Integrated Membrane Processes for Separation and Purification of

Article pubs.acs.org/IECR

Integrated Membrane Processes for Separation and Purification of Organic Acid from a Biomass Fermentation Process Young Hoon Cho, Hee Dae Lee, and Ho Bum Park* WCU Department of Energy Engineering, Hanyang University, Seoul 133-791, Korea ABSTRACT: During biomass (e.g., waste wood chips) decomposition under an anaerobic fermentation process, organic acids such as acetic acid and butyric acid are continuously produced from controlled microbial activity. Since the accumulation of organic acids hinders the microbial metabolism in the fermentation broths, the organic acids should be removed by using appropriate separation processes. A few separation processes such as extraction, electrodialysis, and distillation have been reported, but they still have many limitations such as high energy input and environmental problems (e.g., toxic chemical effluents). The integrated membrane processes proposed here, including the three steps of (1) clarification of fermentation broth, (2) organic acid separation, and (3) dewatering, can be applied to achieve energy-efficient and environmentally friendly organic acid removal and recovery. First, microorganisms and large insoluble particles in fermentation feed can be mostly removed by clarification steps using microfiltration or ultrafiltration processes. In this study, we focused only on organic acid separation and dewatering processes using nanofiltration and forward osmosis membrane processes. Using nanofiltration (or high-flux reverse osmosis) membranes, aqueous organic acids can be selectively separated from pretreated fermentation feed solutions while other organics and many salts can be rejected using these processes by varying pH conditions in the feed. Finally, a low-energyconsuming forward osmosis process was applied for dewatering in the aqueous organic acid solutions to concentrate organic acid. The concentrated organic acid was successfully obtained by using conventional desalination and/or commercial forward osmosis membranes.

1. INTRODUCTION Biomass fermentation by microbial activity is a promising process to extract many valuable organic products from various biomass resources such as crops, animal residues, or even food waste and sewage.1,2 Typical examples are organic acids, alcohols, and methane fermentation processes, and for these purposes, specific microbes and their metabolic pathways can be used to produce desirable target products.2−5 One class of high value products from fermentation processes is organic acids such as citric acid, lactic acid, acetic acid, and butyric acid. They can be easily found in a living organism or its metabolites.6 These organic acids are used in important applications, including the chemical, pharmaceutical, food, and cosmetics industries.7 Moreover, organic acids (e.g., acetic acid and butyric acid) from a fermentation process were successfully converted into biofuels.8 Among them, butyric acid (BAc) is one of the useful products obtained from the anaerobic fermentation process using the acetogenesis of specialty microorganisms such as Clostridium acetobutyricum and Clostridium tyrobutyricum and various carbon sources such as glucose, xylose, sucrose, and other biomass as feeds for microorganisms.9−14 However, a high concentration of BAc in the fermentation feed often inhibits bacterial growth, resulting in lower productivity and selectivity of BAc.15 In general, fermentation broths consist of a complex mixture including various inorganic salts, organic carbons, acids, and other byproducts, and it is difficult to separate BAc continuously from fermentation broth while maintaining microbial activity. Thus, the separation process for BAc recovery is an important factor to achieve successful fermentation processes. © 2012 American Chemical Society

Several separation processes have been proposed to separate the target products from the fermentation broth. Not only conventional separation processes (e.g., distillation and extraction) but also membrane separation processes (e.g., membrane extraction,15 pervaporation,16 and electrodialysis17) have been studied for energy efficient recovery of organic products during fermentation. Membrane processes have several advantages such as process continuity and easy scaleup compared to conventional technologies. Membrane extraction, commonly considered in this application, is similar to liquid−liquid extraction in the way that they use organic solvents to extract organic acids from fermentation broth. Since many organic extractants are not biocompatible, i.e., toxic, and affect the microbial activity fatally, the direct contact of extractants and microorganisms should be prevented from contacting between two phases, i.e., fermentation broth and extractant. However, an additional extraction process for recovery of BAc from organic solvents using strong alkali solution as another extractant is also required to extract BAc from the solvent followed by energy intensive concentration and separation processes such as distillation. Furthermore, biochemical waste from mass fermentation−separation processes can cause harmful environmental effluent problems (e.g., discharge, posttreatment). On the other hand, pressure-driven membrane processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration Received: Revised: Accepted: Published: 10207

April 20, 2012 June 19, 2012 June 28, 2012 June 28, 2012 dx.doi.org/10.1021/ie301023r | Ind. Eng. Chem. Res. 2012, 51, 10207−10219

Industrial & Engineering Chemistry Research

Article

Figure 1. Illustration of integrated membrane processes for BAc separation and dewatering to selectively recover BAc from fermentation broth.

