Effluent organic matter removal by Purolite®A500PS: Experimental performance and mathematical model

Separation and Purification Technology 98 (2012) 46–54

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Effluent organic matter removal by PuroliteÒA500PS: Experimental performance and mathematical model Rana Tanveer Ahmad a, Tien Vinh Nguyen a, W.G. Shim b, Saravanamuthu Vigneswaran a,⇑, H. Moon b, Jaya Kandasamy a a b

Faculty of Engineering, University of Technology Sydney (UTS), P.O. Box 123, Broadway, NSW 2007, Australia School of Applied Chemical Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea

a r t i c l e

i n f o

Article history: Received 6 March 2012 Received in revised form 24 April 2012 Accepted 15 June 2012 Available online 23 June 2012 Keywords: Fluidized bed Mathematical model Organic removal Organic fraction PuroliteÒA500PS

a b s t r a c t In this study, the performance of PuroliteÒA500PS in effluent organic matter (EfOM) removal was evaluated through adsorption equilibrium, kinetics and fluidized bed experiments. It was found that the maximum EfOM removal capacity of PuroliteÒA500PS calculated by the Langmuir isotherm was 50.9 mg DOC/g PuroliteÒA500PS. The results also showed that fluidized bed operational conditions strongly affected the EfOM removal efficiency. A fluidized bed packed with PuroliteÒA500PS can maintain a consistent EfOM removal efficiency of more than 80% with more than 800 bed volumes from 10 mg DOC/L of synthetic wastewater. A majority of hydrophilic compounds (76.4%) and a significant amount of hydrophobic compounds (55%) were removed by the PuroliteÒA500PS fluidized bed. The PuroliteÒA500PS fluidized bed was also found to remove a majority of biopolymer (98.5%), humic substances (86.5%), and low molecular weight neutrals (83.3%). Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Wastewater reuse is increasingly seen as an essential strategy for making better use of limited fresh water. Membrane processes are now being successfully applied to obtain water of recyclable quality but the major challenge for membrane systems is the fouling of membranes. One of the main factors that cause membrane fouling is residual organic matter in wastewater. The residual organic matter is known as effluent organic matter (EfOM) and they are very difficult to remove because of their recalcitrant nature. EfOM can react with chlorine during the disinfection process and form disinfection by-products (DBPs), which are cancerogenic [1]. EfOM also increases the disinfectant demand and therefore increases the treatment cost. Furthermore, these organic compounds can provide a food source for the micro-organisms in the receiving water and cause the re-growth of the micro-organism in the pipelines. Therefore, further removal of EfOM is necessary. EfOM can be removed by different processes such as flocculation, adsorption, and ion exchange. Flocculation and adsorption are effective in removing hydrophobic organic compounds. However, the biologically treated sewage effluent (BTSE) also contains a significant portion of hydrophilic organic compounds [2,3]. These

⇑ Corresponding author. Tel.: +61 2 95142641; fax: +61 2 95142633. E-mail addresses: [email protected] (W.G. Shim), [email protected] (S. Vigneswaran), [email protected] (H. Moon). 1383-5866/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2012.06.025

compounds can successfully be removed by an ion exchange process. Croue et al. [4] found that strong anion exchange resins removed dissolved organic carbon (DOC) better than weak anion exchange resins and the increase in ionic strength enhanced the removal of natural organic matter (NOM). Gottleib [5] showed that lightly cross linked gel resins were more efficient in removing organic materials than those of highly cross linked ones, and were also better than highly cross linked macroporous resins. Kabay et al. [6] investigated the optimum dose of resin for boron removal from seawater RO plants. According to their data, 1 g resin/L (both Diaino and Dowex sorbents) was the optimum resin amount for the elimination of boron from H3BO3 added to seawater RO permeate. Magnetic Ion Exchange (MIEXÒ) resin, another ion-exchange resin can effectively remove dissolved organic matter from BTSE and produce high-quality water [7–8]. MIEXÒ resin could effectively remove small molecular weight DOC (345–688 Dalton) and a fluidised bed MIEXÒ contactor could remove more than 60% DOC from BTSE. The purolite resins, product of the Purolite Company, have been applied in several water treatment plants. Different types of purolite resins can be used to remove toxic ions such as ammonia, nitrate, cyanide, lead, iron, cerium [9–12]. However, there has not been much study on the use of purolite in removal of organic matter from wastewater. This main aim of this study is to evaluate the performance of different particles sizes of PuroliteÒA500PS in removing EfOM through batch and dynamic experiments. Several models

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(Langmuir, Homogeneous Surface Diffusion) were used to study the adsorption behavior of PuroliteÒA500PS.

