Using SDI, SDI+ and MFI to evaluate fouling in a UF

Desalination 285 (2012) 153–162

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Using SDI, SDI + and MFI to evaluate fouling in a UF/RO desalination pilot plant A. Alhadidi a, A.J.B. Kemperman a,⁎, R. Schurer b, J.C. Schippers c, M. Wessling a, W.G.J. van der Meer a a b c

Membrane Technology Group, MESA+ Institute for Nanotechnology, Faculty of Science and Technology, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands Evides Waterbedrijf, Berenplaat 10, 320 LB Spijkenisse, The Netherlands UNESCO-IHE Delft, Westvest 7, P.O. Box 3015, 2601DA Delft, The Netherlands

a r t i c l e

i n f o

Article history: Received 30 March 2011 Received in revised form 29 September 2011 Accepted 29 September 2011 Available online 16 November 2011 Keywords: Silt Density Index Normalized SDI Case study RO feed water fouling potential

a b s t r a c t This paper assesses the performance of a UF/RO demonstration plant located in the Oosterschelde estuary in the south-western part of the Netherlands. Spring blooms in the seawater pose a challenge to the plant because of the resulting increased fouling potential of the water. Determinations of the fouling indices SDI, SDI + and MFI0.45 were carried out at the plant with different operational conditions, such as of coagulant addition and pH correction. Eight different membranes were used in the tests. In general, the UF performance was found to be good as the SDI values were around 1, provided standard membranes were used, and the MFI0.45 values lower than 1 s/L2. The MFI0.45 showed the same tendency as the SDI in most cases. As expected, whereas the SDI showed marked sensitivity to used membrane type and operational conditions, the SDI + did not display this dependency and hence appear to be a more reliable fouling index than the SDI. Storing the RO feed overnight in the feed tank increased the fouling potential of the RO feed, likely caused by continued coagulation. © 2011 Elsevier B.V. All rights reserved.

1. Introduction An alternative to established desalination methods like vacuum distillation and multi-stage flash distillation is reverse osmosis (RO). It can be used to separate dissolved solutes from brackish water and seawater in the preparation of drinking water. RO is more energetically favorable as no phase transformation is required, only electrical energy to drive the pumps to overcome the osmotic pressure, but is hampered by membrane fouling. Several types of fouling can occur in RO membrane systems, e.g. inorganic fouling or scaling, particulate and colloidal fouling, organic fouling, and biofouling. This results in a decline of the permeate flux and loss of product quality. To predict the colloidal fouling behavior of RO feed water, fouling indexes can be used. The Silt Density Index (SDI) and the Modified Fouling Index (MFI0.45) are the two most popular fouling indexes. The ASTM established a standard protocol for measuring the SDI [1]. From a practical point of view and the membrane manufacturers' specifications, the SDI of fine hollow fiber membrane RO feed water preferably should be lower than 3 [2]. The SDI has been routinely applied worldwide for many decades, and is currently considered the ultimate test for measuring the fouling potential of feed water for reverse osmosis and nanofiltration membranes. Main advantage of the SDI test is that it is simple to execute even by non-professionals. However, there are growing doubts about the reliability of SDI results, e.g. several manufacturers of micro-and ultrafiltration membranes are

⁎ Corresponding author. Tel.: + 31 53 4892956; fax: + 31 53 4894611. E-mail address: [email protected] (A.J.B. Kemperman). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.09.049

frequently confronted with the phenomenon that the SDI of the filtered water is above 3. From a theoretical point of view, it is hard to explain that water filtered through membranes with pores smaller than 0.02 μm has an SDI higher than 3. Therefore, these observations may have to be attributed to deficiencies of the SDI test such as that it does not correct for variations in pressure, temperature, and pore size and membrane resistance of the used filters [3–7].Consequently, in the most recent standard (D 4189–07) ASTM states that the SDI is not applicable for the effluents from most RO and ultrafiltration (UF) systems. Therefore, we previously proposed corrections for the SDI, resulting in a normalized fouling index called SDI+ [8]. The MFI0.45 is based on the cake filtration model, and can be corrected for pressure and temperature. However, measuring an MFI0.45 is more difficult and not very suitable for being performed in the field due to the need for accurate and expensive equipment to collect the filtration data ( V , t) [9–11]. We examined the performance of a UF unit with RO pretreatment by using the SDI test with different operational regimes. We also studied the effect of using different commercial membranes with a 0.45 μm pore size. We normalized the SDI results for testing parameters and membrane resistance (leading to the SDI+) with a model for the relation between the SDI and the MFI0.45 [12]. In addition, we assessed the sensitivity of SDI to the particle concentration experimentally. 1.1. Plant description The seawater UF/RO demonstration plant is located in the southwestern part of the Netherlands, and operated by water supply company Evides. The site is in the Oosterschelde estuary, as shown in