Figure 2. Schematic drawing of FO dewatering process to concentrate BAc solution.

fermentation processes. However, only a few studies on the recovery of organic acids such as lactic acid and succinic acid from fermentation broth were reported by using NF and or RO membranes.20,21,27,28 As such, the purpose of these studies was the retention of the organic acids or their salts (e.g., lactic acid, lactate, succinate) by NF or RO membranes.

(NF), and reverse osmosis (RO) have gained much attention in biochemical industry (e.g., pretreatment of fermentation broth or concentration of downstream) due to their simplicity, high selectivity, low energy cost, and reduced chemical usage.18−21 MF and UF membranes have pore sizes from a few nanometers to submicrometers. Large molecules such as proteins, viruses, and microorganisms in the biological environment can be selectively removed by these membranes via a size sieving mechanism.22,23 However, such microporous membranes cannot reject small molecules such as organic acids and salts, and thus RO or NF membranes should be used to separate them efficiently. NF and RO membranes have a dense, ultrathin skin layer on the microporous polymeric supports. In general, NF membranes have looser porous structures than RO membranes, but have more charged groups on the membrane surface. The salt rejection mechanism in NF membranes is the combination of size sieving, solution−diffusion, and Donnan exclusion.24 Numerous studies on the separation of small molecules such as dyes and organic acids, using NF membrane processes, have been reported.25−28 In general, fermentation broths contain many different inorganic solutes and organic species. A majority of the solutes are carbon sources such as glucose, sucrose, and other biomasses. Microorganisms can digest and then decompose them into various small organic species such as alcohols and organic acids that are small enough to pass through many NF membranes, while large undecomposed or other macromolecular components are selectively rejected by NF membranes. From this point of view, NF membranes can be considered as an efficient separator to permeate water and some organic acids in continuous

2. INTEGRATED MEMBRANE PROCESSES Here, we propose integrated membrane processes including pretreatment, organic acid separation, and dewatering membrane processes to selectively recover organic acid (e.g., BAc) from fermentation broth as illustrated in Figure 1. First, we applied NF or RO process to selectively recover organic acid and water from fermentation broth while other organic and inorganic solutes are retained. We anticipated that small organic acids can permeate to some extent through the NF (or RO) membranes by adjusting the pH in the feed stream. In general, the rejection ratio of organic acids with carboxylic acid groups is strongly dependent on the pH of the feed solution. At high pH, the membrane surface is negatively charged due to carboxylic acid groups on the polyamide membrane surface. BAc with a carboxylic acid group also experiences the same effect and changes to butyrate form. Thus, membranes tend to be more selective to anionic species including organic acids due to Donnan exclusion. Conversely, the membrane surface charge becomes positive at low pH since polyamide also has an amine group. Organic acid molecules can penetrate the positively charged membrane layer while neutral large organic molecules are retained. 10208

dx.doi.org/10.1021/ie301023r | Ind. Eng. Chem. Res. 2012, 51, 10207−10219


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Finally, we applied forward osmosis (FO) processes for dewatering of downstream (i.e., NF/RO permeate) since the additional concentration processes are needed to obtain a highly concentrated product. Dewatering by using FO membrane processes has been studied for desalination, wastewater treatment, and food processing.29−32 As compared to conventional distillation processes, FO processes have extremely low energy requirements to remove water from the mixture solution. The energy consumption to eliminate the water from the downstream is high in thermal distillation processes (25−120 kWh/m3),33 which lowers the economic feasibility. Basically, thermal or electrical energy is not necessary to dewater feed solution since the driving force of water transport in forward osmosis is just the chemical potential difference between feed and draw solutions. Moreover, the draw solution used in forward osmosis can have extremely high osmotic pressure, which is much higher than the hydraulic pressure used in reverse osmosis, depending on the concentration and kinds of draw agents such as common inorganic salts or other low molecular weight organics. The concentrated BAc can be obtained using an FO dewatering process shown in Figure 2. We first successfully demonstrated an FO membrane process for concentrating downstream in bioprocesses by using commercial desalination membranes.