Table 3 Physical properties of different sizes of puroliteÒA500PS. Parameters

2. Materials and methods 2.1. Material 2.1.1. Wastewater Synthetic wastewater was used in this study. The compositions of the synthetic wastewater are presented in Table 1. It represents the BTSE [13]. The DOC concentration of this synthetic wastewater was about 10 mg/L and the pH was 7.3.

2.1.2. Purolite resin PuroliteÒA500PS resin was used in this study. This purolite is a premium quality macroporous type strong base anion exchange resin. The basic features of puroliteÒA500PS are presented in Table 2. In the experiment, purolite was ground by mortar and separated by sieve into four different sizes: smaller than 150 lm, 150–300 lm, 300–420 lm and 420–600 lm. The surface areas of different sizes of purolite were analyzed by an automated surface area analyzer (Micromeritics Gemini 2360, USA) by means of nitrogen (N2) adsorption–desorption. The specific surface area (SBET) was determined by a multipoint BET method. In order to calculate the micropore volume and micropore size, the Dubinin–Radushkevich (DR) method was applied. The Barret– Joyner–Halender (BJH) method was also used to estimate the mesopore volume and the average mesopore size for the mesopore region using nitrogen desorption data. The physical properties of different sizes of puroliteÒA500PS is presented in Table 3. As expected, the BET specific surface area of puroliteÒA500PS increased when it was ground into smaller sizes. The BET of the particles size of 420–600 lm was 20.7 m2/g and this value increased to 25.3 m2/g for the smallest particle size (below 150 lm). Similarly, the mesopore volume and average mesopore

Table 1 Constituents of the synthetic wastewater. Compounds

Weight (mg/L)

Beef extract Peptone Humic acid Tannic acid (Sodium) lignin sulfonate Sodium lauryle sulfate Acacia gum powder Arabic acid (polysaccharide) (NH4)2SO4 KH2PO4 NH4HCO3 MgSO4.3H2O

1.8 2.7 4.2 4.2 2.4 0.94 4.7 5.0 7.1 7.0 19.8 0.71

SBET (m2/g) VolumeDR-micro (cm3/g) VolumeBJH-meso (cm3/g) Diameter-DR (nm) Diameter-BJH (nm)

PuroliteÒA500PS <150 lm

150–300 lm

300–420 lm

420–600 lm

25.3 0.01

24.9 0.01

22.6 0.01

20.7 0.01

0.44

0.35

0.26

0. 19

0.87 8.97

0.87 7.95

0.87 7.33

0.87 7.6

size for the mesopore region also increased when puroliteÒA500PS was ground to smaller particle sizes (Table 3). 2.2. Experimental methods 2.2.1. Adsorption equilibrium experiments An adsorption equilibrium study was undertaken with different particle sizes of PuroliteÒA500PS. Different doses of PuroliteÒA500PS resin was added in 200 mL of synthetic wastewater. The flasks containing synthetic wastewater and different doses of resin were then mixed (Ratek Platform Mixer) at 110 rpm for 72 h continuously. Upon completion of the experiments, samples were filtered through a 0.45 lm filter and the DOC was analyzed. 2.2.2. Adsorption kinetics experiments Kinetics experiments were conducted to investigate the DOC removal rate of different particle sizes of PuroliteÒA500P. A fixed amount of resin (0.5 g/L) was added to a known quantity (200 mL) of synthetic wastewater and the concentration was measured at different times. Synthetic wastewater with predetermined amounts of PuroliteÒA500P resin was mixed at 110 rpm. Samples were collected at different times, from 5 to 420 min. The supernatant was filtered through a 0.45 lm filter before analyzing for DOC. 2.2.3. Fluidized bed contactor The effect of PuroliteÒA500PS ion exchange resin in a fluidized bed contactor (a continuous flow system) in removing organic matter was studied. Four different PuroliteÒA500PS particle sizes (<150, 150–300, 300–420 and 420–600 lm) were used to pack the fluidized bed column. A predetermined quantity of resin was added to a column having a diameter of 2 cm and a vertical length of 1.4 m. The wastewater was pumped through the resin at fluidization rates of 2, 6 and 10 m/h from bottom using a dosing pump and the effluent was collected at the top of the column. Samples were collected on an hourly basis. The fluidized bed height was measured using a measuring tape fixed to the column wall. The schematic diagram of the PuroliteÒA500PS fluidized bed column is shown in Fig. 1. 2.3. Analytical methods

Table 2 Characteristic of puroliteÒA500PS. Parameters

Values

Polymer structure

Microporous polystyrene crosslinked with divinylbenzene R-(Me)3N+ 16–49 mesh 63–70% 0.8 eq/L min 1.04 g/mL 0–14 (Stability) 100 °C

Functional group Screen size range Moisture retention Total capacity Specific gravity pH limit Temperature limit