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Fig. 1. The net desalinated water production capacity of the plant is 13.5 m 3/h. Fig. 2 shows the flowchart of the UF/RO demonstration plant. Via an open intake, the seawater is pumped to a buffer tank inside the plant. The raw water is filtered with a 50 μm micro-strainer followed by an optional coagulation and acidification step, in which coagulant and acid are added to the mixing tank. Both dosages are applied according to actual raw water quality and resultant UF behavior. After this mixing step, the UF feed is collected in a buffer tank and then pumped to the UF unit, which contains Norit Seaguard UF membranes. UF permeate is collected in another buffer tank and then pumped to the RO unit. Part of the UF permeate is used for UF backwash. Due to backwash discharge regulations, backwash treatment and recycling is mandatory and consists of application of coagulant in a sedimentation process, followed by partial recycling to the UF buffer tank. Two RO stages operate in the plant: a seawater RO step for desalination and a subsequent, seasonally required, brackish-water RO step for boron removal (Dow Filmtec, which requires an SDI of less than 5 [13]). The RO permeate for the first RO stage is collected in a buffer tank and then pumped to the second stage. To protect the pumps and membrane units, each feed tank is followed by cartridge filter. For research and analysis purposes, there are 54 sampling points in the plant. The water samples for our SDI tests were collected at the sampling points shown in Fig. 2. 1.2. Raw water characteristics The plant faces an algae bloom challenge for three months in the spring season. During their active period, the algae produce a polymeric material called Transparent Exopolymer Particles (TEP). This causes fouling in UF and RO membrane installations [14–17]. Fig. 3 shows a large amount of foam generated by the algae close to the Jacobahaven plant intake in May 2010. 1.3. Fieldwork and plant operation The spring period is the most critical operation period due to biological activity in the raw water, hence this field work was carried out from 6 April 2010 to 13 May 2010. During our study, the plant experienced a great variation in the raw water quality. Several quality parameters were monitored such as turbidity, pH and temperature. The raw water temperature during the field work varied between 8 °C and 10 °C at sampling point 20 (see Fig. 2). The raw water was filtered by a strainer to remove colloidals larger than 50 μm. The turbidity varied between 5 and 40 FTU (Formazin Turbidity Unit), with an average of approximately 10 FTU.

The raw water had a pH of 8 to 8.4 during the fieldwork period, and the plant was operated with ferric coagulant during the entire fieldwork. When necessary, the pH was adapted to the pH required for the coagulation process (pH 8) or to prevent RO scaling (pH 6.6) [18]. The pH was also changed in order to study the effect of the pH on UF operation. The coagulant dosage was varied between 2 and 3.5 g Fe/m 3 depending on the degree of fouling (Table 1). The variations in the operational conditions were not only due to algal blooms resulting in extreme UF fouling, but also to accidental damage to some of the plant equipment. UF performance was monitored continuously as permeability, transmembrane pressure (TMP) and flux, as plotted in Fig. 4. Fig. 4 describes the operational conditions of the UF unit, with a near-constant flux of 55 L/h/m 2 (LHM) and permeability maintained between 200 and 400 L/h/m 2 bar. During filtration, the transmembrane pressure increased from 120 mbar to 400 mbar to maintain a constant flux. A chemically enhanced backwash cleaning, using regular chemicals (HCl, NaOH and NaClO) was performed at 200 LHM and 1 bar [19]. 2. Theory and background 2.1. SDI definition To determine the SDI, the rate of plugging of a membrane filter with pores of 0.45 μm at 207 kPa is measured. The measurement is done as follows. a) The time t1 is determined, which is the time required to filter the first 500 mL. b) 15 min (tf) after the start of this measurement, time t2 is measured which is the time required to filter another 500 mL. c) The index is calculated with the following formula:

SDI ¼


 100% t %P 1− 1 ¼ tf tf t2

ð1Þ

Here, SDI is the Silt Density Index in%/min. If the plugging ratio (%P) exceeds 75%, a shorter period tf has to be taken, e.g. 10, 5 or 2 min. 2.2. MFI The Modified Fouling Index (MFI0.45) was derived by Schippers and Verdouw in 1980 [20]. For determination of the MFI0.45, the flow through the membrane filter is measured as a function of time. t μ⋅RM μ⋅I ¼ þ ⋅V V dP⋅A 2⋅ΔP⋅A2M

Fig. 1. Location of Jacobahaven in Zeeland, the Netherlands. Sources: Google.

