An experiment with different pretreatment methods

DESALINATION Desalination

ELSEVIER

156 (2003) 5 l-58 www.elsevier.comiIocate/desal

An experiment with different pretreatment methods C.K. Teng, M.N.A. Hawlader*, A. Malek Department

qfMechanical

Engineering, National University of Singapore, 9 Engineering Tel. + 65 874 2218; Fax + 65 779 1459; [email protected]

Received

Drive I, Singapore

II 7576

13 February 2003; accepted 18 February 2003

Abstract The use of reverse osmosis (RO) method for the desalination of seawater to produce fresh water is gaining momentum over the last decade. Although there have been considerable improvements in membrane materials, the fouling of membrane creates a significant problem yet to be overcome. Performance of systems employing different pretreatment methods used in the RO desalination process was investigated experimentally. Field tests included advanced membrane filtration techniques: ultrafiltration (UF) and microfiltration (MF). During the pilot testing, silt density index (SDI) of the filtrate samples were regularly measured to quantify the performance of pretreatment systems. Measurements of other important parameters included filtrate flux, transmembrane pressure (TMP), total suspended solids, colloidal silica, total organic carbon, etc. Test results showed that membrane pretreatment consistently produced filtrate of a good quality. SD1 of the filtrate produced by membrane pretreatment method was consistently below 3.0, a prerequisite for proper operation of a RO desalination plant. Improved maintenance procedures, such as filtrate backwashing and air scouting coupled with periodic use of chemicals, resulted in significant flux and pressure recoveries during the pilot tests. Ease of operation of the membrane pretreatment systems was also noted by the authors. bywords:

Reverse osmosis; Fouling; Pretreatment; Ultrafiltration; Microfiltration; Silt Density Index

1. Introduction In general, there is a shortage of fresh water in many parts of the world, and Singapore is no exception. This is attributed to the limited natural water resources and the shortage of catchment ~~ “Corresponding author.

areas in the nation. In order to be more self-sufflcient and meet increasing fresh water demands, Singapore has devoted considerable effort in the last three decades to monitor progress in the field of desalination, a drought-proof source of water. Reverse osmosis (RO) is one such techniques that is fast gaining attention due to its simplicity

Presented at the European Conference on Desalination and the Environment: Ewopenl? Desalination Sociep, International Water Association.

OOl l-9164/03/$-

Fresh Water for All, Malta, 4-X May 2003.

See front matter 0 2003 Elsevier Science B.V. All rights reserved

PII: SO01 l-9 164(03)00324-2

in operation and significant improvements in membrane technology. However, despite improvements in cost effectiveness and breakthrough in membrane technology, performance reliability of RO desalting technique has been greatly affected by the problem of membrane fouling. With fouling. replacement rate of the RO membrane and downtime of the plant will increase. Consequently, operating costs of the plant will inflate. Membrane fouling is attributed largely to the seawater and RO process chemistry. To control and reduce the extent of membrane fouling, besides maintaining proper operating conditions, IIILICII can be done in the area of pretreatment upstream of the RO membrane in providing better quality feed water. I. 1. An improved technique: integrated membrane system (IMS) A quality feed pretreatment process is instrumental to successful operation of a SWRO plant [I]. With proper feed pretreatment, fouling potential of the seawater feed can be reduced, leading to better survivability of the RO membranes. However, performance of media filtration, which has been the conventional pretreatment technique for many years, is still lacking in many aspects [2-4]. Intensive consumption of chemicals and inconsistency in performance are some of the main disadvantages of media-filtration. More recently, a double membrane barrier concept has emerged as a possible alternative. By installing microfiltration (MF)/ultrafiItration (UF) membrane upstream of the RO membrane in an Integrated Membrane System (IMS) to filter off bigger suspended solids, the feed water quality may improve. Consequently, useful life ofthe RO membrane may be extended, leading to lower replacement rate. There are even cases where performance of RO systems exceeded their initial design specifications after adopting the membrane pretreatment techniques [5]. Considering its potential and positive spin-off effects, the current research will focus on the evaluation of perform-

ante of membrane filtration techniques in the pretreatment of seawater. In order to ensure realistic results close to actual operating conditions, coastal pilot trials were conducted. The membrane pretreatment techniques considered in the study comprised of UF and MF. The quality of seawater in Singapore will be briefly discussed in the next section. The remaining sections of the report will cover the research findings. It should be noted that SD1 was used as a primary yardstick of filtrate quality during the pilot testing of the pretreatment systems.

