upflow MDC,12 recirculated MDC13,14 and microbial electrodialysis cell (MEDC),15−17 have also been reported. However, most of previous studies were conducted on milliliter-scale MDCs (500 mv, the external resistance was gradually decreased to 100, 50, and 10 Ω with a resistance box (0.1−99 999X; ZX21, Tianshui, China) to facilitate the acclimation of electrochemically active bacteria on anode and cathode. After the start-up, a NaCl solution of 0.5 g/L (∼1.08 mS/cm, pH 7.0), as a representative of secondary effluent from practical municipal WWTP,3 was used as salt solution for the desalination and concentrated chambers. The salt solution was circulated separately in the desalination and concentrated

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MATERIALS AND METHODS SR-MDC Construction. The SR-MDC is a cubic bioreactor comprising an anode chamber, a biocathode chamber, three desalination chambers, and two concentrated chambers (Figure 1). Both the anode and biocathode chambers are made of polyvinyl chloride (PVC), and possess the same dimension of 15 × 45 × 6 cm (empty bed volume of 4050 mL). The thickness for each desalination and concentrated chambers are 10 mm and 5 mm, respectively. Therefore, their empty bed volumes are 675 and 338 mL (total volume are 2025 and 675 mL) respectively. The anode, biocathode and desalination cells are separated by alternating anion-exchange membranes (AEMs) (1.8 mol/kg, Shanghua, China) and cation-exchange membranes (CEMs) (2.0 mol/kg, Shanghua, China). For each desalination chamber, the AEM is set adjacent to anode and the CEM is set on the other side close to biocathode. Both membranes possess a sectional area of 675 cm2. O-rings are inserted between membranes and chambers to prevent water leakage. All the cells are clamped together by stainless steel blots with steel plates pressed at both end of the reactor (Figure 1A). Both the anode and biocathode chambers were filled with activated carbon granules (∼2 mm in diameter and ∼5 mm in length, Weishimei Environmental Technology Co., Ltd., China) as electrode materials. The porosity of the packed bed was about 46%. To enhance the electron conduction from the activated carbon, two-sides electron collection mode was adopted in both anode and biocathode chambers. Titanium meshes (screen mesh size of 0.8 cm × 0.8 cm) were used as inner collecting material on the side adjacent to the stacked desalination cells. Titanium plates were set on the other side of the two electrode chambers. The titanium mesh and plate on both sides of the chamber were connected as one anode or biocathode (Figure 1B). To decrease internal resistance, mixed ion-exchange resins (diameter of ∼0.60−0.75 mm) were packed in both 9918

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Figure 2. (A) Voltage and power density curves at the beginning and the end of a desalination cycle. (B) Conductivity changes of diluate and concentrate during a desalination cycle. (C) Instant current generation and current efficiency during a desalination cycle. (D) Conductivity changes of diluate and concentrate in open circuit control experiments with initial NaCl of 0.5 g/L (for both diluate and concentrate) and 0.02 g/L (for diluate) and 0.98 g/L (for concentrate).

chambers with two reservoirs of 2.4 L. The diluate and concentrate were replaced every 12 h to investigate the desalination performance. The anolyte and catholyte were replaced every 24 h to ensure a sufficient supply of nutrients and stable solution pH. The SR-MDC was operated at a constant resistance of 10 Ω. Analysis and Calculation. Cell voltage (U) across resistor and anode potential versus saturated calomel electrode (SCE, 0.242 V vs standard hydrogen electrode (SHE), Leici, China) were measured automatically every 5 min by a data acquisition system (DAQ2213, ADLINK, China). Current (I) was determined according to ohm’s law: I = U/Rex, where Rex is the external resistance. Power density (P) and internal resistance (Rin) were obtained from polarization curve and P was calculated based on the anode chamber volume (4 L). Ohmic resistance was measured at the beginning of each desalination cycle using electrochemical impedance spectroscopy (EIS, CHI 660D, Chenhua, China). As NaCl concentration was confirmed having a linear correlation with solution conductivity within the range tested in the study, the conductivities of diluate and concentrate in two reservoirs were measured in 5 min interval to evaluate salt concentration change during desalination. The percentage of salt which was removed from the desalination chamber or recovered in the concentrated chamber after one cycle (12 h) was evaluated by calculating desalination efficiency (DE) or recovery efficiency (RE) as follows: DE =