ð2Þ

A. Alhadidi et al. / Desalination 285 (2012) 153–162

Acid+ coagulant dosage and mixing

Buffer tank

Filter

Cartridge

155

Buffer tank

UF

Strainer 20

Intake

30

23

21

31

32

UF backwash UF backwash treatment Cartridge

Buffer tank

RO1

37

47 48

Cartridge

RO2

Production tank

54

RO concentrate

Fig. 2. The flowchart of the UF/RO demonstration plant. The numbers indicate the sampling points used.

Where V t AM dP μ RM I

accumulated filtrate volume (L or m 3) time (s) membrane area (m 2) applied pressure (Pa) water viscosity (Pa.s) clean membrane resistance (m −1) fouling potential (m −2)

The MFI0.45 is derived from the slope of t/V versus V. This slope tgα is by definition equal to MFI0.45, when it has its minimum and under the conditions that the temperature is 20 °C, the pressure is 207 kPa and the membrane surface area equals 13.8 × 10− 4 m 2 (47 mm diameter). The MFI0.45 is corrected for T and P, and is therefore independent of temperature and pressure. 2.3. SDI + The relation between the SDI and the MFI0.45 was described before [12]. It was used for normalizing the SDI to the reference testing condition parameters and the membrane resistance. By definition, the SDI has no linear relationship with the colloidal concentration, but the fouling index (I) as used in the MFI definition does. The SDI test yields the filtration data t and V, more specifically times t1 and t2 for collecting sample volumes V1 and V2. Furthermore, the membrane resistance RM is considered as R at t = 0. The SDI-measured is determined using Eq. (1). The fouling parameter I is estimated from the curve of t/V versus V. By incorporating I, RM and the reference testing

Table 1 Evides UF/RO plant operation data for the period 6 April 2010 to 12 May 2010.

Fig. 3. Open intake of the Jacobahaven plant, showing the foam caused by algae.

Raw temperature [°C]

Strained water turbidity [FTU]

UF permeate pH

Coagulation dosage

8–10

5–20

8 or 6.6

2.5–3.5

[g Fe/m3]

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16

filter, the water to be tested was flushed through the apparatus in order to remove any contaminants. From the raw data from the SDI setup, the membrane resistance and filtered volume were calculated.

14

Permeability

12 10

3

8 2

6 TMP

4

UF flux [×102 LHM]

4

TMP [×102 mbar]

UF permeability [×102 LHM at 1 bar]

5

3.2. Membranes Eight different 0.45 μm MF membranes were used in this study, including membrane filters meeting the ASTM standards (Table 2). 3.3. Reference testing parameters

1 2 0

Flux

08-Apr 12-Apr 16-Apr 21-Apr 11-May 12-May

In order to study the effect of the testing condition parameters, the following reference testing parameters were defined (Table 2).

0

Fig. 5 schematically describes the methods for determining SDImeasured and SDI +.

a. Clean-water membrane resistance RM: In the updated (2007) version of the ASTM standard, the membrane filter was better specified. The pure water flow time should be 25–50 s for 500 mL under an applied pressure of 91.4–94.7 kPa. The calculated specific/clean water membrane resistance RM subsequently ranges from 0.39 × 1010 to 2.65 × 1010 m−1. An average value of 1.29 × 1010 m −1 is defined as the reference membrane resistance RMO. b. Feed temperature T: The reference temperature was set to 20 °C. c. Applied pressure dP: The standard ASTM pressure of 207 kPa was defined in this study as reference pressure. d. Membrane area AM: A diameter of 47 mm was considered standard membrane size, implying the reference membrane area AMO was equal to 13.4 × 10 −4 m 2. Different membrane diameter can be used (25 mm and 90 mm). e. SDI: The commonly applied limitation for RO feed water, SDIO = 3, was defined as the target value.

