Novel tri-bore PVDF hollow fiber membranes for the control of dissolved oxygen in aquaculture water

Journal of Water Process Engineering xxx (xxxx) xxx–xxx

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Novel tri-bore PVDF hollow fiber membranes for the control of dissolved oxygen in aquaculture water ⁎

Jincai Su , Yanyan Wei School of Life Sciences & Chemical Technology, Ngee Ann Polytechnic, 535 Clementi Road, 599489, Singapore

A R T I C L E I N F O

A B S T R A C T

Keywords: Tri-bore hollow fiber Dissolved oxygen Mass transfer Aquaculture Deoxygenation

Novel tri-bore hollow fiber membranes have been developed from polyvinylidene fluoride (PVDF) for the control the dissolved oxygen (DO) in aquaculture denitrification process. The fabricated hollow fibers are characterized in terms of morphology, porosity, hydrophobicity and mechanical strength. Two membrane modules, each including 200 pieces of hollow fibers, are connected in series or parallel in order to determine the optimum operation mode. The deoxygenation test is firstly conducted for DI water and then for aquaculture water. Various methods including water flushing, air blowing or chemical cleaning have been applied to assure the cleaning efficiency after membrane fouling. A mathematical model has been developed by using the resistance-in-series concept by taking into account boundary layer and membrane characteristics. Overall mass transfer coefficient, radial and axial concentration profiles, and molar flux of oxygen at different water flow rates are calculated. This work has demonstrated that the developed tri-bore hollow fiber membranes are applicable for the control of dissolved oxygen in aquaculture water. The observations have provided solid evidence for the development of membrane-based denitrification system for recirculating aquaculture system (RAS).

1. Introduction

nutrients is discharged everyday. Discharge of nitrate into receiving water courses would adversely affect the existing aqueous ecosystems and incur unexpected situations such as algal booming [7,9]. Another method is to convert nitrate through a biological process named as denitrificaiton. Denitrification depends on facultative heterotrophic bacteria which reduces nitrate NO3− to nitrite (NO2−), nitric oxide (NO) and nitrous oxide (N2O), before eventually converted to N2 [2,7,9–12]. Though relatively expensive compared with water replacement, biological denitrification attracts more attention recently because it offers high rate of nitrate removal and minimizes the discharge of wastewater and consumption of new water. Constructed wetland, algal pond, and aquaponics have also been used for nitrate removal but they are mainly for other culturing systems [13,14]. Facultative bacteria need a carbon source as food to live while facultative bacteria get their oxygen by taking dissolved oxygen (DO) from water or taking it off nitrate molecules. If both DO and nitrate are present, facultative bacteria tend to prefer oxygen. It is commonly perceived that DO acts as an inhibitory and toxic agent to anaerobic treatment because the invasion of oxygen influences the activity of denitrifying bacteria [15–17]. In the study of Tan et al., the optimum DO level for simultaneous nitrification and denitrification was 0.5–1.0 ppm and the total nitrogen removal efficiency was observed at 70.6% [17]. The nitrogen removal efficiency was compromised when

There is growing interest in recirculating aquaculture system (RAS) in recent years due to the worsening pollution of the sea, lakes and rivers and the expectation on more intensified production to feed the growing population. RAS enables minimum water consumption, offers improved control of culture conditions, and allows accurate quantification of culturing conditions and their effects on physiological rates such as aeration, feeding, fresh water, and waste accumulation and disposal [1–3]. RAS makes it possible to place the farms in locations where water resources are limited and offers the flexibility of switching the culturing species to follow the market demand or preference for seafood products [4,5]. Nevertheless, the accumulation of nitrate in RAS facilities as the end product of nitrification affects the growth of culturing species and it is more serious in the systems where nitrifying biofilters are used [6]. Water consumption and environmental impact are also driving forces for nitrate control in RAS [7]. To control the nitrate level in RAS, two methods have been commonly practiced. One method is to exchange a fraction of water in the culturing system each day with water low in nitrate [8]. Easy to be implemented, water replacement is being used in many RAS facilities. Except for the consumption of a large quantity of water, the same amount of wastewater containing dissolved solids and dissolved

⁎

Corresponding author. E-mail address: [email protected] (J. Su).

https://doi.org/10.1016/j.jwpe.2018.02.007 Received 11 October 2017; Received in revised form 5 February 2018; Accepted 5 February 2018 2214-7144/ © 2018 Elsevier Ltd. All rights reserved.