2.3.1. Dissolved organic carbon (DOC) measurement DOC was measured after filtering samples through a 0.45 lm filter using a Multi N/C2000 TOC analyzer. 2.3.2. Molecular weight distribution For molecular weight distribution (MWD) analysis of organic matters, a high-performance liquid chromatography (HPLC, Jasco LC-2000Plus) with a size exclusion chromatography (SEC) column (Protein-pak 125, Waters Milford, USA) was used. The separation of different components in a mixture of sample was done in the SEC column on the basis of size. As the sample to be analyzed was subjected to the column, it was elutriated with a solvent,

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Ds qp

r ¼ rp ;

@q ¼ K f ðC  C s Þ @r

ð5Þ

where q is the adsorbed phase concentration of adsorbate, mg/g; Ds is the surface diffusion coefficient of adsorbate, m2/s; r is the radial distance from the center of adsorbent particle, Kf is the external mass transfer coefficient, m/s; qp is the density of adsorbent particle, kg/m3; C is the liquid phase concentration of adsorbate, mg/L; and Cs is the liquid phase concentration at particle surface, mg/L. 2.4.3. Mathematical modeling of batch Here following assumptions were used: (1) To simplify the system, we have assumed that the system followed complete back mixing of adsorbent and liquid in the reactor. (2) No adsorbents are withdrawn or added and (3) Adsorbents have the same residence time. The mass balance in the batch can be described as follows:

V

Fig. 1. Schematic diagram of PuroliteÒA500PS fluidized bed column.

which is normally an organic buffer. The organic components diffuse inside the column gel at rates depending on their molecular size. As they emerge from the column, they are detected by a UV spectrometer and/or a fluorescence spectrometer (Jasco FP-2020). This method is known as gel permeation chromatography. 2.3.3. Liquid chromatography-organic carbon detection Liquid chromatography-organic carbon detection (LC-OCD) can identify the different classes of organic compounds present in wastewater and it gives both qualitative and quantitative information of the organic matter. LC-OCD was employed in this study to obtain information of organic matter present in wastewater before and after treatment. 2.4. Modeling 2.4.1. Langmuir isotherm Adsorption equilibrium of PuroliteÒA500PS with organic matter can be described by the Langmuir isotherm:

qe ¼

qm bC e 1 þ bC e

ð1Þ

where Ce is the equilibrium concentration (mg/L), b is a Langmuir constant related to the binding energy of adsorption (L/mg), and qm is the saturated maximum monolayer adsorption capacity (mg/g). 2.4.2. Homogeneous surface diffusion model (HSDM) The HDSM used to study the PuroliteÒA500PS adsorption kinetics is presented in Eqs. (2)–(5). HSDM consists of a three-step process: (i) the adsorbate diffuses through a stagnant liquid film layer surrounding the adsorbent particle; (ii) the adsorbate adsorbs from the liquid phase onto the outer surface of the adsorbent particle; and (iii) the adsorbate diffuses along the inner surface of the adsorbent particles until it reaches its adsorption site [14,15]. The equation is expressed as:

! @q @ 2 q 2 @q ¼ Ds þ @t @r 2 r @r

ð2Þ

The above equation can be numerically solved using the following initial and boundary conditions:

t ¼ 0;

q¼0

ð3Þ

r ¼ 0;

@q ¼0 @r

ð4Þ

 @C b dq ¼0 þM dt @t

t ¼ 0;

C b ¼ C b0

ð6Þ ð7Þ

where Cb is the organic concentration in the bulk phase in the reactor, mg/L; Cb0 is the initial organic concentration in the bulk phase in the reactor, mg/L; V is the volume of the bulk solution in the reactor, m3; and M is the weight of purolite present in the reactor, g.  is the average value of adsorbed phase concentration of Here q adsorbate in the particle, mg/g, which is given in Eq. (8):

¼ q

3 R3p

Z

Rp

qr 2 dr

ð8Þ

0

The equilibrium relationship can be related with the Langmuir isotherm Eq. (1). 2.4.4. Mathematical modeling of fluidized bed The material balance of organic matter concentration in the fluidized bed can be described by Eq. (9) [16]:

 dC b Q M dq ¼  ðC in  C b Þ   V V dt dt

ð9Þ

where Cb is the organic concentration in the bulk phase in the reactor, mg/L; Cin is the organic concentration in the feeding tank, mg/L; Q is the flow rate, m3/s; V is the volume of the bulk solution in the reactor, m3; and M is the weight of purolite used, g. Here =dtÞ represents the adsorption of the organics onto purðM=VÞ  ðdq olite in suspension. The equilibrium relationship can be also related with the Langmuir isotherm (1). To solve the above mathematical model equations in this study, namely the HSDM, for the batch and the fluidized bed, the orthogonal collocation method (OCM) and the variable coefficient ordinary differential equation solver (VODE) were used. In addition, the Nelder–Mead simplex method, which is based on minimizing the differences between the experimental and predicted results, was used to obtain the best fit parameters (Ds and Kf). In this study, the following object function was used.