3. Material and methods

4. Results and discussion

3.1. SDI setup

4.1. SDI determination using different membranes

Fig. 6 shows the apparatus used for the SDI tests. The applied pressure was maintained by the feed pump in the automatic setup. The feed tank was insulated to keep the water temperature constant (±1 °C) throughout the test. A 0.45 μm membrane filter (25 mm in diameter) was placed on the support plate of the holder. The membrane filter was touched only with tweezers to avoid puncturing or contamination. It was checked whether the O-ring was in a good condition and properly placed. The trapped air was bled out through a relief air valve in the filter holder. Before installing the membrane

The latest version of the ASTM standard considers cellulose acetate/ cellulose nitrate membranes the standard membranes to be used in SDI tests. However, 0.45 μm MF membranes can be obtained from several manufacturers, all with different properties and various membrane materials. Table 2 lists the different membranes used in this study. They differ in properties such as pore size distribution, porosity, surface charge, roughness, hydrophilicity and cross-sectional morphology (depth filter, tracked-edge, spongy, or reinforced). In an earlier discussion of the influence of membrane properties on the SDI [21], it was suggested to use the membrane resistance as a lump sum parameter representative of the physical properties of the membrane. For a comparison between the SDI values obtained with the different membranes, a constant feed quality was necessary. For each SDI test with a 25-mm-diameter membrane, 6 to 7 L of feed water was consumed, and 60 L was required for the full set of experiments. To assure a constant feed water quality, a feed tank (60 L) was filled batch-wise with UF permeate within several minutes. The SDI values determined on 11 May 2010 and 13 May 2010 for the UF permeate using different membranes are plotted versus the membrane resistance in Fig. 7 (due to the lack of time on 13 May, some membranes were not used). It shows that an increase in membrane resistance results in a lower measured SDI value. This is explained as follows. The total volume that is filtered through the membrane within a certain amount of time depends on the flow rate. A higher specific membrane resistance decreases the flow through the membrane; as a result, the accumulated fouling load on the membrane decreases. This results in a lower SDI value, but it does not represent the actual fouling capacity of the water. One exception is the PVDF membrane (M1), which gives an SDI value of zero. This can be due to hydrophilization of the membrane, which decreases the fouling rate and consequently

Fig. 4. UF performance: permeability, transmembrane pressure (TMP) and flux in the period 6 April 2010 to 12 May 2010.

condition, the normalized SDI (SDI +) is calculated using the SDI/MFI relation [12]: 0

1

B C B C μ⋅RM 1 μ⋅I⋅Vc2 B C ⋅V þ ⋅ C 100 B dP⋅AM c 2 dP⋅A2 B C SDI ¼ 0 1 0 1 B1− C qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi   2 C tf ð minÞ B 2 2 2 2 B C −μ⋅RM þ μ ⋅RM þ 2⋅μ⋅I⋅dP⋅A⋅tf ⋅AM C 1 μ⋅I B −μ⋅RM þ μ ⋅RM þ 2⋅μ⋅I⋅dP⋅A⋅tf ⋅AM C μ⋅RM B B C @Vc þ Aþ @Vc þ A −tf A @ 2 dP⋅A2M dP⋅A μ⋅I μ⋅I

ð3Þ

Experimental data t&V

Estimated Fouling parameter

t1, t2, tf

Reference: TO dPO RMO Model

SDImeasured

SDInormalized

Fig. 5. Diagram of SDI (measured) and SDI+ (normalized).

A. Alhadidi et al. / Desalination 285 (2012) 153–162

157

Clean water tank

Isolated feed tank

Clean water pump pH T Κ

Flushing outlet P

F

Air-Relief valve

T

0.45μm membrane Feed pump Fig. 6. Flowchart of the SDI setup. Feed tank and clean water tank are shown. pH, temperature (T) and conductivity (κ) are measured in the feed tank as well as in the feed line. Pressure (P), flow rate (F) and temperature (T) are measured in the feed line.

lowers the obtained SDI value excessively [22]. Main conclusion from these experiments is that the measured SDI value is too dependent on the chosen test membrane, which makes it an unreliable parameter for judging the fouling potential of RO feed water. 4.2. Validation of SDI + in practice 4.2.1. UFfeed diluted with UFpermeate In this set of experiments, the UF feed was diluted with UF permeate in different ratios to obtain a range of different particle concentrations without affecting the initial water salinity. The following UF feed volumes were diluted in 25 L of UF permeate: 25 mL, 50 mL, 100 mL, 200 mL and 500 mL. The SDI/MFI0.45 tests were performed with three different membranes (M4, M7 and M5) in parallel in order to compare the SDI values for different membrane resistances (low, average and high membrane resistance, respectively). Membranes M4, M7 and M5 have different membrane resistances (0.64 ×1010, 0.85 ×1010 and 2.65× 1010 m−1 respectively) which result in different SDI values as shown in Fig. 8 (a). The measured SDI values for M4 are the highest whereas the measured SDI values for M5 are the lowest. Fig. 8 (b) shows the SDI values when normalized for membrane resistance and testing condition parameters, using the SDI/MFI0.45 theoretical relationship and assuming a cake filtration mechanism. The normalized SDI values (SDI+) were calculated using the reference values and membrane resistances listed in Table 3. 4.2.2. UFfeed diluted with ROpermeate Another way for obtaining a broad range of different colloid concentrations is by diluting the UF feed with RO permeate in different ratios.

