Electrodeionization (EDI) is the most common method to produce high purity water used for boiler feed water, microelectronic, and pharmaceutical industries. Commonly, EDI is combined with reverse osmosis (RO) to meet the requirement of EDI feed water, with hardness less than 1 ppm. However, RO requires a relatively high operating pressure and ultrafiltration (UF) as pretreatment which results in high energy consumption and high complexity in piping and instrumentation. In this work, UF was used as the sole pretreatment of EDI to produce high purity water. Tap water with conductivity 248 μS/cm was fed to UF-EDI system. The UF-EDI system showed good performance with ion removal more than 99.4% and produced water with low conductivity from 0.2 to 1 μS/cm and total organic compounds less than 0.3 ppm. Generally, product conductivity decreased with the increase of current density of EDI and the decrease of feed velocity and UF pressure. The energy consumption for UF-EDI system in this work was 0.89–2.36 kWh/m3. These results proved that UF-EDI system meets the standards of high purity water for pharmaceutical and boiler feed water with lower investment and energy consumption than RO-EDI system.

INTRODUCTION

High purity water is greatly important, especially for pharmaceutical and boiler feed water. Based on US and European pharmaceutical regulations, pharmaceutical industries require water with conductivity <1.3 μS/cm, total organic carbon (TOC) < 0.5 ppm as C, heavy metal <0.1 ppm as Pb, and aerobic bacteria <100 CFU/mL (Wang et al. 2000; Harfst 1993; Bennett 2009), while boiler feed water demands water with conductivity <1.1 μS/cm, TOC < 0.5 ppm as C, and silica <1 ppm (Scott 1995; Singh 2009). The conductivity of high purity water is less than 1 μS/cm at 25°C, with low quantities of TOC and total dissolved solids (TDS) (Bennett 2009; Bohus et al. 2010). Traditionally, ion-exchange system is used to produce high purity water in industry, but nowadays membrane processes are becoming popular as replacements for ion-exchange systems (Hernon et al. 1994). One of the membrane processes often used for high purity water production is electrodeionization (EDI) (Strathmann 2010).

EDI has been applied for high purity water production at a large industrial scale (Khoiruddin et al. 2014b). EDI combines ion-exchange resins and ion-selective membranes with direct current to remove ionized species from water (Hernon et al. 1994; Strathmann 2004; Alvarado et al. 2009; Lee & Choi 2012). It was developed to overcome the limitations of ion-exchange system, which needs regeneration of the resins and low quality product of electrodialysis (Bouhidel & Lakehal 2006; Nagarale et al. 2006; Wardani et al. 2017). Compared to conventional ion-exchange system, EDI has the advantage of being a continuous process with stable product quality, which is able to produce high purity water without the need for acid or caustic regeneration (Helfferich 1962; Hernon et al. 1994; Lee et al. 2007). EDI is the most common method for producing high purity water that typically achieve more than 99.5% salt rejection with a resistivity of 1–18 MΩ cm and low quantities of TOC (Franken 1999; Grabowski et al. 2006; Wood et al. 2010).

An EDI device contains alternating permselective anion-exchange membranes and cation-exchange membranes between two electrodes (Wood et al. 2010; Arar et al. 2014). The compartments in EDI stack consist of diluate or product compartments, concentrate compartments and electrode compartments. The compartments are filled with mixed-bed ion-exchange resins, which enhance the transport of ionic components from bulk solution toward the ion-exchange membranes under the force of a direct current (Helfferich 1962; Widiasa et al. 2004). When EDI is used for the production of high purity water, the ion-exchange resin beads enhance mass transfer, facilitate water splitting, and reduce stack resistance (Strathmann 2004). The direct current electrical field splits water into hydrogen and hydroxyl ions, which in turn continuously regenerate the ion-exchange resins. The exchanged ions are transferred through the membranes to the concentrate compartments and flushed from the system (Widiasa et al. 2004; Yeon et al. 2004).