Minimum ¼

X ðC exp;j  C cal;j Þ2

ð10Þ

j

where Cexp,j is the experimental concentration data and Ccal,j is the calculated concentration result. A detailed explanation is given elsewhere [17–20]. The value of Ds is dependent on the organic concentration. In batch mode Kf is a function of the agitation speed and in a fixed or fluidized bed it is a function of Reynolds (Re) and Schmidt (Sc) numbers. It is difficult to obtain the exact values of these parame-

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ters because wastewater (synthetic or real) used in this study contains various components. Those unknown parameters (Ds and Kf) were first optimized by matching the experimental batch kinetic data with theoretical kinetic equations. To simulate the fluidized bed experiments, the Ds values derived from the experimental batch kinetic data may be used as it is a function of concentration. Kf will not be the same as that derived from batch kinetics data. Kf was determined by matching the fluidized bed experimental data with the theoretical equations.

3. Results and discussion 3.1. Adsorption Equilibrium Adsorption equilibrium experiments were conducted to predict the adsorption capacity of PuroliteÒA500PS resin of different particle sizes. In experiments, the dose of PuroliteÒA500PS was kept in the range of 0.05–3.75 g/L. The equilibrium results were then fitted with the Langmuir isotherm equation. In this work the constants, qm and b, were estimated using a nonlinear optimization approach. The simulations of adsorption equilibrium with synthetic wastewater are shown in Fig. 2. The isotherm parameters for the model are shown in Table 4. It can be seen from both Fig. 2 and Table 4 that the adsorption capacity (qm) of purolite increase when the size of purolite is smaller. This can be explained by the increase of surface area and pore

volume of purolite when it was ground to smaller sizes. The surface area of purolite increased 12% when the size of purolite decreased from 300 to 420 lm to below 150 lm (Table 3). In addition, after grinding some active sites of purolite could be more reachable or would have been created or modified. As a result, the DOC removal efficiency increased significantly. 3.2. Adsorption kinetics In the adsorption kinetics, the DOC removal efficiencies increased with smaller purolite particle sizes. As shown in Fig. 3, the simulation curve fitted well with the experimental values. The value of the surface diffusion coefficient Ds varied from 5.01  1012 m2/s to 1.10  1013 m2/s. The external mass transfer coefficient Kf calculated for different particles sizes of PuroliteÒA500PS is summarized in Table 5. Increasing the particle sizes marginally reduces the external mass-transfer coefficients as noted by Choy et al. [21]. This overall trend is also observed in Table 5. The smaller sorbent particle travel faster in a agitated solution and experiences more shear at the particle surface, therefore reducing the boundary layer film. The results show that the surface diffusion Ds is a function of the equilibrium concentration Ce (Fig. 4). This value of Ds was larger with an increase of Ce. The relationship of solid diffusion Ds and equilibrium concentration Ce can be classified into two parts: When Ce was below 3 mg/L the relationship can be expressed as (Eq. (1) in Fig. 4):

Ds ¼ 1:17314  C 5e :901: When the equilibrium concentration Ce was over 3 mg/l, Ds was less affected by Ce. The relationship between Ds and Ce when Ce is above 3 mg/L can be expressed as (Eq. (2) in Fig. 4):

Ds ¼ 3:55512  C 0:195 e 3.3. PuroliteÒA500PS fluidized bed 3.3.1. Effect of particle size of PuroliteÒA500PS on DOC removal Fig. 5 shows the DOC removal efficiency of the fluidized bed column when packed with different size of PuroliteÒA500PS. In this experiment, 28 g of PuroliteÒ A500PS was packed in a column and the upflow filtration velocity was kept at 6 m/h. Here the up-

1.4

size <150 um size = 150~300 um

1.2

size = 300~420 um size = 420~600 um

1.0

C/C0

originnal size Fig. 2. Adsorption equilibrium curves for different sizes of puroliteÒA500PS resin (amount of PuroliteÒA500PS: 0.05–5 g/L, initial DOC of synthetic wastewater: 10 mg/L).

Prediction with HSDM

0.8 0.6 0.4

Table 4 Isotherm coefficients for DOC removal with synthetic wastewater with different sizes of puroliteÒA500PS.

<150 Langmuir

0.0

PuroliteÒA500PS size (lm)

Parameters

qm b r2

50.9 0.146 0.99

150–300 38.1 0.135 0.96

300–420 32.4 0.125 0.98

0.2

420–600 21.1 0.119 0.82

Unground purolite 18.9 0.1 0.84

0

2

4

6

8

Time, h Fig. 3. Adsorption kinetics of synthetic wastewater at different particles sizes of PuroliteÒA500PS (initial DOC: 10 mg/L; PuroliteÒA500PS concentration 0.5 g/L).