Table 2 Microfiltration membranes (pore size 0.45 μm and diameter of 25 mm) used in this work. RM is the measured average clean water resistance [21]. Code

Material

RM [1010 m−1]

M1 M2 M3 M4 M5 M6 M7 M8

PVDF PTFE Acrylic polymer Nitrocellulose⁎ Nylon6,6 Cellulose acetate⁎ Cellulose acetate⁎ Polycarbonate

0.83 0.41 0.66 0.64 2.65 0.74 0.85 0.39

* ASTM standard material.

However, unlike with UF permeate dilution, this simultaneously dramatically decreases the salinity of the SDI feed water. The SDI is related to the interaction between particles and the membrane, which is influenced by the water salinity and acidity. A high ionic strength of the water may result in higher SDI values [23]. Different UF feed volumes (25 mL, 50 mL, 100 mL, 200 mL, 300 mL and 1000 mL) were diluted in 25 L of RO permeate. The measured and normalized SDI values are plotted in Fig. 9. Higher colloid concentrations lead to higher SDI values. Membrane M4 gives higher SDI values compared with M7 and M5 because of the larger membrane resistance. Normalizing the SDI results for membrane resistance and test condition parameters results in less variation in the results as shown in Fig. 9(b). Both Fig. 8 and Fig. 9 show that the use of different membrane materials results in different SDI values for the same water quality. By contrast, the SDI + values do not show any effects of membrane resistance and testing condition parameters, which made is possible to compare the results for the different membranes. 4.3. UF performance under different operation regimes The performance of the UF unit in removing particles was investigated by measuring the SDI and the MFI0.45. The tests were performed with UF permeate samples under different operation conditions such as acid and coagulant dosing, and using cellulose acetate membrane M7. Each result was measured in triplicate. A number of parameters can reduce the removal efficiency of the coagulation step, such as the type of coagulant, coagulant concentration, water pH, residence time and mixing efficiency [24–28]. We assessed this by additional SDI, MFI, and SDI+ determinations under various conditions of coagulation and pH (mixing regime, dosing point, dosage). 4.3.1. Influence of coagulation The SDI and MFI0.45 results for the UF permeate under both conditions (with and without adding coagulant) are displayed in Fig. 10. Fig. 10 (a) shows that adding 1.5 mg/l coagulant to the UF feed led to a slight increase in the SDI value of the UF permeate. Both SDI values are lower than 1 which would indicate a good performance of the UF step in removing particles. The MFI0.45 numbers show the same tendency as the SDI values. The increase of SDI and MFI0.45 values after adding coagulant can be due to residual coagulant which can pass the UF unit. This can increase the particle size, which can lead to fouling of the MF membrane during the SDI measurement.

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7

7

13 May

6

6

5

5

SDI Measured

SDI Measured

11 May

4

M8 M2

3

ASTM standard membrane

2

M6 M4

0

M7

M3

1

M5

M1 0

M8

3 2

M3

1

4

1

2

3

4

0

M6 0

Membrane resistance RM [×1010 m-1]

M7 M1 1

ASTM standard membrane

M5 2

3

4

Membrane resistance RM [×1010 m-1]

Fig. 7. SDI of UF permeate with different membrane materials as measured on 11 May 2010 (left) and 13 May 2010 (right).

4.3.2. Influence of acid addition In this experiment, only acid was added and no coagulant. The UF particle removal efficiency was examined with the SDI/MFI0.45 test. The pH of the UF feed dropped to 6.5 after addition of HCl in the mixing unit. Fig. 11 gives the results of the SDI and MFI0.45 measurements without and with acid dosing.