Please cite this article as: Su, J., Journal of Water Process Engineering (2018), https://doi.org/10.1016/j.jwpe.2018.02.007

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the DO concentration was beyond 1.0 ppm. It was reported that denitrification could occur at the DO level of 3–5 ppm, but the increase in DO levels could cause severe drop of the denitrification performance and the consumption rate of the carbon source was increased [18–20]. As disclosed by Gutieerez-Wing et al., the denitrification rate dropped from 5.5 to 0.5 NO3-N L−1 d−1 when the DO level increased from 0.5 to 4.0 ppm. Xu et al. observed a denitrification efficiency of 50% at DO levels higher than 4 ppm [21]. Therefore, anaerobic environment with proper DO control is essential to the efficiency and stability of denitrification. It is hard to completely avoid the invasion of oxygen into the anaerobic denitrification system because most reactors are operated within an aerobic open environment. However, proper control of DO level favors the denitrification process and is thus preferable. Oxygen scavenging agents such as sodium sulfite and iron sulfide were effective to deplete DO from anaerobic bioreactors [22]. The reaction of reducing agents and DO generates solid products that are contaminants, and it brings environmental and safety hazards due to storing and handling chemicals [23]. More seriously, the reducing agents and the products of reduction reaction would have adverse influences on the aquatic species. Membrane contactors packed with slim hollow fibers have been developed and used for effective control of DO level in semiconductor ultrapure water [24]. However, no suitable membrane contactors are available for anaerobic denitrification systems which contains various foulants. Dual-layer membrane formation using different polymers was reported by Xia et al. [25]. This might be an option to improve the antifouling property of deoxygenation membranes if the fabrication is scalable. The objective of this study was to develop robust tri-bore hollow fiber (HF) membranes for the removal of DO from aquaculture water. The HF membranes were characterized in terms of morphology, hydrophobicity, porosity and mechanical strength. The performance of the fabricated membranes were evaluated for the deoxygenation of deionized (DI) water and aquaculture water. Various cleaning methods were applied to clean the membranes after fouled by aquaculture water.

Table 1 Spinning conditions of tri-bore HF membranes. Polymer concentration (wt%)

15

Additive Dope flow rate (mL min−1) Bore fluid Bore flow rate (mL min−1) External coagulant Air-gap (cm) Take up speed (m min−1) Temperature (°C) Room humidity (%)

PEG200 (5 wt%) 7.5 Water, 25 °C 2.0 Water, 40 °C 10 Free-fall 25 65–75

were made and the results were averaged for report. The membrane porosity was measured by following the protocol described in [26]. Porosity ε is calculated from:

mfiber / ρfiber ⎞ ε = ⎜⎛1 − ⎟ × 100% V − Vchannel ⎠ fiber ⎝

(1)

where mfiber is the mass of the fiber, ρfiber is the density of the fiber material (1.78 g cm−3), Vfiber is the fiber volume calculated from the fiber outer diameter and fiber length, and Vchannel is estimated from the fiber inner diameter and fiber length. The mean pore radius and the probability density function curve of the pore radius distribution were obtained from the rejections to the neutral solutes. A detailed description of the pore structural characterization and corresponding calculation was given elsewhere [29]. The tri-bore hollow fibers were soaked with ethanol so that they became permeable for liquid water. The Liquid Entry Pressure (LEP) value was calculated using the Cantor-Laplace equation:

LEP =

2. Materials and methods

−2γcosθ rmax

(2)

where γ is the surface tension of the wetting liquid (in this case water at 25 °C, 0.07199 N m−1), θ is the contact angle between the membrane and the wetting liquid (water), and rmax is the maximum radius of the membrane.

2.1. Materials Kynar HSV 900 PVDF supplied by Arkema Inc. was used for the fabrication of tri-bore HF membranes. N-methyl-2-pyrrolidone (NMP, 99.5%) and polyethylene glycol 200 (PEG200, > 99.0%) used in membrane fabrication were supplied by Merck. DI water from Milli-Q (Millipore) system was used in all experiments. The aquaculture water was supplied by a fish farming company in Singapore. Whatman® grade 1 filter paper with pore size of 11 μm was used for the pretreatment of the aquaculture water.