Table 1. Composition of Various Fermentation Batches in BAc Production of Recent Studies and Composition of Prepared Fermentation Broth in This Study carbon source (g/L)

inorganic salts (g/L)

organics (g/L) ref

3. EXPERIMENTAL SECTION Preparation of Synthetic Fermentation Broth. Various carbon sources can be converted to organic acid through fermentation processes. A number of studies on fermentation process for organic acid production have been performed with glucose, xylose, sucrose, and other sources from waste biomass. In this study, we selected glucose as a model carbon source since it is one of the final products of biomass hydrolysis.34 A defined synthetic medium that has a limited carbon source for the growth of microorganisms was used to verify the feasibility of membrane separation processes for organic acid recovery. Table 1 lists the compositions of various fermentation broths commonly used in BAc production and the synthetic fermentation broth used in this study. To minimize undesirable microbial growth in the synthetic fermentation broth, the synthetic fermentation broth was stored in a sealed bath and used within a day. To evaluate individual rejection of each component, single solute solutions were also prepared. Asprepared synthetic fermentation broth and each single solute solution were analyzed by a total organic carbon analyzer (Multi N/C 3100, Analytik Jena, Jena, Germany) for organic species and an ion conductivity meter (inoLab 720, WTW, Weilheim, Germany) for inorganic species to calculate the permeate concentration and rejection ratio. Membrane Characterization. The surface and crosssectional morphologies of the NF and RO membranes were observed using a field-emission scanning electron microscope (thermal FE-SEM, JSM-700F, JEOL, Tokyo, Japan). All the samples were freeze-fractured in liquid nitrogen for the crosssectional observation. The samples were coated with platinum for 100 s to prevent electron charging. Overall membrane performances strongly depend on membrane surface properties in NF and RO membranes. In this study, commercially available membranes were used but the exact information of the membrane materials and compositions was unknown. Therefore, basic studies on membrane surface properties were performed. All the samples, for the surface analysis, were completely dried in a vacuum

glucose

30

glucose

60

glucose

50

glucose

15

glucose

10

MgSO4·7H2O FeSO4·7H2O (NH4)2SO4 K2HPO4 MgSO4·7H2O FeSO4·7H2O (NH4)2SO4 K2HPO4 MgSO4·7H2O FeSO4·7H2O (NH4)2SO4 KH2PO4 K2HPO4 MgSO4·7H2O MnSO4·7H2O FeSO4·7H2O NaCl (NH4)2SO4 KH2PO4 K2HPO4 MgSO4·7H2O MnSO4·7H2O FeSO4·7H2O (NH4)2SO4

0.6 0.03 3 1.5 0.6 0.03 3 1.5 0.6 0.03 3 0.75 0.75 0.4 0.01 0.01 1 2 0.5 0.5 0.2 0.01 0.01 1

yeast extract peptone

5 5

11

yeast extract peptone

5 5

12

yeast extract trypticase

5 5

16

yeast extract aasparagine

5 2

13

yeast extract

1

this work

oven at 50 °C overnight to eliminate water. A static contact angle analyzer was used to measure the surface wettability. The water volume was 0.01 mL per drop, and the tip-to-plate distance was fixed at 5 mm. Membrane surface charge was also observed using a streaming potentiometer (SurPASS electrokinetic analyzer for solid samples, Anton Paar, Graz-Straßgang, Austria) at 0.5 bar from pH 3 to 12 with 1 mM KCl solution. Zeta (ζ) potential calculation was based on the Fairbrother−Mastin approximation. NF and RO Filtration of Single Solute Solution. A deadend filtration experiment at different applied pressures was performed using a high pressure stirred cell (HP4750, Sterlitech, Kent, WA, USA) to measure water permeability. To pressurize the feed solution, nitrogen gas was directly used from a gas cylinder and the pressure was controlled from 100 to 500 psi. The stirring rate was nearly 800 rpm using a magnetic stirrer for every filtration experiment. Deionized water filtration was performed for 1 h to stabilize the membrane flux, and then pure water flux was measured by a computerized digital balance. Since the membrane performance (A and B values) varies with types of solutes, salt concentration, pH, and temperature, it is necessary to evaluate separation performance such as water flux and salt rejection of individual species in fermentation broth at different conditions before actual fermentation broth separation. The solute transport properties in each membrane were measured using a crossflow filtration experiment to minimize the concentration polarization effect and to maintain the feed solution compositions. Individual solute solution filtration experiments were performed in advance by using a crossflow filtration system. A crossflow membrane filtration cell 10209