RE =

Ccon,i

(2)

where Cdil,i, Cdil,f, Ccon,i, and Ccon,f (mg/L) are the initial and final concentrations of diluate and concentrate in two salt reservoirs, respectively. Current efficiency (CE) was defined to evaluate the amount of current that had been used to remove (CEdil) or recover (CEcon) salt from desalination chamber to concentrated chamber. CEdil =

CEcon =

F ·(Cdil,i − Cdil,f ) ·Vdil,f M · ∫ I dt

(3)

F ·(Ccon,f − Ccon,i) ·Vcon,f M · ∫ I dt

(4)

where F is Faraday’s constant (96485 C/mol), Vdil,f and Vcon,f (mL) are the final volume of diluate and concentrate in the desalination chambers, concentrated chambers, and two reservoirs. M is the molecular weight of NaCl (58.5 g/mol). I (A) is current intensity. t (h) is operation time. Instant CE of diluate and concentrate were calculated with above equation in a time interval of 5 min with corresponding current and concentration changes of salt solution. The total desalination rate (TDR) was calculated as TDR =

Cdil,i − Cdil,f Cdil,i

Ccon,f − Ccon,i

(Cdil,i − Cdil,f ) ·Vdil,f t

(5)

The diluate production ratio (DPR) was introduced to evaluate the amount of nonfresh water used to desalinate unit

(1) 9919

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volume of salt water. DPR is expressed in a form of 1:Vt, where 1 stands for unit volume of desalinated water, and Vt (mL) stands for the total volume of nonfresh water, including anolyte, catholyte, and concentrate.

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RESULTS AND DISCUSSION SR-MDC Startup. After the inoculation, the voltage of SRMDC increased to 500 mV in three cycles of operation at 150

Figure 3. Influence of circulation flow rate on SR-MDC performance. (A) Desalination/recovery efficiency, and final diluate concentration (DE: desalination efficiency; RE: recovery efficiency). (B) Average current and current efficiency at each desalination condition.

Figure 4. Performance of the SR-MDC at different diluate/ concentrate volume ratios (L:L). (A) Conductivity changes of diluate and concentrate in one operation cycle; (B) Desalination/recovery efficiency and the final diluate concentration; (C) Average current and current efficiency in one operation cycle.

Ω, the anode potential also exhibited a decrease from −200 to −450 mV vs reference electrode (SI Figure S2A), the maximum power generation was 3.3 W/m3 and internal resistance was 11.2 Ω (SI Figure S2B). Then the SR-MDC was further operated 1 week at external resistance of 150, 100, 50, and 10 Ω to make a better acclimation of electrochemically active bacteria in anode and cathode chambers. After about one month of operation, the SR-MDC achieved a maximum power density of 33.1 W/m3 (SI Figure S2B), the maximum current also increased to 370.2 mA, and the internal resistance was only 1.3 Ω, which were comparable with the performance of similar sized MFC.20,21 In addition, the anode potential of SR-MDC only increased 16.1 mV (from −427.3 to −411.2 mV) when external resistance decreased from 1000 to 1 Ω during polarization curve plotting, indicating that the SR-MDC has finished startup period. SR-MDC Electricity Generation and Desalination Performance. After startup, the salt solutions of desalination and concentrated chambers were decreased from 10 to 0.5 g/L NaCl to simulate the secondary effluent from municipal WWTP, and they were still circulated at a flow rate of 80 mL/min. As shown in Figure 2A, the maximum power density

and current decreased to 11.8 W/m3 and 202.1 mA respectively, and internal resistance presented an increase to 3.2 Ω. However, the power production was still on a relative high level, despite that the solution salinity decreased 95% (from 10 to 0.5 g/L). Figure 2B revealed the conductivity evolution of diluate and concentrate during a desalination cycle. As the ions in desalination chamber were continuously migrated to concentrated chambers, the diluate conductivity presented a decline while that of concentrate increased gradually with operation time. During one desalination cycle, the desalination efficiency of diluate (DE) achieved 80% in 4 h, 90% in 6 h and 96.3% at the end of 12 h (Figure 2B). The final diluate concentration reached 42 μS/cm (∼0.02 g/L), which was comparable with the effluent quality of reverse osmosis.22 Meanwhile, the current generation demonstrated a decrease with the removal of salinity. As shown in Figure 2C, the current at the beginning was 101.5 mA, but it decreased to 35.0 mA in 6 h and 17.2 mA 9920