2.3. Deoxygenation experiments The deoxygenation performance of the freeze-dried tri-bore HF membranes was evaluated through a pilot-scale degassing system as shown in Fig. 1. Prior to the tests, two membrane modules were prepared by bundling the fibers into 1.5 inch PVC casing with the two ends sealed using epoxy resin. Every membrane module contained 200 pieces of fibers with an effective area of 0.3 m2. For the deoxygenation tests, DI water was pumped to the lumen side of the hollow fibers while vacuum was applied in the shell side. Circulation operation modes was adopted, i.e., 4 L water being used and it flowing back to the water tank after exiting from the membrane module. All the experiments were conducted at fixed temperatures controlled by using a water circulator (Julabo F12). The deoxygenation performance under different operation modes and flow rates was firstly examined using DI water feed in order to avoid the possible influence of membrane fouling. Fish pond water was then used in the deoxygenation experiments. The efficiency (E ) of deoxygenation is expressed as

2.2. Preparation and characterization of tri-bore HF membranes Tri-bore HF membranes were fabricated via a dry-jet wet phase inversion spinning process as documented elsewhere [26–28]. Briefly, the dope solution and bore fluid were supplied at specified flow rates by ISCO syringe pumps (Teledyne, 1000D). After entering the coagulation bath, the nascent fibers precipitated and were collected by a take-up roller. Detailed spinning conditions are summarized in Table 1. After spinning, the as-spun tri-bore HF membranes were immersed in tap water for 2 days to completely remove the residual solvent and additives. The fibers were then frozen in a refrigerator and dried overnight in a freeze drier (S61-Modulyo-D, Thermo Electron). Membrane morphology was inspected using a Field Emission Scanning Electron Microscope (FESEM, JEOL JSM-7600F). For FESEM inspection, membrane samples were fractured cryogenically in liquid nitrogen and coated with platinum using a sputtering coater (JEOL JFC1600). Dynamic contact angle of the outer surface of the fibers was measured using a Dataphysics DCAT21 tensiometer. Five measurements

Cl ⎞ E = ⎜⎛1 − out ⎟ × 100% Cinl ⎠ ⎝

(3)

l where Cinl and Cout are the DO concentrations in water at the inlet and outlet of the membrane, respectively.

2

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Fig. 1. The schematic diagram of deoxygenation system.

of water. The velocity profile in z direction can be obtained as:

2.4. Mass transport of oxygen

r 2 vz (r ) = 2v ⎧1 − ⎛ ⎞ ⎫ ⎨ ⎝R⎠ ⎬ ⎩ ⎭

2.4.1. Experimental mass transfer coefficient For deoxygenation experiments, the experimental mass transfer coefficient (kexp) could be determined by [30]:

C − Ci ⎤ Q L ln ⎡ = ⎡exp ⎛−kexp am ⎞ − 1⎤ t ⎥ V⎣ ⎢ C0 − Ci ⎦ v⎠ ⎝ ⎦ ⎣

where v is the average velocity of water in the lumen and Ris the radius of the fiber lumen. The boundary conditions for Eq. (6) are as follows:

(4)

z = 0, C(r ,0) = Cin, b

where C0, Ci and C are the initial DO concentration, the DO concentration that equilibrates with the gas phase at the water-gas interface, and the remaining DO concentration at different experiment time, respectively, Q is the water flow rate, V is the volume of water in the tank, am is the membrane surface area to volume ratio, L is the length of the HF membranes, and vis the water velocity. The value of Ciis estimated by using the Henry’s law:

pi = HCi

(7)

∂C r = 0, C(0, z ) = 0 is finite or ⎡ ⎤ = 0 ⎣ ∂r ⎦0, z

(8)

C(R, z ) = CR = Ci

The following dimensionless form could be considered to express Eq. (6)

(5)

C − Ci r z ,Y= ,Z= C0 − Ci R RPe

where pi is the pressure of oxygen at the water-gas interface.