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(CF042, Sterlitech, Kent, WA, USA) has 42 cm2 of effective membrane area, and the crossflow rate was fixed at 1.5 L/min (Re = 1100, laminar flow) for every filtration experiment. The operating temperature was held at 25 °C (standard filtration condition) and 37 °C (standard fermentation condition). After the 1 h stabilization step for each membrane, 20 L solution of individual solute in fermentation broth was introduced as feed solution. Permeate flux was continuously recorded by a computerized digital balance for 1 h, and the collected permeate was analyzed by the ion conductivity meter and a total carbon analyzer (TOC, Multi N/C 3100, Analytikjena, Jena, Germany). The pressure and pH of feed solutions were controlled to investigate the effect on membrane separation performance. Solutions of 1 M NaOH and 1 M HCl were used for adjusting the pH of feed solution. In NF and RO processes, hydraulic pressure is used as a driving force for water permeation. The solute flux (JS) and water flux (JW) in NF and RO can be described by the simple equations JW = A(Δp − Δπ )

(1)

JS = B(c f − c p)

(2)

a

mol wt (g/mol)

concn (g/L)

180 N/A 132 136 174 120a 151a 152a 88

10 1 1 0.5 0.5 0.2 0.01 0.01 0−5

⎛ Cb ⎞ p recovery ratio of BAc (%) = ⎜⎜ b ⎟⎟ · 100 ⎝ Cf ⎠

(4)

where is the total organic carbon concentration of permeate, Cip is the total organic carbon concentration of permeate without BAc, and Cbp and Cbf are the BAc concentrations in permeate and feed solutions, respectively. Forward Osmosis. Crossflow FO experiments were performed for concentrating BAc aqueous solution as shown in Figure 3. The fully hydrated membranes were mounted in

Figure 3. Scheme of crossflow FO experiments.

the customized FO cell which has a 4 × 10 × 0.5 cm3 channel on each side. The water and salt fluxes of commercial polyamide TFC membrane (XLE) and CTA-based FO membranes (Hydration Technology Innovations (HTI), Albany, OR, USA) provided from HTI were investigated. The CTA composite membrane (pouch type) is made of a cellulose triacetate (CTA) layer and a nonwoven backing layer, while the support embedded CTA membrane (cartridge type) has an embedded polyester mesh in the CTA layer. Both feed and draw solutions were introduced to the FO cell at the same crossflow rate (1.5 L/min) and pressure. For comparison, deionized water and magnesium chloride (MgCl2) solution of differing concentrations were used as a feed solution and a draw solution, respectively. To prevent dilution of draw solution due to water passage, draw solutions of 20 L were used for feed solutions of 1 L and the FO water and salt fluxes were measured by mass and conductivity changes in the feed solution bath. To investigate the feasibility of FO processes for BAc enrichment, 2 g/L BAc solution at pH 6 was introduced to the FO cell as a feed solution and 5 M MgCl2 solution was used as a draw solution. The BAc concentration in the feed solution were periodically obtained by TOC analysis. The concentration factor (CF) at time t was calculated by the equation

Table 2. General Information for the Solutes in a Model Fermentation Broth component

(3)

Ctp

where A is the water transport coefficient, B is the salt permeability coefficient, Δp is the applied pressure difference, Δπ is the osmotic pressure differential across the membrane, and cf and cp are the concentrations of feed and permeate, respectively. Crossflow NF/RO Filtration of Fermentation Broth. The separation of BAc from synthetic fermentation broth via NF and RO processes were examined using a continuous crossflow filtration. To set the filtration conditions similar to fermentation process conditions, the filtration experiment was performed at 37 °C. The operating conditions were fixed at the applied pressure, a crossflow rate of 1.5 L/min, and pH 3 (adjusted by 1 M HCl solution). The synthetic fermentation broth without BAc listed in Table 2 was filtrated for 1 h to

glucose yeast extract (NH4)2SO4 KH2PO4 K2HPO4 MgSO4·7H2O MnSO4·H2O FeSO4·7H2O butyric acid

⎛ Cpt − Cpi ⎞ ⎟ · 100 purity (%) = ⎜⎜1 − Cpt ⎟⎠ ⎝

Molecular weight of dehydrated form.