7

a M4

6

SDI Measured

M7

4 3 2

M4 M5 M7

1

0

1

2

3

4

Fouling index I [×1010 m-2] 7

b

M4

6

SDI Normalized

4.3.3. Influence of simultaneous coagulation and acid dosing The raw water has a pH of 8.4 which is close to the optimum pH for adding ferric aluminum sulfate as coagulant. The SDI/MFI0.45 values for combined acid and coagulant addition are displayed in Fig. 12. Both the SDI and the MFI0.45 value were below 1 The observed differences between the results for acid addition, coagulant addition and the simultaneous addition of acid and coagulant fall within the error margin of the data.

M5

5

0

In general, the UF unit shows good performance as the SDI and MFI0.45 values were low. The minor decrease in the observed MFI0.45 values with acid addition indicates an increase in the UF efficiency.

M5

5

M7

4 3

4.3.4. Influence of RO feed tank storage time on water fouling potential The UF permeate is collected in the RO feed tank. The plant was not working continuously during the fieldwork, and the RO feed was stored for 14 h overnight (from 6 pm to 8 am) in the buffer tank before pumping to the RO installation. Two SDI/MFI0.45 tests were performed. The first test was performed on the fresh water in the tank at 6 pm, and the second one the next morning at 7:30 am, just before operation of the plant started. The results are shown in Fig. 13. Both the SDI and the MFI0.45 results appear to indicate that storing the water in the RO feed tank for 14 h results in an increase of the water fouling potential. Several factors influence the efficiency in removing colloidal matter in the coagulation step, such as pH, coagulant dosage, and mixing intensity [29, 30]. Coagulant passage through the UF unit is mainly a function of the solubility of the coagulant which is determined by pH and temperature [31]. Therefore, some residual coagulant can be present in the UF permeate and coagulation might continue after the UF step [32]. Villacorte et al. investigated polymeric material removal in the Evides UF/RO plant and found that 60–80% of the initial amount of polymeric material was still present in the UF permeate [33]. During regular operation, the residence time in the RO feed tank is only ~30 min, but during our experiments, coagulation may have taken place during the 14 h between UF and RO.

2

0

Table 3 Reference parameters.

M4 M7 M5

1

0

1

2

3

4

Fouling index I [×1010 m-2] Fig. 8. a: Measured SDI values b: SDI normalized for membrane resistance and testing condition parameters T = 10 °C and κ = 53,600 μS/cm (SDI+).

Parameter

Reference value

RMO TO dPO AMO SDIO

1.29 × 1010 m−1 20 °C 207 kPa 13.4 × 10−4 m2 3

A. Alhadidi et al. / Desalination 285 (2012) 153–162

7

a

6

M4

6

M7

a

7

b

6

5

5

4

4

M5

SDI

SDI Measured

5 4 3 2

M4 M5 M7

1 0

0

1

2

3

4

5

6

7

8

3

No Acid

With Acid

No Acid

With Acid

3

2

2

1

1

0

MFI [s/L2]

7

159

0 M7

M7

Fig. 11. (a) SDI and (b) MFI0.45 values at 10 °C after the UF unit using cellulose acetate membrane M7 without and with acid dosing. pH 8.3, 6.3. Samples taken on 14 April 2010.

9

Fouling index I [×1010 m-2] 7

b 5 4 3 2

M5 M7 M4

1 0

0

2

4

6

Fouling index I [×10

8 10

10

m-2]

Fig. 9. (a) Measured SDI values (b) SDI normalized for membrane resistance and testing condition parameters T = 10 °C (SDI+).

4.4. Fouling potential at different locations in the plant We scanned the Evides plant with SDI and MFI0.45 tests using the 0.45 μm cellulose acetate membrane M7; see Fig. 14. The indices were measured on two different days, May 11 and May 12, 2010. Due to the time constraints, SDI and MFI tests for the UF and RO permeate were not carried out on May 12. The raw water as well as the UF feed samples have a high fouling potential (SDI N 5), and the plugging ratio (%P) exceeded the 75% limit

during the measurements. The XIGA UF membrane has a molecular weight cutoff (MWCO) of 150 kDa and can remove particles larger than 20 nm [34]. This results in a drop in the SDI to values below 1. The slight increase in SDI values in the RO feed can be explained by the effect of ongoing coagulation in the RO feed tank as discussed before (polymeric material post-coagulation). Due to the sensitivity of the SDI for measurement accuracy errors at low SDI values (b1), drawing detailed conclusions from the individual SDI values of the UF permeate, RO feed, RO permeate and RO concentrate is not possible, except that the obtained range is in line with expectations and targets (b3). The MFI0.45 values show the same trend as the SDI. The MFI0.45 values for raw water as given in Fig. 14 (b) are 5000–6600 s/L 2. The MFI0.45 value increased to around 7500 s/L 2 after addition of the coagulant to the UF feed. The UF membrane then removed the particles and consequently, the MFI0.45 value dropped to values below 1. RO feed, concentrate and permeate have MFI0.45 values below 1 as well. Fig. 15 shows water samples from different locations in the plant. The raw water had a high turbidity due to the algal bloom present in the seawater. Adding the ferric coagulant to the UF feed increased the turbidity and changed the sample color to yellowish. There are no visible particles in the RO feed owing to the UF filtration step.