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Table 1. Comparison of Power Generation and Desalination Performance of Various MDC Systemsa chamber volume (ml)

chamber number

An:Des:Con:Cat

Des: Con

salt concentration (g/L) initial

4000:2000: 667:4000m

3:2

0.5

27:3: -: 27 21.2:14.2:7.1:7.1 21:14:7: 7

1:0 2:1 2:1

300:300: -: 600m

1:0

130:15: -: 130m 14:14: -: 14 60:9: -: 60 1187n: 350: -: -c 28:14: -: 14d

1:0 1:0 1:0 1:0 1:0 5:5

20 20 20 2 5 10 0.72 20 2.1b 30 20 35 35 35/14f 10 5.8 + 8.4h 9.3n 10 35 35 35 10 35 35

30:5.2:5.2:18e 30:4: 4:15 U‑1.2 25:17:-: 36 U‑0.8 140:60: -: 140g 1100:850:-:-i 23:10: -: 27j 49:39: -: 40 49:39: -: 40 49:39: -: 49 60:75: -:60k 160:18:18:53 24:4000: -: 24l

4:4 1:0 1:0 1:0 1:0 1:0 1:0 1:0 1:0 3:3 1:0

final 0.23 (in 0.12 (in 0.07 (in 0.03 (in 1.2 6 7n 1.5n 3.2n 4.2n 0.04 7.4n 0.1b,n 0.3n 13.2n 15.4n 5.0n 0.12n 4.83n 0.9n 3.06n 2.8 8.16 7.91 4.22n 25.9n 34.98n

2 h) 4 h) 6 h) 12 h)

DE (%) 53.4 75.3 85.2 93.4 94 ± 3 70.0 65 25 36 58 94.4n 63 ± 2 95 ± 1 99 34 ± 1 44 97.6 98.8 66 90.3n 69.4 92.0 76.7 77.4 57.8 26.0 ± 0.5 0.056n

TDR (mg/h)

current (mA)

CE (%)

332.2 232.0 174.3 95.4 2.3n 25.2 7.6n 9n 27n 42n 0.9n 3.5n 0.8n 106.9n 3.2n 92.4n 68.3n 33.9 7.0n 1.4n 151.2n 1.4n 2.8 2.3 2.2 6.0n 107.4n 2.9−3.3

77.7 65.7 56.8 41.0 3.0 max 6.4 max 1.9n 3.6 9.5 12.3 1.4 max 0.6 max 13.2 max 62.6 0.6 max 3- 5 2- 3 2.1n 2.2n max 0.7 max 74.4 2.5 max 3.1 max 2.4 max 3.2 max 4.6 36.5max 1.6 ± 0.1max

228.5 188.7 164.2 124.4 100 223 150−190 74.9 90.1 83.9 >29.8n >283.5n 28.2n 81 >266.5n 86 ± 5

>147.3n >75.8n 93.1n >25.4n >42.1n >44.5n >31.3n 60.0n 96 ± 7

DPR

cathode type

reference

1:0.9

Biocat

this study

1:100n 1:92.0n 1:4.5n 1:10.4n 1:10.4n 1:33.3n 1:5n 1:2n 1:13.3n 1:61.7n 1:3 1:4.5n 1:2n 1:12.2n 1:27.1n 1:4.7n 1:40n 1:5n 1:11.3 1:12.5 1:12.5 1:19.6n 1:5.8n 1:0.3n

Fe(CN)63− Pt air Pt air

ref8 ref9 ref13

Biocat

ref10

Fe(CN)63− Pt air Fe(CN)63− Pt air Pt air

ref28 ref11 ref29 ref12 ref14

Pt air

ref27

Pt air Pt air Pt air Pt air Fe(CN)63− biocat air Fe(CN)63− Fe(CN)63− Pt air Pt air