stabilize the membranes. Then, BAc was added continuously at the rate of 2 g/L·h (i.e., the BAc production rate in a real fermentation reaction). The membrane flux was observed simultaneously and the permeate samples were collected every 30 min of filtration. The collected samples were analyzed by the ion conductivity meter and TOC analyzer. The recovery ratio of BAc and its purity were calculated by the equations

concentration factor =

Ct C0

(5)

where Ct and C0 are the feed solution concentration at time t and the initial concentration, respectively. The permeate 10210



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Figure 4. Surface and cross-sectional morphologies of commercial NF and RO membranes: (a) LE RO; (b) XLE RO; (c) DR NF; (d) HL NF.

processes. The components and their compositions of synthetic fermentation broth used in this study are summarized in Table 2. Inorganic salts such as ammonium sulfate, potassium phosphate, and other minor sulfates exist in the fermentation batch to promote the microbial growth. Glucose and yeast extract are organic species commonly used as nutrition and feed carbon sources for BAc production. There are also other carbon sources, such as sucrose and waste biomass. Since glucose obtained from the hydrolysis of other biomass is the most common carbon source for BAc fermentation, glucose was selected as a model carbon source in this work. The

volume ratio is the ratio of the total amount of water loss and the initial feed volume. When ideal dewatering occurs without BAc loss, CF would be 2 if half the feedwater were removed (permeate volume ratio = 0.5).

4. RESULTS AND DISCUSSION Fermentation Broth. The components of a fermentation broth vary with microbial species, carbon source operation conditions such as processing time, pH, and temperature in the fermentation processes. Moreover, the compositions of each species in the fermentation broth are also changed continuously with microbial activity especially in batch fermentation 10211



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Table 3. Specification of NF and RO Membranes Used in This Study type

name

manufacturer

stand. water flux (L/m2 h bar)

rejection (%, solute)

oper press. (bar)

pH range

NF

HL NF Duraslick NF XLE LE

GE GE Dow Filmtec Dow Filmtec

6.59a 5.74a 7.05b 5.34c

95, MgSO4 98.6, MgSO4 99, NaCl 99, NaCl

∼41 ∼41 ∼41 ∼41

3−9 3−9 2−11 2−11

RO

Testing conditions: 2000 ppm MgSO4, 7.6 bar, 25 °C, pH 7.5, 15% recovery. bTesting conditions: 500 ppm NaCl, 6.9 bar, 25 °C, 8% recovery. Testing conditions: 2000 ppm NaCl, 10.3 bar, 25 °C, 15% recovery.

a c

concentration of glucose is generally much higher than that of other solutes since a large amount of glucose is used in fermentation processes to maximize the productivity of BAc. For the rejection study of glucose in NF and RO experiments, the model glucose concentration was fixed at 10 g/L. In the fermentation processes, however, the carbon sources are consumed by microorganisms and converted to other bioproducts. The BAc concentration in fermentation broth is changed as a function of time, pH, and temperature. In this study, the permeate flux and rejection were investigated in terms of BAc concentration in the feed solution. Membrane Characteristics. Currently, most of the commercial NF and RO membranes are prepared from interfacial polymerization of polyamide and polypiperazine on the support layer to enhance the membrane flux. The surface and cross-sectional morphologies of NF and RO membranes in this study are shown in Figure 4. The thin film composite membranes have a thin active skin layer (2−300 nm) on the top surface of the membrane, supported by a thick porous support layer (50 μm) and a nonwoven (100 μm) layer. Membrane surface morphologies are quite different due to different chemical compositions (e.g., monomers) and fabrication methods. Aromatic polyamide based RO membranes show a rough surface, while semiaromatic based NF membranes have a relatively smooth surface. Different monomers having amine, carboxylic, and other functional groups can be used to form the active layers of membranes, and the structure and chemical properties of the active layer determine the overall performance. The specifications of NF membranes (HL, DR) and RO membranes (XLE, LE) are summarized in Table 3, and the chemical properties of each membrane were well described in recent literature.35 The surface contact angle can be affected by membrane surface composition, surface roughness, or porosity. Originally, polyamide and poly(piperazinamide) are hydrophilic materials because of ketone, amine, and extra carboxylic acid groups. Therefore, the measured contact angles of polyamide based NF and RO membranes have generally low contact angles. In Figure 5, XLE and LE RO membranes have higher contact angles than HL and DR NF membranes. Moreover, HL membranes showed water spreading and complete wetting due to their hydrophilic surface and support layer. Since aromatic polyamide based RO membranes have dense chemical structures mainly composed of aromatic groups and quite rough surface morphologies as confirmed in Figure 4 while NF membranes have looser structures (partially aromatic) and smoother surfaces than RO membranes, the contact angles of NF membranes tend to be smaller than those of RO membranes. Membrane Performances on Solute Rejection. The water fluxes in NF and RO membranes are linearly increased with applied feed pressure as shown in Figure 6. NF membranes have higher water fluxes than RO membranes

Figure 5. Contact angle analysis of NF and RO membrane surfaces. Experimental conditions: water drop volume = 0.01 mL; five points of each sample.