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R.T. Ahmad et al. / Separation and Purification Technology 98 (2012) 46–54

Table 5 The mass transfer coefficients in synthetic wastewater at different particles sizes of PuroliteÒA500PS. PuroliteÒA500PS size (lm)

Ce (mg/L) Kf (m/s) Ds (m2/s)

<150

150–300

300–420

420–600

Unground purolite

1.530 2.95  104 1.10  1013

1.783 5.49  104 5.15  1013

2.763 9.50  104 4.29  1012

4.857 9.75  105 4.99  1012

6.493 6.85  106 5.01  1012

1E-10

Table 6 PuroliteÒA500PS fluidized bed depths, detention times and DOC removal efficiencies with different PuroliteÒA500PS sizes.

Eq 2

Ds, m2/s

1E-11

PuroliteÒA500PS size (lm)

Bed height before fluidization (cm)

Bed height during fluidization (cm)

DOC removal efficiency after 1 h (%)

Detention time (min)

<150 150–300 300–420 420–600

23 17 15 13

72–10a 35 24 20

77.2 74.5 71.4 41.0

7.2–1.0 3.5 2.4 2.0

1E-12

Eq 1 1E-13

a Bed height changed from 72 to 10 cm after 1 h of operation with PuroliteÒA500PS particle sizes <150 lm.

1E-14 Ds

v¼

1E-15 0

1

10

Q

pD2

ð11Þ

100

Ce, mg/l

where v is the upflow filtration velocity (m/h), Q is the flow rate (m3/h), and D is the inside diameter of fluidized column (m)

Fig. 4. Variation of surface diffusivity with equilibrium concentration.

t¼

Fig. 5. Effect of PuroliteÒA500PS size on DOC removal efficiency (Initial DOC of synthetic wastewater: 10 mg/L, fluidization velocity: 6 m/h, amount of PuroliteÒA500PS used: 28 g).

flow filtration velocity refers to vertical fluidization velocity and is typically much larger that the minimum fluidization velocity. This is evidenced by the bed expansion of at least 60% (Table 6). The results show that smaller sizes generally are better in removing DOC from synthetic wastewater. The reason for this phenomenon is firstly due to the higher surface area (SBET) and mesopore volume (VBJH-meso) of a smaller particle size PuroliteÒA500PS (Table 3). In addition, during the fluidizing process, the fluidized bed depth of a smaller particle size was also larger than that of a bigger particle size. This led to a longer detention time for a fluidized bed with a smaller particle size PuroliteÒA500PS. The detention times and bed depths of PuroliteÒA500PS before and during fluidization are presented in Table 6. Here the upflow filtration velocity and detention time can be calculated by Eqs. (11) and (12) respectively.

H

v

ð12Þ

where t is the detention time (h), H is the column bed height (m), and v is the upflow filtration velocity (m/h) During the first hour of operation, the smallest size (<150 lm) PuroliteÒA500PS (corresponding to a detention time of 7.2 min) removed 77.2% DOC but after that the DOC removal efficiency decreased dramatically. The reason for this phenomenon was the rapid escape or washout of PuroliteÒA500PS particles from the column at a upflow filtration velocity of 6 m/h. As a result, after 6 h of experiments, only 9% of PuroliteÒA500PS particles remained in the column (Table 6). The PuroliteÒA500PS of 150–300 lm particle size was more consistent in removing DOC till the end of the experiment and it removed almost 74.5% DOC continuously during a 6 h experiment. Thus the particle sizes of 150–300 lm was used in the subsequent experiments. As expected, larger sizes of 300–420 lm and 420– 600 lm resulted in lower DOC removal efficiency of 71.4% and 41.0% respectively in the first hour of experiments and these values decreased slightly during the experiments.Fig. 6 shows the model simulation with different particle size of PuroliteÒA500PS. The model parameters are given in Table 7. Here, values of Ds were determined from the batch kinetic data and the assumption that it was related to the equilibrium concentration Ce was based on the experimental results (Fig. 4). The amount of purolite after the fluidised bed experiment was also taken into account in this modeling. On the whole, the DOC removal efficiency was closely related with the adsorption equilibrium amount.

3.3.2. Effect of fluidized bed depth The effect of fluidized bed depth of PuroliteÒA500PS in the column on the organic removal efficiency was experimentally investiÒ gated. The Purolite A500PS bed depths were 4.5, 9, 17 and 26 cm (corresponding to 9.3, 18.6, 28 and 37.3 g of PuroliteÒA500PS) before fluidization. During 5 h of experiment, approximately 9 L of

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Table 8 Summary of PuroliteÒA500PS fluidized bed depth, DOC removal efficiency and detention time in terms of bed height.