ref30 ref16 ref32 ref18 ref31 ref33 ref34 ref35 ref36

a

An:Des:Con:Cat: Anode, desalination, concentrated and cathode chambers in MDC; DE: Desalination efficiency; TDR: Total desalination rate; CE: Current efficiency; DPR: Diluate production ratio; Biocat: Biocathode. -: No corresponding chambers. bHardness as CaCO3. cTubular MDC. d Recirculation microbial desalination cell (rMDC). eFour MDCs connected in series. f35 g/L NaCl in desalination chamber and 14 g/L NaCl in concentrated chamber. gReal wastewater as anolyte. h5.8 g/L NaCl + 8.4 g/L NaHCO3 as salt influent. iJust the performance of tubular MDC in a Osmotic MFC coupled desalination system. jCapacitive adsorption. kOsmotic microbial desalination cell. lSpatially decoupled anode and cathode. U0.8, U-1.2: Applied voltage of 0.8 or 1.2 V. mMixed ion-exchange resin packed in desalination chamber; max: Maximum current. nData calculated or estimated according to figures and profiles presented in corresponding articles.

μS/cm) were simplified as pure NaCl. As two pairs of membranes were in contact with concentrate and diluate, theoretically the total junction potential could reach as high as 426 mV in the SR-MDC, which to some extent, explained the sharp decline of open circuit voltage at the end of desalination cycle (Figure 2A). As there are three desalination chambers and two concentrated chambers in the SR-MDC (Figure 1B), the theoretical current efficiency in terms of diluate (CEdil) and concentrate (CEcon) are 300% and 200% respectively. In this experiment, the instant CEdil at the beginning of the cycle was ∼250% (Figure 2C), which was very close to the theoretical value of 300%. However, as time went by, the CEdil decreased sharply to 91%, respectively. However, the CEcon (red column) decreased with the decline of concentrate volume. At the concentrate volume of 2.4 L, the CEcon and CEdil were 186.0% and 124.5%, respectively, indicating that more salt was recovered in concentrate than that removed from the diluate. In fact, salt permeation (0.53 g) from two electrode chambers to desalination chambers was observed at this concentrate volume (SI Figure S5), because the concentrations of anolyte and catholyte were higher than that in the diluate. However, when concentrate volume decreased to 0.1 L, the CEcon decreased to 112. 8% and was lower than CEdil (125.0%). The total salt of diluate and concentrate decreased 0.23 g at the end of a desalination cycle, indicating that 0.23 g of NaCl was

After one cycle of desalination, the volume of concentrate kept almost unchanged but a decrease of 0.27 L in diluate was observed, indicating that water was transferred from desalination chamber to electrode chamber under osmotic pressure. The water osmosis caused 11.3% loss of diluate production and 12.2% loss of current efficiency according to eq 3. Moreover, as ion species and concentrations in anolyte (10.2 mS/cm) and catholyte (10.8 mS/cm) were much higher than those in diluate (1.08 mS/cm) and concentrate (1.08 mS/cm), salt permeation from electrode chamber to middle desalination stack was also observed. As shown in Figure 2B, the final conductivity of concentrate was 2590 μS/cm, and its pH was 4.1 ± 0.4. Ion chromatography showed that it mainly contained Na+ (457.4 mg/L), K+ (47.8 mg/L) and Cl− (865.4 mg/L), which was much higher than the theoretical maximum concentration (1 g/L NaCl, 2050 μS/cm) when all salt was removed from desalination chamber to concentrated chamber. The fluctuation in pH was potentially linked to water hydrolysis, as the ion mobility in electric field and permeability in membrane were various for multi-ions in desalination stack and electrode chambers.23 As the salt in electrode chambers first permeated to desalination chamber under concentration difference and then migrated to concentrated chamber driven by electric field, it would decrease CEdil (or increase CEcon), which explained the phenomenon that the CEcon was much higher than CEdil after 2 h in Figure 2C. To further confirm the salt permeation without current generation, two groups of open circuit control experiments were conducted with initial diluate concentration of 0.5 and 0.02 g/L, correspondingly the initial concentrate concentration were 0.5 and 0.98 g/L. As shown in Figure 2D, the conductivity of concentrate at both concentrations kept almost stable but that of diluate increased gradually with operation time. After 12 h of operation, the diluate conductivity increased from 1076 μS/cm (0.5 g/L NaCl) to 1233 μS/cm and from 42 μS/cm (0.02 g/L) to 417 μS/cm, respectively. Similar ion compositions were detected in the final diluate of 417 μS/ cm: Na+ (56.1 mg/L), K+ (32.7 mg/L), and Cl− (115.4 mg/L), indicating that except salt ions like Na+ and Cl−, other ion species would also permeate under concentration difference. Impact of Flow Rate on SR-MDC Performance. To evaluate the influence of circulation flow rate on SR-MDC performance, the SR-MDC was intermittently operated at circulation flow rates of 3.3, 10.0, 20.0, 40.0, and 80.0 mL/min (one cycle for each condition). As shown in Figure 3A, the desalination/recovery efficiency increased with circulation flow rate. At circulation flow rate of 3.3 mL/min, the desalination efficiency was only 49.8%, but it increased to 86.6% at 10 mL/ min and finally to 96.3% at 80 mL/min. The average current during a desalination cycle also presented an increase from 29.7 to 41.2 mA (Figure 3B), indicating that high circulation flow rate promoted the desalination performance and electricity generation. As reported in previous reverse electrodialysis systems,25 additional mixing by adding spacer or increasing flow rate could reduce diffusive boundary layer, which would decrease internal resistance and improve power generation. As presented in SI Figure S3, the ohmic resistance of the SRMDC was 3.57 Ω at 3.3 mL/min, but decreased to 2.8 Ω when flow rate increased to 80 mL/min. The average current efficiency during each desalination cycle also increased with flow rate (Figure 3B). At 3.3 mL/min, the CEdil was 93.7%, but it increased to ∼130% when circulation flow rate increased to >10 mL/min, due to the stacked 9922