Figure 6. Pure water flux of NF and RO membranes. Experimental conditions: dead-end filtration; 25 °C; feed, 250 mL of deionized water (conductivity = 0.6 μS/cm, pH 5.8); stirring rate, 800 rpm.

since they have a loose skin layer structure compared to RO membranes. Because the energy requirement for membrane separation processes tends to increase with applied pressure, the applied pressure should be minimized with high productivity and the operating cost. The solute rejection of the membrane is also a key parameter as is the membrane flux. The solute rejections vary with not only the type of membranes but also applied feed pressure, pH, and temperature. Since the fermentation broth has a variety of solutes including inorganic 10212



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Figure 7. Effect of operating pressure on the rejection of various solutes (LE,XLE, DR, HL). Test conditions: crossflow filtration; 25 °C; crossflow rate, 1.5 L/min; pH 6; KxHySO4, 1000 ppm; (NH4)2SO4, 1000 ppm; glucose, 10 000 ppm; yeast extract, 1000 ppm.

and organic species, it is important to investigate each solute rejection and transport properties at different operation conditions. Various individual solute solutions of major inorganic salts and organic species commonly used in a fermentation reactor were filtrated at different applied pressures, temperatures and pHs of feed solution. Figure 7 shows the effect of applied feed pressure on solute rejection of NF and RO membranes. The rejection ratios of all species tend to increase with increasing applied feed pressure at the same operation conditions (25 °C and pH 6.0). The difference between applied pressure and osmotic pressure of feed solution is the net driving force for permeation in both NF and RO processes as shown in eq 1. On the other hand, solute transport is mainly governed by the concentration gradient and is theoretically independent of applied pressure (eq 2). Therefore, the solute rejection is generally increased with increasing applied pressure. The rejection ratios of both inorganic and organic species were generally higher in RO membranes than in NF membranes. Although the concentration of glucose in the feed solution was the highest (10 000 ppm), the rejection of glucose was on the high level (>99.5%) for RO membranes. Since both NF and RO membranes have charged groups on the surface active layer, the pH of feed solution strongly affects solute transport especially for ionic species. Figure 8 shows the

effect of pH on the rejection of various solutes in NF and RO membranes. The rejections of inorganic solutes tend to be higher at higher pH than at lower pH due to electric repulsion to divalent anions (PO42− and SO42−) caused by a negative surface charge of the membranes at high pH. However, the rejection of inorganic salts in all membranes is relatively low at low pH. The surface potential of a polyamide skin layer of NF (HL) and RO (LE) membranes in hydrated state depends on pH as shown in Figure 9. The amount of charged groups on the membrane surface varies with chemical composition of the active layer material (i.e., polyamide); thus, NF and RO membranes have different magnitudes of the ζ potential. Generally, NF membranes have more negatively charged groups on their surface than RO membranes, which helps reject divalent ions via the Donnan exclusion mechanism in spite of their loose active layer structure. Therefore, NF (HL) membrane showed a larger absolute value of the ζ potential than RO (LE) membrane in streaming potential measurement. In addition, the ζ potential of the membrane surface becomes negative at high pH and it continuously increases positively as the pH decreases. Eventually, the surface charge turns into positive below pH 5−6 (isoelectric point). At the isoelectric point, the rejections of ionic species usually show minimum values due to the lack of electrostatic repulsion.35 Below the isoelectric point, cations such as K+ and NH4+ are rejected by 10213



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Figure 8. Effect of pH of feed solution on the flux and rejection of various membranes (LE, XLE, DR, HL). Test conditions: crossflow filtration; 25 °C; crossflow rate, 1.5 L/min; 15 bar; KxHySO4, 1000 ppm; (NH4)2SO4, 1000 ppm; glucose, 10 000 ppm; yeast extract, 1000 ppm.