1.50

1.25

C/C0

1.00

420~600 um

300~420 um

150~300 um

<150 um

pre420-600

pre300-420

pre150-300

pre150

0.75

Bed height before fluidization (cm)

Bed height during fluidization (cm)

DOC removal efficiency after 1 h (%)

Detention time (min)

4.5 9 17 26

16.5 22 35 45

67.5 73.4 75.5 78.7

1.6 2.2 3.5 4.5

0.50

0.00 0

1

2

3

4

5

6

Time, h Fig. 6. Fluidized bed model simulation with different particle size (initial DOC of synthetic wastewater: 10 mg/L, fluidization velocity: 6 m/h, amount of PuroliteÒA500PS used: 28 g).

Table 7 The mass transfer coefficients in synthetic wastewater at different particles sizes of PuroliteÒA500PS (bed depth: 17 cm, Upflow filtration: 6 m/h). PuroliteÒA500PS particles sizes (lm)

Ce (mg/L) Kf (m/s) Ds (m2/s)

<150

150–300

300–420

420–600

7.05 1.81  103 5.18  1012

2.48 1.78  103 2.49  1012

4.02 1.65  103 4.65  1012

5.11 1.53  103 4.87  1012

synthetic wastewater was passed through the column at constant upflow filtration velocity of 6 m/h. As can be seen from Fig. 7, a larger bed height led to higher DOC removal from synthetic wastewater due to a longer detention time. The DOC removal efficiency reduced from 78.7% at a bed height of 26 cm to 67.5% at a bed height of 4.5 cm after the first hour of experiment. In addition, the smaller bed height also led to a faster decline of DOC removal efficiency due to the faster saturation of PuroliteÒA500PS. In experiments with a larger bed height of 26 cm, the DOC efficiency of PuroliteÒA500PS fluidized column still remained high (71.6%) until 5 h (or 5 h). This bed height at a upflow filtration velocity of 6 m/h corresponds to a detention time

of 4.5 min through the fluidized column (Table 8). However, with a bed height of 9 and 4.5 cm the DOC removal efficiency decreased faster to 57% and 54% respectively as the experiment progressed (corresponding to a detention time of 2.2 and 1.6 min respectively, Table 8). On the other hand, it can be observed from Fig. 7 that there is not much difference in DOC removal efficiency at bed heights of 17 and 26 cm during 5 h of experiment. The bed depth of 17 cm (28 g) of PuroliteÒA500PS was used in subsequent experiments.Fig. 8 shows the model simulation for different bed depths of PuroliteÒA500PS. The model parameters are given in Table 9.

3.3.3. Effect of upflow filtration Influent flow rate or upflow filtration velocity is one of the factors that can directly affect the quality of treated effluent during the treatment process. The effects of different fluidization velocities on the DOC removal of PuroliteÒA500PS fluidized column was determined experimentally. The experiments were run with different fluidization velocities of 2, 6 and 10 m/h. Experiments were operated for 6 h and samples were collected after every hour.

1.50

37.3 g 18.6 g pre 37.3 pre 18.6

1.25

1.00

C/C0

0.25

28.0 g 9.3 g pre 28 pre 9.3

0.75

0.50

0.25

0.00 0

1

2

3

4

5

6

Time, h Fig. 8. Fluidized bed model simulation with different bed depth (PuroliteÒA500PS particle size: 150–300 lm, initial DOC of synthetic wastewater: 10 mg/L, fluidization velocity: 6 m/h).

Table 9 The mass transfer coefficients in synthetic wastewater with different bed depth of PuroliteÒA500PS (particle size 150 lm, Upflow filtration: 6 m/h). Bed depth (cm) Fig. 7. Comparison of DOC removal efficiency of PuroliteÒA500PS fluidized bed for synthetic wastewater with different bed heights (PuroliteÒA500PS particle size: 150–300 lm, initial DOC of synthetic wastewater: 10 mg/L, fluidization velocity: 6 m/h).

Ce (mg/l) Kf (m/s) Ds. (m2/s)

9.3

18.6

28.0

37.3

3.37 1.01  103 4.49  1012

3.28 1.01  103 4.47  1012

2.48 4.50  103 2.49  1012

2.56 5.61  103 3.03  1012

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from 3.5 min to 1.2 min (Table 10). As a result, the DOC removal at an upflow filtration velocity of 10 m/h was only 56.1%. Further at a high upflow filtration velocity of 10 m/h, more PuroliteÒA500PS was also observed to washout from the column (only 30% of PuroliteÒA500PS left after the experiment). This led to a decrease in DOC removal to only 32% by end of the experiment.The Fig. 10 shows the model simulation with different upflow filtration velocity of PuroliteÒA500PS. The model parameters are given in Table 11. In summary the change of upflow filtration velocity or flow rate of the feed water and bed height of the PuroliteÒA500PS resin during operation affected the organic removal. As expected, PuroliteÒA500PS fluidized column resulted in a good DOC removal when it was operated at a low upflow filtration velocity and a large bed height. Fig. 9. Comparison of DOC removal efficiency of PuroliteÒA500PS fluidized bed with synthetic wastewater at different fluidization velocities (PuroliteÒA500PS particle size: 150–300 lm, initial DOC of synthetic wastewater: 10 mg/L, amount of PuroliteÒA500PS used: 28 g).