θ=

2.4.2. Concentration profile Fig. 2 is a schematic diagram of the mass transport of oxygen with water flowing in the lumen side of the hollow fiber membrane as well as the concentration profile at different phases. The steady state two-dimensional flow in the lumen side can be written as [31–33]:

where Pe is the Peclet number defined as Pe = Rv0/D. Substituting these dimensionless variables into Eq. (6) gives

vz

∂C ∂ 2C D ∂ ⎛ ∂C ⎞ = Dl 2 + r ∂z ∂z r ∂r ⎝ ∂r ⎠

(1 − Y 2)

(9)

∂θ 1 ∂ 2θ 1 ∂ ⎛ ∂θ ⎞ = + Y ∂Z Pe 2 ∂Z 2 Y ∂Y ⎝ ∂Y ⎠

(10)

Boundary conditions for Eq. (10) are as follows: (6)

θ (Y , 0) = 1, θ(1, Z ) = 0, θ(0, Z ) is finite or

where C, Dl, r and vz denote the local concentration of oxygen, the diffusivity of oxygen in water, the radial distance and the axial velocity

∂θ (0, Z ) = 0 ∂Y

(11)

The values for Pe are generally large(Pe > 100) . Therefore, it is valid to

Fig. 2. Schematic of the concentration profile of oxygen at different phases.

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J. Su, Y. Wei 1 ∂2θ

assume 1/Pe2 ≪ 1 and 2 2 ≈ 0 . Neglecting the item Pe ∂Z the solution of the simplified equation is: ∞

∑

θ (Y , Z ) =

1 ∂2θ Pe 2 ∂Z2

1/3

d 2v kl d i = 1.62 ⎜⎛ i ⎟⎞ Dl ⎝ LDl ⎠

in Eq. (10),

The mass transfer coefficient in the hydrophobic membrane is [34]:

2

An e−λnZ Φn (Y )

(12)

n=1

kp = Dp

In Eq. (11), Φn (Y ) is the eigenfunction of a proper Sturm-Liouville system:

1 ∂ ⎛ ∂Φ ⎞ λ2 (1 − Y 2) Φ + Y =0 Y ∂Y ⎝ ∂Y ⎠

(13)

dΦ (0) = 0 or Φ(0) is finite ; Φ(1) = 0 dY

Dp =

(14)

Defining X = λY2 and substituting it into Eq. (13) gives

∂ 2W dW λ 1 X + (1 − X ) + ⎛ − ⎞W = 0 ∂X 2 dX 2⎠ ⎝4

(

−

and W (0) , only M

1

An =

∫0 1 ∫0

Cb (z ) =

λ

)

− 4 , 1, X is the valid solution of Eq. (15): (16)

1 vS

R

∫

C (r , z ) vz (r )2πrdr (24)

0

where S is the cross-section area of the fiber lumen. The local molar flux of oxygen along at different position of the hollow fiber may be written as:

p N = k 0 ⎛Cj, b (z ) − b ⎞ H⎠ ⎝

(17)

(25)

where pb is the bulk partial pressure of oxygen.

and b = 1. The item An in Eq. (12) might be de-

3. Results and discussion

Φn (Y ) Y (1 − Y 2) dY 3.1. Characteristics of tri-bore HF membranes

Φn 2 (Y ) Y (1 − Y 2) dY

(18) As shown in Table 2, the inner diameter and outer diameter of the tri-bore hollow fibers are 670 and 1600 μm, respectively. The porosity is about 75%, which is beneficial for fast transport of oxygen. A water contact angle of 94° indicates necessary hydrophobicity, which helps to prevent the entry of liquid water into membrane pores under operating conditions. It should be noted that the contact angle is measured for the outer surface because it is very hard to measure it for the inner surface. As the contact angle is mainly determined by the material (PVDF for this case), the measured contact angle may be considered as an indicator of the hydrophobicity of the membrane. The tri-bore HF membranes are strong and stretchable as indicated by the maximum load at break of 4.04 N, the tensile stress of 6.77 MPa and the elongation of 52.3%. Typical morphology of the as-developed fibers is shown in Fig. 3. The fiber wall and the junction at the center have thickness of about 120 and 200 μm, respectively. The thin fiber wall is favorable for the reduction in the resistance to oxygen transport. The relative thicker junction provides strong support to the membrane which could stand the pressure difference under operation and maintain its integrity. A layer of finger-like macrovoids is present underneath the inner surface probably due to the rapid phase inversion and nonsolvent (i.e., water)