2. Silt Density Index (SDI) of seawater SDI of seawater was observed to be fairly consistent during the pilot testing, varying from 6.1 to 6.5. Since operation of RO membranes requires feed quality with SD1 not more than 3.0-4.0. pretreatment of seawater was necessary. In order to characterize the impact on SD1 by different sizes of particles existing in seawater, a 5 urn and I pm cartridge pre-filters were subsequently incorporated upstream of the SD1 measuring instrument and the test was repeated. The results of the SD1 tests are displayed in Table 1. According to the results shown in Table I. particles having sizes varying from I urn to 5 pm had a higher impact on SD1 than particles having sizes greater than 5 urn in seawater. Though a SD1 test does not give any information on the number of particles present, the above data indicated that a substantial quantity of particles would probably exist in the sub-micron range. This was because the SD1 of seawater was rather high even after passing through the 1 pm pre-filter.

Table I SD1 of seawater with and without pre-filters Description __~__ _~ ~~~~ ~~~_~~ _ Seawater Seawater with 5 pm cartridge filter Seawater with I pm cartridge filter

SDI,, 6.1-6.5 6. L-o.5 4.262

C.K. 7kng et al. i Desalination 156 (2003) 51-58 3. Performance

of UF pretreatment

3.1. Features of UF pretreatment

system

system

An UF pilot system, having a maximum production capacity of 1.2 m3/h, was tested on local seawater. The UF membrane has a hollow fibre configuration with an ID/OD of l/l .25 mm and an effective surface area of 15 m’. It is manufactured from polyethersulfone (PES) and has a nominal pore size of 0.01 urn. During the experiment, a cross-flow process mode of filtration was adopted, as advised by the system supplier. Fig. I shows the schematic diagram of the UF pilot system. The maintenance of the UF pilot system relies on periodic backwashing coupled with sodium hypochlorite disinfection. Backwash cycle is normally activated every 30-90 mitt, depending on the quality of the feed stream. In addition, 1 ppm of sodium hypochlorite is dosed in tank T03 during backwashing to prevent biological fouling of the UF membrane.

1LEGEND P

PUMP

T

TANK

FI FLOWINDICATOR

53

3.2. Chemical analyses on filtrate produced the UF pretreatment system

by

The performance of the UF pretreatment system is considered in this section. In the pilot study, rejection characteristics and consistency in performance of the UF pilot system were evaluated. The results of the chemical analysis performed on filtrate samples of the UF pilot system are displayed in Table 2.

Table 2 Results of chemical analyses on filtrate produced by UF pretreatment system receiving seawater feed without prior pretreatment Description

Seawater feed UF filtrate

Silica as SiOz (reactive) Silica as SiO, (colloidal) Total organic carbon, mg/L Oil and grease, mg/L Total coliform, c&/ml

0.65 0.75 2.15 3.4 4

I PRODUCT

-

BV BALL VALVE _ ~~~ .-~~~~ -

BV-01

r~ i% $j BV-12-

YDRAIN Fig. I. Schematic diagram of the UF membrane pretreatment pilot system.

0.75 Not detected 1.81 3.6
54

C. K Teng et al. i Desalination 156 (2003) 5/-58

experimental data collated during the course of operation are shown in Fig. 3.

From the results shown in Table 2, it was evident that the removal of colloidal silica and coliform group of bacteria could be significant with UF treatment. However, there was only a small percentage (16%) removal of organics and no rejection in the case of oil and grease. The higher reactive silica level recorded after UF treatment could be due to experimental errors. Other than chemical analysis on the filtrate, SDI test was also performed regularly. The test results obtained are displayed graphically in Fig. 2. SDI of the UF filtrate varied from 0.7 to 3.0, with the poorer readings coinciding with periods of operation when the filtrate flux was high. SD1 of filtrate from the sand filter varied from 2.8 to 6.3 during the same period of operation. These results were comparable to those obtained by van Hoof et al. [6]. In the pilot study carried out by the researchers, majority of the SD1 measurements varied between i .O and 2.0 when the UF system was operated at a filtrate flux of 57 I/m’.h.

List of@dings. Operating on seawater directly, instead of sandfiltered filtrate (used during period of operation before direct seawater testing on Fig. 2), put more strain on the UF pilot system. This was indicated by a 10% drop in filtrate flux (i.e. 78 l/m2.h to 70 I/m2.h) and a slowly increasing TMP trend during the experiment. Low flux-high recovery operation (57.6 l/m*.h and 80%) was more suitable than high fluxlow recovery approach (higher than 70 I/m2.h and 50%) for direct application on seawater. High flux-high recovery (80% and 80 l/m2.h) operation was not suitable as increasing TMP trend was observed during the experiment. Recirculation of feed led to an increasing TMP trend during the experiment.