4.5. Reduction in SDI values and MFI0.45 The results presented in Fig. 14 (a) and (b) in Section 4.4 were used to calculate the average reduction in the fouling potential of the UF permeate relative to the UF feed. We defined the reduction

7 7

7

a

b

6

6

a

7

b

6

6

4

4

3 2

No With Coagulant Coagulant

No With Coagulant Coagulant

3

SDI

5

MFI [s/L2]

SDI

5 5

5

4 3 2

4 No With Coagulant Coagulant No Acid With Acid

No With Coagulant Coagulant No Acid With Acid

1

1 1

0

0

0 M7

2

2

1 M7

3

MFI [s/L2]

SDI Normalized

6

Fig. 10. (a) SDI and (b) MFI0.45 values at 10 °C after the UF unit using cellulose acetate membrane M7 without and with coagulant dosing. Fe+ 3 1 mg/L. 14 and 15 April.

M7

M7

0

Fig. 12. (a) SDI and (b) MFI0.45 values at 10 °C after the UF unit using cellulose acetate membrane M7 when both acid and coagulant are added. pH 8.5, Fe+ 3 1 mg/L. 14, 15 and 16 April.

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A. Alhadidi et al. / Desalination 285 (2012) 153–162

7

7

5. Conclusions

The SDI is related to the change in the flow after 15 min during constant pressure filtration, and is related to the change in the total

In this work, the SDI and the MFI0.45 were used to evaluate the performance of the Evides UF/RO seawater desalination demonstration plant in the Netherlands. This plant receives raw water from an open intake with great variation in the water quality. In the spring, the plant faces an algae bloom challenge and requires a coagulation step. We obtained markedly different SDI values when we used different membranes; this indicates that the standard SDI, however practical, is not an ideal fouling index. We previously developed a mathematical model for normalizing the SDI, and suggested using this SDI+ instead. In this fieldwork, we also determined the SDI+ values, and found that they were not sensitive to differences in membrane resistance and testing condition parameters It was not possible to draw conclusions with regard to which combination of acid and coagulant application resulted in better UF performance on the basis of SDI values alone. We ascribe this to the sensitivity of the SDI at low values (SDI b 1) to accuracy errors in the measurements. The RO feed water is required to have SDI values of less than 3. The UF performance can be considered good, as the SDI values were lower than 1, provided the tests were carried out with an ASTM standard membrane. The MFI0.45 results showed the same tendency as the SDI values in most cases, and were generally lower than 1 s/L 2. The reduction in SDI and MFI0.45 values due to the UF pretreatment was 90.86 ± 5%% and 99.947 ± 0.053%, respectively. Storing the UF permeate water in the RO feed tank for 14 h during the night increased the fouling potential of the RO feed, most likely due to post coagulation of polymeric material.

b

6

6

5

5

4

4 3

3 Night

Night

Morning

Morning

2

2

1

1

0

M7

MFI [s/L2]

4.6. Total resistance at different sampling points

SDI

in SDI and MFI 0.45 values as the ratio of the different between feed and permeate values to the feed value. The particle removal based on the average SDI was 90.86 ± 5% and based on the MFI0.45, was 99.947 ± 0.053%. That the MFI0.45 values indicate more particle removal is likely related to the sensitivity of the SDI for measurement accuracy errors at these low values.