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desalination chamber, wastewater treatment in anode chamber, power generation, and stability during long-term operation. As to the further up-scaling, enlarging the width and height or increasing stack number of desalination/concentrated chambers may be considered. However, further optimization on reactor design and configuration is still needed based on commercial membrane size, hydraulic condition and internal resistance.

removed from diluate to two electrode chambers under electric field (SI Figure S5). Therefore, a conclusion could be drawn that the change of diluate/concentrate volume ratio influenced the salt migration direction of two electrode chambers. With a low diluate/ concentrate volume ratio (high concentrate volume), the anode and cathode chambers functioned as “salt donors” with salt migrating to the desalination chambers, then being further removed to concentrated chambers. However, at a high diluate/ concentrate volume ratio (low concentrate volume), the two electrode chambers functioned as “salt acceptors” which recovered salt form desalination chambers under an electric field, as the salt could not further migrate to the concentrated chamber with very high concentration. Comparison with Previous Studies. Table 1 summarized the power generation and desalination performance of various MDC systems. In previous studies, no-resin-packed MDC succeeded in reaching a desalination efficiency of >90% with NaCl ranging from 10 to 35 g/L. However, the desalination of low-salinity water (10 L to date, and it achieved excellent power generation and desalination performance, suggesting a promising application of the SR-MDC for low salinity water treatment. Nevertheless, several challenges still remain to be overcome in the further experiments. For instance, the high concentration brine water needs further disposal. The organic matter and bacteria can not be effectively removed in SR-MDC diluate. Moreover, the introduction of organic matter may cause fouling of membrane and mixed ion exchange resins. To make this device more applicable, future work will utilize real wastewater and secondary effluent as anolyte and salt influent, and the performance of SR-MDC will be systematically evaluated in terms of desalination, organic matter and bacteria rejection in

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ASSOCIATED CONTENT

S Supporting Information *

Fabrication of SR-MDC (Figure S1), voltage and anode potential evolution during start-up period (Figure S2A), voltage and power density curves after one-week and five-weeks of acclimation (Figure S2B), ohmic resistance of SR-MDC at the beginning of desalination cycle at each circulation flow rate (Figure S3), salt permeation and volume change of diluate and concentrate at each circulation flow rate (Figure S4), and salt permeation and volume change of diluate and concentrate at each diluate/concentration volume ratio (Figure S5). This material is available free of charge via the Internet at http:// pubs.acs.org.”

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AUTHOR INFORMATION

Corresponding Authors

*Phone: (+86-10) 62796790; e-mail: [email protected]. cn. *Phone: (+86-10) 62772324; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the Key Program of the National Natural Science Foundation of China (No. 51238004), Tsinghua University Initiative Scientific Research Program (No.20121087922) and Program for Changjiang Scholars and Innovative Research Team in University.

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REFERENCES

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dx.doi.org/10.1021/es502075r | Environ. Sci. Technol. 2014, 48, 9917−9924