The rejection of sucrose is independent of the pH of feed solution due to its electroneutrality. On the other hand, the rejection of yeast extract varied with feed pH. Yeast extract is a mixture of various organic compounds such as amino acids, peptides, water-soluble vitamins, and carbohydrates, and the molecular weights of the components lie in a broad range. Because some organic compounds such as amino acids in yeast extract can be dissociated, the rejections of these components show behaviors similar to those of ionic salt species. Operating temperature can also affect the membrane performance. Since the temperature of most fermentation processes is about 37 °C, the same temperature may be more favorable than room temperature for continuous separation processes. The change in membrane flux and rejection of each component with operation temperature are shown in Figure 10 and Table 4, respectively. The membrane fluxes were dramatically increased, by about 30−40%, at 37 °C as compared to 25 °C. This is caused by the membrane swelling effect and the increase of activity of the feed solution by increasing temperature. Also, the rejection decreases slightly at 37 °C due to the swelling of polyamide structure and the increased solute activity. Transport Behavior of Butyric Acid in NF and RO Membranes. BAc is the target product that should penetrate through NF and RO membranes at a certain level while other species, especially carbon sources, should be rejected. BAc has lower molecular weight than other components in the fermentation broth and is a weak acid with pKa of 4.82. The

Figure 9. ζ potential of RO membranes at different pH conditions.

electrostatic repulsion and anions such as PO42− and SO42− are also rejected due to electroneutrality. However, the existence of protons lowers the rejection at the low pH. Since protons have a higher ionic mobility and smaller size than other cations and can penetrate the membranes without hindrance, more anions will also permeate through the membranes to maintain electroneutrality in the permeate.36 Thus the rejection of ionic species at pH 3 showed a relatively lower value as shown in Figure 8. 10214



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Figure 10. Effect of temperature on membrane flux. Test conditions: crossflow filtration; crossflow rate, 1.5 L/min; 15 bar; pH 6; 1000 ppm KxHySO4 was used as a feed solution.

Table 4. Effect of Temperature on Solute Rejectiona rejection (%) KHPO4 LE XLE DR HL

(NH4)2SO4

glucose

yeast extract

25 °C

37 °C

25 °C

37 °C

25 °C

37 °C

25 °C

37 °C

99.6 99.6 96.6 95.6

99.4 99.6 94.8 93.9

99.2 99.4 92.6 91.2

99.0 99.3 88.7 87.0

99.7 99.5 91.4 87

99.7 99.5 84.6 79.2

99 98.9 91 94.1

98.8 98.7 89.1 91.9

a Test conditions: crossflow filtration; crossflow rate, 1.5 L/min; 15 bar; pH 6; KxHySO4, 1000 ppm; (NH4)2SO4, 1000 ppm; glucose, 10 000 ppm; yeast extract, 1000 ppm.

Figure 11. Effects of pH and pressure on rejection of BAc in NF and RO membranes. Test conditions: crossflow filtration; crossflow rate, 1.5 L/min; feed, 20 L of 1000 ppm BAc. (a) Pressure, 15 bar. (b) pH 3.

formation of BAc is susceptible to solution pH as are other ionic species: the acid form at low pH and the salt form (butyrate) at high pH. The membrane transport and rejection behavior of BAc is also similar to that of ionic solute. Figure 11a shows the rejection change of BAc depending upon pH and the applied feed pressure. The rejection of BAc changed dramatically with pH due to its low molecular weight (Mw = 88.11 g/mol) and the change in acidity especially in NF membranes. At a low pH, negatively charged BAc can permeate through the positively charged active layer of the membrane. In RO membranes, the selective permeation of BAc was as not much as in NF membranes due to the highly dense skin layers of RO membranes. Since the NF membrane has a loose skin layer (typical MWCO (molecular weight cutoff) is 100−300 Da for NF membranes) and the rejection is governed by Donnan exclusion, the rejection of BAc lay on a low level and significantly changed with pH condition. The effects of pressure on the rejection of BAc were similar to those of other solutes as shown in Figure 11b. However, the rejection change of BAc in NF membranes was negligible at a low pH due to the low hindrance effect of NF membranes to BAc permeation at a low pH condition. The results imply that BAc recovery from fermentation broth with NF membranes will be more efficient at high pressure and low pH condition. As the applied pressure increased, the rejections of the ionic salts and organics generally increased in the NF membranes while the permeation of BAc was maintained at the same level (93−95% BAc permeation for DR and HL membranes). Moreover, the effect of pH on glucose rejection was not significant for both NF and RO