4. Performance of the MF pretreatment system

3.3. Flux und TMP trends oJ’UF pretreatment system

4. I. Features qf the MF pretreatment

A MF membrane pretreatment system, having a production capacity of 5.2 mj/h, was installed and tested for its performance. The filtering media

The influence of different operating parameters on the flux and TMP trends were considered during the pilot testing of the UF system. The

90

6.0

J&O1

,9-Jut-01

26-J&01

31-J&01

system

6-Aug-01

Fig. 2. SDI trend of filtrate produced by UF pretreatment system.

,2-Aug.01

16.Aug-01

244ugbl

30~Aug-01

55

C.K. Teng et al. / Desalination 156 (2003) 51-58

--%--Filtrate Flux ---rtRecovery

y

2%Jun-01

;'

TMP

6-Jul-01 14-Jul-0122-Jul-0130-Jul-017-Aug-01 1.5Aug-0123-Aug-01

Date Fig. 3. Filtrate flux, recovery and TMP trends of the UF pretreatment system.

consists of four hollow fibre membrane modules manufactured from polyvinylidenefluoride (PVDF) with nominal pore size of 0.1 pm. Operation of the pilot system is based on a direct-flow concept with filtration taking place from the outside to inside of the hollow fibre. To prevent against an uncontrollable increase in TMP, air scouring and filtrate backwashing are carried out periodically. Despite these maintenance procedures, TMP will build up with time and a thorough chemical clean is required to restore the TMP back to its initial level. The entire chemical cleaning process com-

prises of citric acid cleaning, followed by sodium hypochlorite cleaning. Schematic of the UF pilot system is shown in Fig. 4.

3.2. Chemical analyses on filtrate produced by the MF pretreatment system During pilot testing, seawater and MF process filtrate samples were tested to give a baseline of system performance. The results of the chemical analyses are shown in Table 3. Filtrate Storage Tank

Filtrate -_----t__

MF Membrane Filtrate Backwash Tank

Feed Water Tank

Air Scouring --

&--=l J__j

.--_j

Fig. 4. Schematic diagram of the MF membrane pretreatment pilot system.

Chemical Mixing Tank

C. K. Teng ef nl. I Llesalination

56

Table 3 Results of chemical analyses on filtrate produced by the MF pretreatment system receiving seawater feed without prior pretreatment Description

Seawater feed

MF filtrate

Total suspended solids, mg/L Silica as SiO? (colloidal), IllgIL Silica as SiO? (reactive), mg/L

6

1.3

0.2

0.07

0.27

0.29

156 (2003) 51-58

Consistency in performance ofthe MF system was closely monitored during the trial with regular SDI checks. SD1 during initial plant startup was found to be 3.8. With continuous operation, SD1 stabilized and fluctuated within a performance band of 2.0-3.0, comparable with the results of the pilot trial at Doha Research Plant, Kuwait [7]. It was noted that the turbidity of seawater during this period of operation was stable, varying from 1.5 to 3.0 NTU, as shown in Fig. 5.

Particle size distribution, counts/ml

4.3. Flux and TA4Ptrends ofthe MFpretreatment

Q pm <2-5 pm <5- 15 pm I00 pm Total organic carbon, mg/L Oil and grease, mplL Total coliform, cWlOOm1

system

Remaining counts 323 49,394 6 1900 2 27 0 0 0 0 0 0 2.49 I .43
Preliminary operation of the MF pilot system started with a conservative filtrate flux of 80 I/m’.h to allow for functional tests to be carried out. This persisted for a period of two days and filtrate flux was increased to 100 I/m’,h after proper functioning of the MF pilot unit was ascertained. With reference to the performance data shown in Fig. 6, a fluctuating TMP trend was observed. Sudden drops in TMP were accounted for by the chemical cleaning performed, while the remaining fluctuations were due to changing feed water conditions, which would be elaborated. During the initial period of operation before the first chemical cleaning was performed, a steep TMP trend was observed. This rapid escalation of TMP was attributed to a high operating flux of 100 I/m’.h and operation on seawater feed without

According to the above data, moderate removal of colloidal silica and suspended solids were possible through MF pretreatment. There was no rejection of reactive silica and, for organics, a percentage rejection of approximately 40% was achievable during the experiment. The experimental results were non-conclusive with regards to the rejection of oil and grease, and coliform. ~______“ _.__ .._ “__“l._“l._-l _-_

,_-.----

-.-...__.“...” .--- --. _---

/

0.0

,0 9

6.0 *SDI

of MF Filtrate X Turbidity of Seawater

8

1

6-Ott-01

40 16-Ott-01

26-Ott-01

5-Nov-01

15-No+01

Date Fig. 5. SDI of MF filtrate and turbidity of seawater.