membrane resistance. It may be useful to compare not only the two slopes but also the entire curves for a better understanding of the fouling behavior. The total resistance during the SDI and MFI0.45 tests was calculated from the raw data and plotted in Fig. 16 for RO permeate, RO concentrate, RO feed and UF permeate. Fig. 16 shows that with the RO permeate, the total resistance had a small and constant slope after 1.5 min, which is an indication of a low fouling potential. Furthermore, the RO concentrate resulted in the highest total resistance (2.2 × 10 10 m −1 after 17 min). Some differences can be observed between the resistance curves of the RO feed and the UF permeate. The UF permeate curve has a more constant slope after 2 min compared with the RO feed curve. Moreover, the UF permeate leads to a higher total resistance in the first 2 min than the RO feed water. This difference can be explained by additional coagulation caused by remaining coagulant in the RO feed tank as discussed in Section 4.3.4. In Fig. 17, resistance curves for RO feed and RO concentrate are plotted for two different days. The difference between the values for the two days can be due to the high turbidity in the raw water on May 12. The RO unit was operated at a recovery of 40%. Therefore, the particle concentration in the RO concentrate is 67% higher than in the RO feed assuming 100% particle retention. The resistance curves in Fig. 17 (a) and (b) are in agreement with that.

a

0

M7

Fig. 13. (a) SDI and (b) MFI0.45 values of the RO feed tank at 10 °C using cellulose acetate membrane M7 as measured at 6 pm (‘night’) and 7.30 am next morning (‘morning’)i.e. after being stagnant for 14 h.

7

a

11 May 12 May

6

SDI Measured

5 4 3 2 1 0

8000

Raw water

UF feed

UF per

RO feed

RO per

b

RO con

11 May 12 May

7000

MFI [s/L2]

6000

5000 0.8 0.6 0.4 0.2 0.0

Raw water

UF feed

UF per

RO feed

RO per

RO con

Fig. 14. SDI (a) and MFI0.45 (b) values in the Evides UF/RO plant on 11 and 12 May 2010 using cellulose acetate membrane M7. The feed samples were taken at sampling points 20, 30, 31, 37, 47 and 48 in Fig. 2. Per = permeate; con = concentrate.

Nomenclature AM Membrane area [m 2] AM0 Reference membrane area 13.4 × 10 −4 [m 2] C Scaling factor proportional to the foulant concentration dP Applied pressure [Pa] dPo Reference applied pressure 207 [kPa] J Flux [m 3/m 2 s bar] JO Initial flux [m 3/m 2 s bar] I Fouling potential [m −2] m Fouling mechanism parameter (0, 1, 1.5 and 2)

A. Alhadidi et al. / Desalination 285 (2012) 153–162

161

Fig. 15. Water samples from different locations in the plant: raw water, after the 50 μm strainer, UF feed, RO feed, RO concentrate and RO permeate, taken at sampling points 20, 21, 30, 37, 48 and 47 in Fig. 2, respectively.

2.4

Total resistance R [×1010 m-1]

Total resistance R [×1010 m-1]

2.4 RO permeate

RO concentrate

2.2

2.2

2.0

2.0

1.8

1.8

1.6

1.6

1.4

1.4

0 2.2 RO feed

0 UF permeate

2.2

2.0

2.0

1.8

1.8

1.6

1.6

1.4

1.4

1.2

0

4

8

12

Time [min]

16

0 20

4

8

12

16

1.2 20

Time [min]

Fig. 16. Total resistance (R) at different sampling points in the plant using cellulose acetate membrane M7 at 9–10 °C.

n %P R RM RMo SDI SDI + SDIO %P t1,2 tf t V

Number of data points Plugging ratio [%] Total Resistance [m −1] Membrane resistance [m −1] Reference membrane resistance 1.29 × 10 10 [m −1] Silt Density Index [%/min] Normalized Silt Density Index [%/min] Reference Silt Density Index 3 [%/min] Plugging ratio [%] Time to collect the first and second samples [s] Elapsed filtration time [s] Time [s] Filtration volume [L or m 3]

Greek letters μ Viscosity [Pa.s]

Acknowledgments The authors of the paper would like to acknowledge the scientific and financial support of Vitens, Evides and Norit X-Flow B.V. Part of this work was carried out within the framework of the InnoWATOR subsidy regulation of the Dutch Ministry of Economic Affairs (project IWA08006 ‘Zero Chemical UF/RO System for Desalination’).

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2.4

Total resistance R [×1010 m-1]

a

11 May RO concentrate

2.2 RO feed

2.0

1.8

1.6

1.4

0

4

8

12

16

20

Time [min]

Total resistance R [×1010 m-1]

2.4

b

12 May RO concentrate

2.2

2.0 RO feed

1.8

1.6

1.4

0

4

8

12

16

20

Time [min] Fig. 17. Total resistance (R) for RO feed and RO concentrate at different days using cellulose acetate membrane M7. (a) 11 May 2010; (b) 12 May 2010.

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