The experimental results from Fig. 9 reveals that at a lower upflow filtration velocity of 2 m/h, PuroliteÒA500PS removed more DOC (80%) from synthetic wastewater in a consistent manner over the entire period of experiment as compared to a higher upflow filtration velocity(10 m/h). The DOC removal efficiency dropped from 78.5% to 74.5%, when the upflow filtration velocity increased from 2 m/h to 6 m/h. At an upflow filtration velocity of 6 m/h, the performance of the PuroliteÒA500PS column remained constant during the experimental period. When the upflow filtration velocity was increased from 6 m/h to 10 m/h, the detention time decreased

Table 10 Summary of PuroliteÒA500PS fluidization bed depth and DOC removal efficiency. Upflow filtration velocity (m/ h)

Bed height before fluidization (cm)

Bed height during fluidization (cm)

DOC removal efficiency after 1 h (%)

Detention time (min)

2 6 10

17 17 17

45 35 20

78.5 74.5 56.1

13.5 3.5 1.2

3.4. Long term performance of PuroliteÒA500PS fluidized column with synthetic wastewater In this study, a PuroliteÒA500PS fluidized bed column was operated for an extended period (5 days) to evaluate its performance in removing DOC over a long period. PuroliteÒA500PS resin with particle size of 150–300 lm was employed in this experiment because previous results showed that this size led to a superior DOC removal from synthetic wastewater. Experiments were carried out at different fluidization velocities of 2, 6 and 10 m/h and the total amounts of wastewater treated were 75, 225 and 365 L respectively over 5 days of operation. An amount of 28 g (bed height of 17 cm) of PuroliteÒA500PS resin was packed in the column. The effectiveness of upflow filtration velocity on the performance of PuroliteÒA500PS fluidized bed column in removing DOC from synthetic wastewater is presented in Fig. 11. The results show that a lower upflow filtration velocity of 2 m/h led to a higher DOC removal with a 67% DOC removal after 1350 bed volume. Due

Table 11 The mass transfer coefficients in synthetic wastewater with different velocity of PuroliteÒA500PS (particle size 150 lm, bed height: 17 cm). Velocity (m/h)

Ce (mg/l) Kf (m/s) Ds (m2/s)

2

6

10

1.47 1.03  103 1.12  1013

2.48 4.51  103 2.49  1012

4.24 4.26  106 4.70  1012

1.50

1.25

10 m/hr

6 m/hr

2 m/hr

pre 10

pre 6

pre 2

C/C0

1.00

0.75

0.50

0.25

0.00 0

1

2

3

4

5

6

Time, h Fig. 10. Fluidized bed model simulation with different velocity (PuroliteÒA500PS particle size: 150–300 lm, initial DOC of synthetic wastewater: 10 mg/L, amount of PuroliteÒA500PS used: 28 g).

Fig. 11. DOC removal efficiency of PuroliteÒA500PS fluidized bed with synthetic wastewater (PuroliteÒA500PS particle size: 150–300 lm, Initial DOC of synthetic wastewater: 10 mg/L, amount of PuroliteÒA500PS used: 28 g).

R.T. Ahmad et al. / Separation and Purification Technology 98 (2012) 46–54

53

to the slow upflow filtration, most of PuroliteÒA500PS remained inside the column during the 5 days of operation. Thus the DOC removal efficiency during the treatment of the first 800 bed volumes was consistent with more than 80% DOC removal. The performance at the upflow filtration velocity of 6 m/h was also good with more than 55% DOC removal even after treating 1620 bed volumes. After this stage, DOC removal efficiency reduced gradually not only due to the saturation of the PuroliteÒA500PS but also due to the washout of some of the PuroliteÒA500PS from the column. The DOC removal efficiency of the PuroliteÒA500PS fluidized bed column operated at a higher fluidized velocity of 10 m/h was too low because of the large amount of PuroliteÒA500PS washed off (9% left after five days of operation) and the short detention time. Therefore, the DOC removal efficiency after treating 1400 bed volumes was only 30%.Fig. 12 shows the model simulation for long term operation of PuroliteÒA500PS fluidized bed column. The model parameters are given in Table 12. The MWD of organic matter in the synthetic wastewater before and after treatment by the PuroliteÒA500PS fluidized bed was analyzed. Fig. 13 indicates that the MWD of synthetic wastewater ranged mostly from 890 to 130 Da. The MWD of organic matter following the treatment of PuroliteÒA500PS fluidized bed after a long period of operation (1 and 5 days) showed that low and medium fluidization velocities were more effective in removing both large and small MW compounds, whereas a high fluidized velocity of 10 m/h is less effective in organic removal because resin washed out from the column. Liquid Chromatography-Organic Carbon Detection (LC-OCD) was used to measure the MW to identify the different classes of organic compounds present in wastewater before and after treatment. The synthetic wastewater was found to contain mainly