The quality of product water obtained by EDI process depends much on the characteristics of feed. It is usually required in present EDI technology that the hardness of feed water should be less than 1.0 ppm (as CaCO3) (Fu et al. 2009). Therefore, reverse osmosis (RO) is often compelled to be adopted for pretreatment of the EDI (Liang et al. 1992; Auerswald 1994; Wang et al. 2000; Song et al. 2005; Arar et al. 2013; Wenten & Khoiruddin 2016). Generally, RO needs ultrafiltration (UF) as pretreatment to filter out particles that may otherwise clog or damage the RO membrane. RO also requires a relatively high operating pressure, which results in high energy consumption and high complexity in piping and instrumentation (Liberman 2004; Fritzmann et al. 2007; Lee et al. 2011; Kucera 2015). This work aims to use UF as the sole pretreatment of EDI for high purity water production by varying UF pressure, EDI feed velocity, and current density. UF membrane was used to replace RO membrane due to its low operating pressure, less than 2 bar. UF can remove organic compounds and produce crystal clear water too (Aryanti et al. 2015, 2016). In addition, the energy consumption and investment cost of UF-EDI system is much lower compared to RO-EDI system.

EXPERIMENTAL

Experimental set-up

UF-EDI system was used in this work to produce high purity water from tap water (Table 1). Polysulfone capillary UF membrane (GDP Filter, Indonesia) with pore size ±10 nm and effective area 2.51 m2 was used as pretreatment to produce EDI feed water. This pretreatment aimed to remove hardness and organic compounds before fed to EDI module. Meanwhile, the EDI stack consists of one diluate compartment, two concentrate compartments, and two electrode compartments (one anode and one cathode) (see Figure 1). Electrodes used in this work were stainless steel SS-304. Cation-exchange membrane (MC-3470) and anion-exchange membrane (MA-3475) from Ionac Chemical Company (USA) were used as ionic selective barriers of the EDI stack. Properties of the membranes have been explained in the literature (Khoiruddin et al. 2014a). Each membrane had effective area of 200 cm2 with the internal spacer for each concentrate and electrode compartments was 4 mm and for diluate compartment was 8 mm. Mixed ion-exchange resins with volume ratio 1:1 were filled to the diluate and concentrate compartments. The main characteristics of the ion-exchange resins used in this work are presented in Table 2. An adjustable power supply (homemade power supply) was used to produce direct current on EDI. It could supply voltage and direct current in the range of 0–100 V and 0–50 A, respectively. TDS and electrical conductivity of diluate and concentrate was measured every 10 minutes for 3 hours.
Table 1

Quality of feed water

 Feed water
TDS (ppm) 136 ± 0.8 
Conductivity (μS/cm) 248 ± 1.0 
Hardness (ppm) 6.1 ± 0.2 
TOC (ppm) 4.5 ± 0.4 
pH 7.1 ± 0.2 
 Feed water
TDS (ppm) 136 ± 0.8 
Conductivity (μS/cm) 248 ± 1.0 
Hardness (ppm) 6.1 ± 0.2 
TOC (ppm) 4.5 ± 0.4 
pH 7.1 ± 0.2 
Table 2

Properties of ion-exchange resin (Dow Chemical Company)

 Amberlite™ IR120-NaAmberlite™ IRA900-Cl
Type Strong acid Strong base 
Matrix structure Styrene divinylbenzene Styrene divinylbenzene 
Function group Sulfonate Trimethyl ammonium 
Ion-exchange capacity ≥2.00 eq./L ≥1.00 eq./L 
Moisture holding capacity 45–50% 58–64% 
 Amberlite™ IR120-NaAmberlite™ IRA900-Cl
Type Strong acid Strong base 
Matrix structure Styrene divinylbenzene Styrene divinylbenzene 
Function group Sulfonate Trimethyl ammonium 
Ion-exchange capacity ≥2.00 eq./L ≥1.00 eq./L 
Moisture holding capacity 45–50% 58–64% 
Figure 1

Schematic diagram of UF-EDI system (CM: cation-exchange membrane and AM: anion-exchange membrane).

Figure 1

Schematic diagram of UF-EDI system (CM: cation-exchange membrane and AM: anion-exchange membrane).