The value of λ could be determined as follows:

Φ (Y ) = e−

(23)

(

)

1 2

a (a + a 1 λ M ⎛ − , 1, X ⎞ = 1 + X + b b (b + 1)2! 4 ⎝2 ⎠ a (a + 1)…(a + n − 1) X n +… + …+ b (b + 1)…(b + n − 1) n! λ 4

⎟

1, X . Considering the boundary condition of Φ (0)

)

1) X 2

1

⎜

3 ⎝ πMw ⎠

2.4.4. Flux analysis Water is confined in the cylindrical lumen side, so the concentration of DO is not constant along aixal position of the fibers. The bulk average concentration (Cb (z )) of DO in water could be estimated as follows [33]:

1 λ W (X ) = M ⎛ − , 1, X ⎞ 4 ⎠ ⎝2

where a = 2 − termined by:

dp ⎛ 8Rg T ⎞1/2

(15)

(

function T

(22)

where dp is the mean pore diameter and Mwis the molecular weight of oxygen.

Eq. (14) is Known as Kummer’s equation and has two solutions, i.e., 1 λ the Kummer function of the first kind M 2 − 4 , 1, X and the Tricomi λ , 4

{ τbε }

where Dp, τ, and b denote the diffusivity of oxygen in the membrane pore, the tortuosity of the pore and the membrane thickness, respectively. In the membrane pores, the transport of oxygen is through Knudsen diffusion and the diffusivity can be estimated using the following equation:

Boundary conditions of Eq. (13) are

1 2

(21)

λY 2 2 W

(λY 2) at Φ(1) = 0

(19)

2.4.3. Theoretical mass transfer coefficient At the water-gas interface, Henry’s law is applicable (Eq. (4)) as water containing DO can be considered as dilute solution. Since the shell side of the HF membranes is under vacuum and the interface is located at the lumen, the resistance at the shell side (permeate side) is negligible. The overall resistance to the transport of oxygen could be expressed using the resistance-in-series concept [34]:

Rg T 1 1 = + k 0 di kl d i Hkp dm

(20)

where k0, kl and kp are the mass transfer coefficients of oxygen in water and the membrane pore, Rg is the universal gas constant, T is the temperature, and di and dm are the fiber inner diameter and logarithmic mean diameter, respectively. kl can be calculated from Leveque equation under laminar flow condition: Table 2 Characteristics of tri-bore HF membranes. Membrane

Inner diameter (mm)

Outer diameter (mm)

Maximum load (N)

Tensile stress (MPa)

Elongation at break (%)

Porosity (%)

Water contact angle (o)

Tri-bore HF

670

1600

4.04

6.77

52.3

75

94

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Fig. 3. Morphology of tri-bore HF membrane.

intrusion [35–37]. The middle portion and outer edge of the fiber have sponge-like porous structure, which might be due to combined effects from pore forming agent, delayed demixing and stretching. PEG200 is the pore forming agent and it helps to generate pores and porosity. As a non-solvent additive, the addition of PEG200 also brings the dope closer to gelation points so that the phase inversion is accelerated [27]. Using water as the external coagulant for spinning, the outer surfaces of the tri-bore HF membranes is less porous, consisting of interconnected globules. Once the outer skin is formed, the solvent-nonsolvent exchange is retarded and causes delayed demixing, which is favorable for the formation of porous structure [36,38,39]. As well, an air-gap distance of 10 cm was used for the spinning. Appropriate stretching after the nascent fibers are extruded from the spinneret might also contribute to the porous inner surface structure. The inner surface is apparently porous though the bore fluid for spinning is also water. During the spinning, phase inversion is supposed to occur at the bore side rightly after the PVDF solution is extruded from the spinneret. The polymer concentration at the inner surface would increase rapidly and form a relatively dense structure. It should be noted that the outflow of NMP solvent changes the bore fluid from pure water, a strong coagulant, into a dilute aqueous solution of NMP solvent. Even though the amount of outflowed solvent is very small, its impact on the phase inversion of the inner skin layer cannot be neglected as the amount of bore fluid is also very small (2.0 mL min−1). The bore fluid with continuously incoming solvent might make the inner surface not as dense as of the outer surface. Similar phenomenon was reported in another study [40]. As shown in Fig. 4, the pores within the inner skin layer of the tribore hollow fiber membrane are in the range of 2–20 nm with the mean pore radius of 6.3 nm. The membrane is in the category of ultrafiltration and the LEP value for water is determined at 9.9 bar.