1.50

10 m/hr 2 m/hr pre 6

1.25

6 m/hr pre10 pre 2

C/C0

1.00

0.75

0.50

0.25

0.00 0

30

60

90

120

150

Time, h Fig. 12. Fluidized bed model simulation of long term operation (PuroliteÒA500PS particle size: 150–300 lm, initial DOC of synthetic wastewater: 10 mg/L, amount of PuroliteÒA500PS used: 28 g).

Table 12 The mass transfer coefficients in synthetic wastewater of long term operation of PuroliteÒA500PS (particle size 150 lm, Upflow filtration: 6 m/h). Fluidization velocities (m/h)

Average bed height during fluidization (cm)

Ce (mg/l)

2 6 10

32 20 5

1.06 2.02 4.24

Kf (m/s)

8.80  105 1.95  103 3.26  106

Fig. 13. MW distribution of synthetic wastewater before and after PuroliteÒA500PS fluidized bed (PuroliteÒA500PS particle size: 150–300 lm, initial DOC of synthetic wastewater: 10 mg/L, amount of PuroliteÒA500PS used: 28 g): (a) Fluidization velocity: 2 m/h; (b) Fluidization velocity: 6 m/h; (c) Fluidization velocity: 10 m/h.

Ds (m2/s)

1.65  1014 7.25  1013 4.70  1012

hydrophilic organic compound (82.1%) in which the percentage of humic substances (MW  1000 Da), low molecular weight neutrals (MW < 350 Da) and building blocks (MW  350–500 Da) were 38.8%, 17.9% and 15.5% respectively. The biopolymers with MW of about 20,000 Da only accounted for 9%. Table 13 shows the DOC re-

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R.T. Ahmad et al. / Separation and Purification Technology 98 (2012) 46–54

Table 13 Organic fraction removal efficiency (%). Feed water Ò

Purolite A500PS

DOC (%)

Hydro-phobic (%)

Hydro-philic (%)

Bio-polymer (%)

Humic (%)

Building blocks (%)

Neutrals (%)

73.1

55

76.4

98.5

86.5

42.3

83.3

moval of a PuroliteÒA500PS fluidized bed at a upflow filtration velocity of 6 m/h after 1 day of operation. The PuroliteÒA500PS fluidized bed could remove 55% of hydrophobic and 76.4% of hydrophilic matter. In particular, most of the biopolymer, humics and low MW of neutral were removed (98.5%, 86.5% and 83.3% respectively).

4. Conclusions PuroliteÒA500PS fluidized bed can effectively remove organic matter from wastewater. The removal efficiency is strongly dependent on operational conditions (bed depth and upflow filtration). The results show that larger bed depths and lower fluidization velocities led to a superior DOC removal from synthetic wastewater. The particle size of puroliteÒA500PS had a strong effect on DOC removal. PuroliteÒA500PS of a small particle size of 150–300 lm had the highest DOC removal. At a upflow filtration velocity of 6 m/h and bed height of 17 cm, it can remove a majority of hydrophilic compounds (76.4%) including biopolymer (98.5%), humic substances (86.5%), and low molecular weight of neutrals (83.3%). Acknowledgments This research has been funded by an Australian Research Council (ARC) Discovery Grant. The supply of purolite by Vitachem is gratefully acknowledged. References [1] C.Y. Chang, Y.H. Hsiehb, Y.M. Linb, P.Y. Hub, C.C. Liu, K.H. Wang, The organic precursors affecting the formation of disinfection by-products with chlorine dioxide, Chemosphere 44 (2001) 1153–1158. [2] L. Fan, J.L. Harris, F.A. Roddick, N.A. Booker, Influence of the characteristics of natural organic matter on the fouling of microfiltration membranes, Water Res. 35 (2001) 4455–4463. [3] A. Imai, T. Fukushima, K. Matsushige, Y.H. Kim, K. Choi, Characterization of dissolved organic matter in effluents from wastewater treatment plants, Water Res. 36 (2002) 859–870.

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