Analytical method

Total organic compounds and hardness

Organic compounds of feed and product water were measured as TOC by TOC meter (Shimadzu TOC-VCPH, Mandel, Canada). The measurement was conducted at room temperature. Meanwhile, spectrophotometer UV-Vis (Spectronic 20D, ThermoFisher Scientific, USA) was used for measuring the hardness with Eriochrome Black T (EBT) solution as indicator. EBT solution was prepared by dissolving 50 mg of EBT powder into 50 mL of ethanol and placed in a dark and cool place. 10 mL of EBT solution was then diluted into 100 mL ethanol and named as EBT work solution. Optimum reaction between EBT and hardness ion occurred at pH 8–10 (Tachino et al. 2001), so buffer solution was prepared by dissolving 1 g of NH4Cl into 100 mL of NH4OH 12.50% solution. 2 mL EBT work solution and 2 mL buffer were added to 2 mL test solution, and diluted with deionized water until 10 mL, then put in spectrophotometer cuvette. Wavelength 530 nm was used to measure the absorbance value from each solution.

Electrical conductivity and TDS

One of the important characteristics of high purity water is the electrical conductivity. Electrical conductivity was measured using conductivity meter (HI-98303, Hanna Instruments, Mauritius). Meanwhile, ion concentration was measured as TDS using TDS meter (TDS-3, HM Digital, Taiwan). Ion removal was calculated to show the decrease of ions in the diluate compartment using the following equation (Mulder 1996; Lu et al. 2015):
formula
1
where R (%) is the removal of each ion, Cin (ppm) is the feed concentration, and Cout (ppm) is the product concentration.

RESULTS AND DISCUSSION

Determination of optimal feed conditions for EDI using UF membrane

UF membrane was used to remove hardness and organic compounds as a pretreatment of EDI system to produce high purity water. Hardness was measured as concentration of CaCO3, while organic compounds were measured as TOC. In this work, some of hardness compounds as CaCO3 with molecule size 30–60 nm (Jia et al. 2003) were rejected by UF membrane. UF rejection is determined mainly by the size and shape of solutes relative to the pore size in the membrane (Mulder 1996). The pore size of UF membrane used in this work was ±10 nm. Therefore, the molecules with size more than 10 nm were rejected by UF membrane. Hardness removal was also mentioned in the previous work (Tabatabai et al. 1995; Mika et al. 1999), where UF could remove more than 80% of hardness due to high physical–chemical interaction between hardness compounds and membrane surfaces. The other work also showed that UF can remove biodegradable organic compounds from feed solution, while the synthetic organic compounds can hardly be removed (Metcalf 2003).

Figure 2 shows the hardness and TOC removal of UF permeates with pressure and feed velocity. When the pressure was increased, the amount of hardness and TOC in UF permeate also increased. Theoretically, when the pressure is too low, it will be difficult to push the hardness ions and organic compounds through the membrane pores. Thus, the components drift to the UF retentate. Meanwhile, the amount of hardness and TOC decreased when the feed velocity was increased because contact time between feed solution and membrane surface decreased. The effect of velocity is important in the membrane filtration process. A higher velocity can reduce membrane fouling by providing a shear force to sweep away deposited materials (Chen et al. 1997). This can slightly increase the retention of most components (Wei et al. 2008).
Figure 2

Effect of the UF pressure and feed velocity on the hardness and TOC of permeate.

Figure 2

Effect of the UF pressure and feed velocity on the hardness and TOC of permeate.

Based on the results, the product with pressure up to 25 psi meets the standards of EDI feed water, with hardness less than 1 ppm. It implies that UF system was effective as pretreatment to produce EDI feed water. Furthermore, the optimal operating conditions were determined to be 1.25 m/s of feed velocity and 15 psi of UF pressure. These conditions gave the minimum value of hardness and TOC. Under these conditions, the permeate water quality was observed to be 0.921 ppm of the hardness and 0.659 ppm of the TOC.