Fig. 4. Pore size distribution curve of the tri-bore hollow fiber membrane.

3.2. Performance of water deoxygenation The as-prepared membranes were firstly tested for DI water deoxygenation in order to understand the influences of flow rate and operation mode. With two membrane modules in series or parallel, the DO concentration drastically drops in the first 2 min and the change slows down subsequently (Fig. 5). Under both operation modes, the water flow rate of 100 mL min−1 generates much faster drop in the DO concentration than 500 mL min−1. Further increase in the water flow rate above 500 mL min−1 does not significantly influence the DO removal

Fig. 5. Deoxygenation performance of tri-bore HF membranes in series and parallel.

rate as well as the DO concentration in the effluent (data not shown). Pressure buildup on the lumen side was observed when the water flow rate was above 700 mL min−1. To avoid the influence of pressure on the transport of oxygen, only results at relatively low flow rates, i.e., 100 and 500 mL min−1, are discussed here. At the same water flow rate, the 5

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Table 3 Analysis of aquaculture water. Test

pH

Total suspended solids (mg L−1)

Total dissolved solids (mg L−1)

Turbidity (NTU)

Chemical oxygen demand (mg L−1)

Nitrate (mg L−1)

Results

6.4

3245

800

1.39

43.0

45.5

DO removal rate is faster when the membrane modules are operated in series. Under 100 and 500 mL min−1 water flow rates, the DO removal efficiency is 97.5% and 82.2% when the two membranes are operated in series and 87.7% and 75.4% when the membranes are operated in parallel. The experimental mass transfer coefficients are determined as of 2.03 × 10−5 and 4.73 × 10−5 m s−1 for the series mode and 1.17 × 10−5 and 4.01 × 10−5 m s−1 for the parallel mode, respectively. The mass transfer coefficient determined for the deoxygenation of DI water is similar to that for RO water deoxygenation observed by Peng at al. [41]. Pristine aquaculture water taken from RAS system was subjected to deoxygenation test with the two membrane modules in series and under a circulation mode. As shown in Table 3, the aquaculture water is slightly acidic. It shows a turbidity of 1.39 NTU and chemical oxygen demand (COD) of 43 mg L−1 and contains 3245 mg L−1 total suspended solids (TSS), 800 mg L−1 total dissolved solids (TDS) and 45.5 mg L−1 nitrate. Under the four water flow rates tested, the mass transfer coefficient does increase with increasing the water flow rate (Fig. 6). Surprisingly, a flow rate of 100 mL min−1 results in the highest DO removal efficiency of 87.3% which is lower than 97.5% observed in DI water deoxygenation test. The suspended solids and dissolved organic and biological substances adhered to the membrane inner surface might have fouled the membrane, reduced the effective membrane area and increased the resistance for oxygen transport. The DO removal efficiency is 68.1% and 83.2% at 20 and 50 mL min−1, respectively. It might be that the flow is too slow and the foulants stay firmly at the membrane inner surface. Though the flow is more vigorous by increasing the water flow rate to 500 mL min−1, the influence from fouling still cannot be apparently mitigated and a DO removal efficiency of 76.8% is achieved. Based on these observations, a water flow rate of 100 mL min−1 was used for the subsequent deoxygenation tests. Air blowing (1.0 bar, 10 min) and DI water flushing (100 min) were firstly applied to clean the membrane after deoxygenation of unfiltered aquaculture water due to the simplicity to implement in the real RAS. As shown in Fig. 7, air blowing is applied after the 1st run of deoxygenation followed by DI water flushing but it does not completely recover the membrane performance as seen from the 2nd run. Flushing with DI water for 100 min does not show better cleaning efficiency than air blowing, and the DO concentration in the effluent slightly increases. For pristine pond water without any pretreatment, either air blowing or

Fig. 7. Deoxygenation of unfiltered aquaculture water.