membranes even at low pH. Thus, the efficient separation of BAc and glucose can be achieved when pH controlled NF processes are combined with the BAc fermentation processes. NF and RO Filtration of the Fermentation Broth Including Butyric Acid. Microorganisms such as C. tyrobutyricum produce BAc at the rate of 1−2 g/L·h, consuming the carbon source simultaneously.15Therefore, the concentration of BAc in the fermentation broth increases with time and it should be removed where it is under the concentration limit (approximately under 50 g/L in recent studies15) to sustain the BAc productivity of microorganisms. During the filtration, the membrane flux was significantly reduced as shown in Figure 12a due to not only the increasing osmotic pressure with increasing BAc concentration, but also membrane fouling caused by the accumulation of solid particles in the fermentation broth. The membrane flux defined by eq 1 is proportional to the osmotic pressure difference. Therefore, the membrane flux was decreased with increasing BAc concentration in the feed solution. Moreover, the BAc flux, JS, increased with increasing feed concentration as described by eq 2. This means that the BAc rejection would be decreased with increasing BAc concentration in the feed solution. At the point where the flux is not decreased with increasing feed concentration, the effective permeation of BAc will be achieved. 10215



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The BAc recovery ratio and BAc purity calculated by eqs 3 and 4 are shown in parts a and b, respectively, of Figure 13.

Figure 12. Effect of BAc generation in synthetic fermentation broth on membrane flux and rejection. Test conditions: crossflow filtration; 15 bar; pH 3; 37 °C; crossflow rate, 1.5 L/min; feed, synthetic fermentation broth. BAc was added continuously at the rate of 2 g/ L·h.

Figure 13. Effect of BAc concentration of fermentation broth on recovery ratio and purity of BAc downstream. Test conditions: crossflow filtration; 15 bar; pH 3; 37 °C; crossflow rate, 1.5 L/min; feed, synthetic fermentation broth. BAc was added continuously at the rate of 2 g/L·h.

Figure 12b shows the total amount of organic carbon in permeate samples (permeate TOC) at different BAc concentrations in the feed. At the initial stage (no BAc in feed solution), the total organic carbon amount in feed solution was 4800 ppm from glucose and yeast extract. RO membranes exhibited a high rejection ratio above 99.8%, while NF membrane showed 85% organic carbon rejection as expected in the single solute rejection experiment. As the BAc concentration in the fermentation broth was increased, the permeate TOC was increased by the contribution of BAc permeation since BAc can permeate through the membranes at a low pH while other organic molecules were rejected as discussed in the section Transport Behavior of Butyric Acid in NF and RO Membranes. The theoretical organic carbon concentration of 1000 ppm BAc solution is 545.4 ppm (0.545 g/g BAc). When 100% BAc permeation is achieved during the filtration, the increased amount of permeate TOC should be similar to this value. NF membranes showed the highest BAc recovery ratio (above 95% recovery as shown in Figure 11a). In RO membranes, however, the increased amount of permeate TOC was only 120 ppm per 1 g of BAc added due to its high BAc rejection (70−80% rejection even at pH 3).

Although RO membranes showed lower BAc recovery ratios along the different BAc concentrations in feed, the BAc purity was high even at low BAc concentration in feed and increased with its increasing concentration in feed. In comparison, NF membranes showed very high permeation of BAc above 90%. The purity of BAc, however, was only about 40−50% at low BAc concentration due to lower rejection to glucose (90%) than RO membranes and its 10 times higher feed concentration (10 g/L) compared to BAc (1 g/L). By enriching BAc in the feed side, BAc purity increased rapidly because of high BAc permeation in NF membranes. Eventually, the purity reached 80% when BAc concentration reached half the concentration of glucose (5000 ppm). Forward Osmosis for Dewatering of Butyric Acid Solution. After the clarification of fermentation broth via NF or RO processes, BAc and water will be the main components in the downstream. The main issue in downstream processes is energy consumption. The most widely used conventional process for downstream dewatering is the distillation process (600−700 Wh/m 3 for simple distillation), and many 10216



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engineered distillation processes such as multieffect distillation, multistage flash distillation, and vapor compression were proposed to lower the energy consumption (24−37 kWh/ m3) and the product cost. However, the BAc concentration in downstream is low due to the limitation of BAc production in fermentation processes (

Integrated Membrane Processes for Separation and Purification of

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