Voltage–current density characteristics

The voltage–current density curves of EDI with different feed water are shown in Figure 3. The current density increased more rapidly than the voltage at higher voltage for UF permeate and lower voltage for brackish water. The reason is that even at low voltage, a significant amount of H+ and OH ions are produced in the diluate compartment (Wang et al. 2000). Since more H+ and OH are produced at higher voltage, the resistance of the stack is decreased due to the higher conductivity of resin in H+ and OH form (Fu et al. 2009; Xing et al. 2009). Therefore, water with higher conductivity (brackish water) has a steeper curve due to its higher ion concentration.
Figure 3

Variation of current density with voltage in EDI process.

Figure 3

Variation of current density with voltage in EDI process.

Similar to that reported by Song et al. (2004), the voltage–current density curve for this work could fall into two segments, as follows:

  • First segment (0–50 V), the current density increased linearly as the voltage increased. The voltage–current density curve in this section follows Ohm's law, where V = IR.

  • Second segment (50–80 V), the current density increased linearly as the voltage increased like the first segment but more quickly. In this segment, water dissociation took place and a significant amount of H+ and OH ions were produced. Consequently, more charge carriers are present and thus the current density is increased.

From voltage–current density curve, limiting current density can be determined. Limiting current density is the cross-point of the tangents drawn from first segment and second segment (Doyen et al. 2014), and it is about 22.5 A/m2 according to Figure 3. In this work, the current density of 17.5 A/m2 (below the limiting current density), 22.5 A/m2 (limiting current density), and 27.5 A/m2 (above the limiting current density) were chosen to study the characteristics of ionic migration in different segments.

Effect of current density on quality of product water

In this work, the UF permeate with conductivity 237 μS/cm was used as a feed solution. The EDI stack was operated with a constant feed velocity of 0.75 m/s. As shown in Figure 4(a), conductivity of diluate decreased with the increase of the current density. When the current density was increased from 17.5 A/m2 to 27.5 A/m2, the electrical conductivity of diluate decreased from 1 μS/cm (at 17.5 A/m2) to 0.3 μS/cm (at 27.5 A/m2). These results show that high purity water for pharmaceutical and boiler feed water can be obtained by using current density from 17.5 A/m2 up to 27.5 A/m2.
Figure 4

Electrical conductivity of diluate and ion removal as a function of (a) current density and (b) feed velocity.

Figure 4

Electrical conductivity of diluate and ion removal as a function of (a) current density and (b) feed velocity.

The increase in current density also led to the increase in ion removal. At the end of the experiment of 180 minutes, the ion removal was 99.47%, 99.62%, and 99.92% for current density variation of 17.5, 22.5, and 27.5 A/m2, respectively. Theoretically, when the current density is too low, it will be difficult to maintain a desired removal of the ions due to a lower strength of driving force (Arar et al. 2013). When the current density is raised, more electric potential is used for the transport of ions. Therefore, the conductivity of water decreased and ion removal increased, which was in agreement with the previous works (Meyer et al. 2005; Lu et al. 2010; Arar et al. 2011, 2013). At the limiting current density and above, water splitting occurs in the diluate compartment. At this condition, H+ and OH are formed by in situ water dissociation to regenerate the ion-exchange bed continuously (Zhang et al. 2014). When 100% and approximately 120% of the limiting current density (22.5 and 27.5 A/m2) were applied, the number of the generated H+ and OH ions from the water dissociation could be too high. Therefore, they not only helped to regenerate the ion-exchange resins, but also reduced stack resistance and participated in the ion transport through membranes.

Effect of EDI feed velocity on quality of product water

It was found that product conductivity and ion removal depended not only on the current density, but also on the feed velocity. In this work, the velocity of the UF permeate was varied from 0.5 to 1 m/s with a constant current density of 22.5 A/m2. When the feed velocity was increased, conductivity of the diluate increased and conductivity of the concentrate decreased. After 180 minutes, the electrical conductivity of diluate was 0.4, 0.7, and 0.9 μS/cm for feed velocity of 0.5, 0.75, and 1 m/s, respectively (Figure 4(b)). Increasing feed velocity also affected the ion removal. When the velocity was increased from 0.5 m/s to 1 m/s, the ion removal decreased from 99.77% (at 0.5 m/s) to 99.55% (at 1 m/s). When the velocity was increased, the residence time of the solution in the resin bed decreased. Thus, the diffusion kinetics of the ions from the solution to the ion-exchange resin declined. This led to the decrease in the transport of ions to the concentrate compartments (Wen et al. 2005; Xing et al. 2009; Arar et al. 2013).