DI water flushing could not effectively remove the foulants deposited at the membrane inner surface and could not maintain the deoxygenation performance. In subsequent experiments, the aquaculture water was filtered to remove the suspended solids before deoxygenation performance evaluation using the same membrane modules. Flushing with DI water is tried firstly and used as the reference. As seen from Fig. 8, the DO level in the effluent still increases gradually after every experiment and the DO removal efficiency slightly drops. It seems that the fouling is not apparently mitigated after the suspended solids are removed. The fouling might have mainly contributed by dissolved organic substances and microorganisms. Cleaning with NaOH solution (pH10) followed by DI water flushing works when the flushing time is increased to 60 min but the efficiency is still quite low (Fig. 9). A mixture of NaOH (pH10) and SDS (1 mM) is effective to remove the foulants from the membrane surface. Better recovery of the membrane performance is achieved with lengthening the cleaning time from 20 to 40 and 60 min. As well, the cleaning with NaOH/SDS solution could help to maintain the performance of the HF membranes, i.e., the exiting aquaculture water not higher than 1 ppm DO which is expected for biological denitrification. 3.3. Mass transport of oxygen 3.3.1. Mass transfer coefficient The mass transfer in vacuum deoxygenation involves the diffusion of oxygen in liquid water, membrane pores, and surrounding vacuum or gas stream [30,42]. Using Eqs. (21)–(23), the mass transfer coefficients in the liquid phase and membrane pore as well as the overall mass transfer coefficient are determined and shown in Fig. 10. With the

Fig. 6. Deoxygenation of unfiltered aquaculture water at different flow rates.

Fig. 8. Deoxygenation of filtered aquaculture water.

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membrane are directly or indirectly connected with the bulk gas phase. Only gas molecules (e.g., oxygen and water vapor) could enter the pores resulted from the hydrophobic nature of the membrane while they are immediately taken away upon continuous suction in the shell side. Within the membrane pores, the resistance to the movement of oxygen only comes from the tortuous or interconnected pore walls. The mass transfer coefficient in the liquid water phase (lumen side) is 15–90 times lower than that in the membrane phase. In the lumen side, DO needs to diffuse in water towards the water-gas interface and pass through the interface before entering the membrane pores as gas molecules. The slow transport of oxygen is directly resulted from the low oxygen diffusivity in the bulk water and the boundary layer near the fiber wall. Even at a water flow rate of 1 L min−1, water is still at laminar flow and the influence of mass transfer from the boundary layer is significant. As a result, the overall resistance to the transport of oxygen is dominated by the liquid water phase and the overall mass transfer coefficient (k 0) is determined by the mass transfer coefficient in water (kl ) . 3.3.2. Concentration profiles At a water flow rate of 100 mL min−1, the concentration profiles of DO in radial and axial directions were calculated and are shown in Figs. 11 and 12, respectively. Showing obviously similar trend, only radial concentration profiles at water flow rates of 100 and 500 mL min−1 are presented and discussed here. At both water flow rates, the DO concentration at the center of the fiber lumen (r = 0) is higher than that at the fiber wall (r = R) while the difference is more significant at a higher water flow rate. At 100 mL min−1 water flow rate, the radial DO concentration varies more significantly near the entrance region of the membrane and the variation becomes less with water flowing through the membrane module. It drops slowly in the later stages. At the half length point (L/2) , the DO concentration drops by 68.6% and 83.3% at the center and fiber wall, respectively. Around the exit region of the membrane module, the DO reduction is only 83.2% at the center and 85.3% at the fiber wall. It seems that at low water flow rate the existing DO concentration at the center almost reaches the same level as that at the fiber wall. At 500 mL min−1 water flow rate, the DO removal rate becomes slow as seen from the small change in the radial DO concentration (Fig. 11). Even though a higher water flow rate means enhanced mass transfer coefficient (Fig. 10), the significantly shortened residence time within the membrane determines lower the ultimate DO removal efficiency. For the concentration of DO in axial direction for different radial points, the influence of flow rate on the deoxygenation performance is more clearly seen from Fig. 12. At the center of the fiber lumen (r = 0) , the axial DO concentration falls slowly for 500 mL min−1 water flow rate but falls rapidly at 100 mL min−1 flow rate. Apart from the lumen center, the axial DO concentration at different radial points changes slower at 500 mL min−1 water flow rate. Near the fiber wall, there is a sharp decline in the DO concentration for both cases. The exiting DO concentration corresponding to 500 mL min−1 flow rate has a broad distribution, which is very different from what seen from another case (Fig. 12). The possible reason for this phenomenon is that the diffusion of DO from the bulk to the fiber wall where the water-gas interface is located is too slow.

Fig. 9. Performance of tri-bore HF membranes: (a) effectiveness of cleaning and (b) efficiency of deoxygenation.

Fig. 10. Theoretical mass transfer coefficient at different flow rates.

3.3.3. Molar flux profile The molar flux of DO at different axial positions of the membrane module is shown in Fig. 13. Obviously, the molar flux of DO increases with increasing the water flow rate. At low flow rates (e.g., 20 mL min−1), the molar flux is only high in the entrance region (z / L = 0.2) and it is almost constant thereafter. For relatively higher water flow rates, the molar flux of DO gradually decreases with water flowing long the membrane module and reaches the minimum value at the exiting point. Calculating from the average DO concentration, the DO removal efficiency is about 85.6%, 84.3%, 52.4% and 35.1% at

objective of understanding the mass transfer, the theoretical study here only considers the case that fresh clean water (without foulants) enters the membrane module and leaves at the exiting point without circulation (i.e., one-pass mode). Generally, the calculated mass transfer coefficient is in line with the experimental results shown in Fig. 6. Clearly, the mass transfer coefficient in the membrane pores does not change with increasing the water flow rate and it is much higher than that in the liquid phase. The reason is as follows. The shell side of the fibers, i.e., the bulk gas phase, is under vacuum. The pores of the 7

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Fig. 11. The concentration of DO in radial direction for different axial points.

water flow rates of 20, 100, 500 and 1000 mL min−1, respectively. Consequently, slow water flow rate is preferred as it results in better deoxygenation performance. It should be noted that these theoretical calculations are based on the assumption that the feed water is clean and there is no membrane fouling. If some foulants exist in the feed water, they tend to deposit on the membrane surface to form an extra layer, which not only reduces the effective membrane surface area for oxygen to transport but also increases the resistance. The fouling is more serious if the flow is slow. For a real scenario, the operation conditions should be optimized by considering the mass transfer rate as well as the membrane fouling propensity simultaneously.

4. Conclusions Novel and robust tri-bore hollow fiber membranes have been developed for water deoxygenation. Its performance is better when two membranes are connected in series. Based on DI water feed, a DO removal efficiency of 97.5% is achieved at a water flow rate of 100 mL min−1. When applied for aquaculture water, the deoxygenation performance slightly decreases and a DO removal efficiency of 87.3% is obtained. Membrane fouling is observed whether the aquaculture water is pre-treated or not. DI water flushing, air blowing or cleaning with

Fig. 13. Molar flux of DO at different water flow rates.

NaOH (pH) solution are not effective to clean the fouled membranes. A combination of NaOH (OH) and 1 mM SDS shows satisfactory cleaning efficiency and the deoxygenation performance of the membrane could be maintained.

Fig. 12. The concentration of DO in axial direction for differential radial points.

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Mathematic modeling has been conducted to investigate the oxygen transport from water to the gas phase through the membrane. With water flowing in the lumen, the mass transfer coefficient and molar flux of DO increase with increasing the water flow rate. The DO concentrations in radial and axial directions show very different features at low and high water flow rates. However, the ultimate DO removal efficiency would be higher at low water flow rates due to longer residence time, which allows the diffusion of DO from the fiber lumen to the boundary layer and the water-gas interface. The DO removal efficiency could be reduced from 85.6% to 35.1% with increasing the water flow rate from 20 to 1000 mL min−1. For the deoxygenation of relatively clean water without or with minimum foulants, operation of the membrane modules at low flow rate would be preferable.

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Acknowledgments

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We acknowledge Ministry of Education, Singapore for funding the research through the project “High throughput multi-bore hollow fiber membrane module for deoyxgenation” (Grant number MOE2014-TIF-1G-020). We thank Professor Tai-Shung Chung and his team members (National University of Singapore) for providing the facilities and valuable suggestions.

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