All variations of feed velocity in this work produce high purity water suitable for pharmaceutical and boiler feed water. However, it is important to look for the optimum feed velocity since feed velocity is related to the product capacity. To achieve same product capacity, using higher feed velocity is more profitable due to fewer numbers of modules needed, which leads to a reduction in investment cost.

UF-EDI product characteristics and energy consumption

The UF-EDI system showed good performance in producing high purity water. As shown in Table 3, water product has low conductivity from 0.2 to 1 μS/cm with TOC less than 0.3 ppm. These results showed that UF-EDI product for each operating condition meets the requirements of pharmaceutical and boiler feed water, with conductivity <1.1 μS/cm and TOC < 0.5 ppm (Scott 1995; Singh 2009).

Table 3

Product quality and energy consumption of UF-EDI system

 Current density (A/m2)EDI feed velocity (m/s)Ion removal (%)Conductivity (μS/cm)TOC (ppm)Total energy (kWh/m3)
Feed water – – – 248 2.452 – 
Product 17.5 0.75 99.47 1.0 0.296 0.89 
22.5 0.75 99.62 0.7 0.274 1.45 
27.5 0.75 99.92 0.3 0.263 2.23 
27.5 0.5 99.94 0.2 0.248 2.36 
22.5 0.5 99.77 0.4 0.255 2.12 
22.5 0.75 99.62 0.7 0.274 1.45 
22.5 99.55 0.9 0.281 1.12 
 Current density (A/m2)EDI feed velocity (m/s)Ion removal (%)Conductivity (μS/cm)TOC (ppm)Total energy (kWh/m3)
Feed water – – – 248 2.452 – 
Product 17.5 0.75 99.47 1.0 0.296 0.89 
22.5 0.75 99.62 0.7 0.274 1.45 
27.5 0.75 99.92 0.3 0.263 2.23 
27.5 0.5 99.94 0.2 0.248 2.36 
22.5 0.5 99.77 0.4 0.255 2.12 
22.5 0.75 99.62 0.7 0.274 1.45 
22.5 99.55 0.9 0.281 1.12 

The optimum operating parameters including UF pressure, current density, and feed velocity were obtained according to the experimental results. In order to give an economic evaluation of the UF-EDI system in this work, a set of representative operating parameters was taken as follows: the UF pressure was 15 psi with feed velocity 1.25 m/s, the current density of EDI was 27.5 A/m2 and EDI feed velocity was 0.5 m/s. The energy consumption for UF process was evaluated by Equation (2) (Mulder 1996):
formula
2
where EUF is energy consumption of UF (kWh/m3), Q0 is UF feed flow rate (m3/h), P is UF pressure (bar), and η is pump efficiency. Meanwhile, the energy consumption for EDI process was calculated using the following equation (Zuo et al. 2008; Lu et al. 2015):
formula
3
where EEDI is energy consumption of EDI (kWh/m3), I is electrical current (ampere), V is voltage (volt), t is operating time (h), and L is water volume (m3). At the optimum conditions, the total energy consumption was 2.36 kWh/m3.

Table 4 shows comparison of UF-EDI system and RO-EDI system from previous works. UF-EDI system had a performance as good as RO-EDI system, but with lower operating pressure. If RO-EDI was operated with same EDI module for UF-EDI system, energy consumption became much higher since operating pressure for RO is 20–30 times operating pressure for UF.

Table 4

Comparison between RO-EDI and UF-EDI system

Operating conditions
 
RO/UFEDI feed flowEDIEDI currentIonConductivity (μS/cm)
Systempressure (psi)rate (m/s)voltage (V)density (A/m2)removalFeedProductReference
RO-EDI – – – – >98% – – Liang et al. (1992)  
– – – – >98% – – Auerswold (1996)  
– – – – >99% 3.5–4.5 0.05–0.06 Prato & Gallagher (2000)  
– 5.5 20 – >99% 14–18 0.005–0.01 Wang et al. (2000)  
40–200 2–20 – 2–30 >99% 50–250 0.006–0.01 Song et al. (2005)  
180 0.11–0.28 20–40 – >99% 1,685 1.1–5.9 Arar et al. (2013)  
112 0.3 30–70 – >99% 40–60 0.3–0.4 Wenten et al. (2013)  
UF-EDI 10–30 10–20 40–60 17.5–27.5 >99% 248 0.2–1 This work 
Operating conditions
 
RO/UFEDI feed flowEDIEDI currentIonConductivity (μS/cm)
Systempressure (psi)rate (m/s)voltage (V)density (A/m2)removalFeedProductReference
RO-EDI – – – – >98% – – Liang et al. (1992)  
– – – – >98% – – Auerswold (1996)  
– – – – >99% 3.5–4.5 0.05–0.06 Prato & Gallagher (2000)  
– 5.5 20 – >99% 14–18 0.005–0.01 Wang et al. (2000)  
40–200 2–20 – 2–30 >99% 50–250 0.006–0.01 Song et al. (2005)  
180 0.11–0.28 20–40 – >99% 1,685 1.1–5.9 Arar et al. (2013)  
112 0.3 30–70 – >99% 40–60 0.3–0.4 Wenten et al. (2013)  
UF-EDI 10–30 10–20 40–60 17.5–27.5 >99% 248 0.2–1 This work 

Generally, high operating pressure of RO not only leads to high energy consumption, but also high complexity in piping and instrumentation (Liberman 2004; Fritzmann et al. 2007; Lee et al. 2011; Kucera 2015). The materials of piping and instrumentation for RO must be able to withstand high pressure condition. It is usually necessary to use metals for the high pressure system (FILMTEC). This high complexity of RO leads to the increase of investment cost. Comparison of investment cost for RO-EDI and UF-EDI system is presented in Table 5. In general, the investment cost for the RO-EDI system is 2–4 times that for the UF-EDI system.

Table 5

Comparison of investment cost of RO-EDI and UF-EDI system

SystemCapacity (m3/hr)Investment cost (US$)Reference
RO-EDI 58.14 925,608 Matzan et al. (2001)  
120 1,120,000 Wenten et al. (2013)  
UF-EDI 120 515,400 Estimated by GDP Filter 
SystemCapacity (m3/hr)Investment cost (US$)Reference
RO-EDI 58.14 925,608 Matzan et al. (2001)  
120 1,120,000 Wenten et al. (2013)  
UF-EDI 120 515,400 Estimated by GDP Filter 

CONCLUSION

This work used UF-EDI system as an alternative process for high purity water production. Such operating parameters as UF pressure, current density of EDI, and feed velocity were investigated in detail. The UF-EDI system showed good performance in producing high purity water, with product conductivity from 0.2 to 1 μS/cm and TOC less than 0.3 ppm. Ion removal of this system was more than 99.4%. Generally, product conductivity decreased with the increase of current density of EDI and decrease of feed velocity and UF pressure. The optimum operating parameters for UF were obtained at pressure 15 psi and feed velocity 1.25 m/s. Meanwhile, current density 27.5 A/m2 and feed velocity 0.5 m/s were the optimum operating parameters for EDI. The energy consumptions for UF-EDI system in this work are 0.89–2.36 kWh/m3. These results proved that UF-EDI system meets the standards of high purity water for pharmaceutical and boiler feed water with lower investment and energy consumption than RO-EDI system.

ACKNOWLEDGEMENTS

Financial assistance for this work has been provided by Lembaga Pengelola Dana Pendidikan (LPDP) Indonesia through Beasiswa Pendidikan Indonesia (BPI) and Program Penelitian, Pengabdian kepada Masyarakat, dan Inovasi (P3MI) Institut Teknologi Bandung. The authors would also like to thank GDP Filter Indonesia for the supporting data.

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