25-Nov-01

B-Dee-01

1%Dee-01

C.K. Teng et al. ,’Desalination 1.56 (2003) 51-58 ,,TMP

(kN/m2) x Filtrate flow (m?h)

I

Feed chlorine level (mg/L)

X Seawater turbidity (NTU)

Set 5 100

90 80 70 N$

60

5

50

;

40 30 20 10 0 24-Sep-01

9-act-01

24-Ott-01

8-Nov-01

23-Nov-01

8-Dee-01

23-Dee-01

Date

Fig. 6. Flux and TMPtrends ofthe MF pretreatment system.

any prior filtration. Since the removal of suspended solids from the membrane surface was usually not perfect even with air scouring and filtrate backwashing, a cake layer would accumulate progressively. The gradual formation ofthis cake layer led to an increase in resistance to flow. As a result, TMP increased with time to offset this increase in resistance. List c&findings. ??

??

Fluctuating TMP trend could be attributed to two reasons: changes in turbidity and free chlorine level of the seawater feed. The first reason was unlikely since the turbidity of seawater was rather stable during this period of operation. On the other hand, free chlorine level ofthe feed appeared to have a significant impact on the TMP trend during the experiment. Referring to Fig. 6, during the period of operation from 26th Oct. to 30th Nov. 0 1, TMP reduced when the free chlorine level in the feed was high and increased when the free chlorine level was low. The rate of increase of TMP was significantly faster after the increase in filtrate flux. At a filtrate flux of 100 l/m’.h, the MF system could

operate continuously for a period of eight days. With reference to Data Set 5 of Fig. 6, the MF system could only operate for a period of three days at a higher flux of 110 l/m*.h, before a chemical clean was required. Therefore, an intended increase in production capacity with higher flux has to be weighed carefully against an increase in chemical cost. According to the experimental data, chemical cleaning was effective in recovering the TMP. A chemical cleaning process was conducted every 2-4 weeks. In the experiment, sodium hypochlorite (i.e. chlorine) was used as the disinfecting chemical and chemical backwashing was performed once every two days. With chemical backwashing, potential benefits include a longer chemical cleaning interval and increased process availability with shorter downtime.

5. Conclusions There was a significant difference between the operating fluxes of the MF and UF pretreatment systems. The higher operating flux of the MF pilot system was achieved at the expense of increasing

58

C’.K. Teng et al.

Desalination 156 (2003) 5/- _V

TMP, which had to be kept constant by a more elaborate maintenance system. comprising of air scouring and filtrate backwashing. However, SD1 of the filtrate produced by the UF pilot system was superior to that produced by the MF pilot system. It should be noted that while filtrate quality of the MF system was inferior to that of the UF pilot system, it is still ofconsiderably good quality. Filtrate SD1 of the MF pilot system was consistently less than 3.0 in the experiment. During the pilot tests, ease of operation of the membrane pretreatment systems was noted by the author. Moreover, membrane pretreatment systems generally require less space and chemicals compared to the conventional pretreatment system. Together with improved maintenance procedures, such as filtrate backwashing and air scouring. significant flux and pressure recoveries are achievable with minimal use of chemicals. Using membrane methods ofpretreatment will also go in tandem with the recent trend of increasing packing density of spiral wound RO elements. With an increasing packing density, propensity to fouling and pore blockage for the new RO spiral element will definitely be higher and greater emphasis will be placed on pretreatment in producing better quality feed. There-

fore, there is an economic incentive to the usage, and continual improvement and research in the area of mem brane pretreatment. Current research also emphasized the importance of pilot testing as measured performance is the combined effect of the membrane, system configuration and the dynamic cake layer that formed OII surface of the membrane during filtration.

Acknowledgement The authors would like to express their appreciation to AquaGen Singapore Pte. Ltd. for their contribution in the success of the pilot tests. References [I]

K. Chida, Desalination,

[Z]

H. Winters, Desalination,

110

[3]

H. Winters, Desalination,

66

[4]

B. Chakravorty (1997)

[5]

110 (1997) 59-64.

( 1997) 93-96. (1987) 3 19-325.

and A. Layson,

Desalination,

I IO

143-150.

J. Murrer

and R. Rosberg,

I18

( 1998)

and B. Mack,

Desa-

Desalination,

14. [6]

S.C.J.M. van Hoof, J.G Minnery lination,

[7]

139 (2001)

S. Bou-Hamad, Mansour 85.-92.

161-168.

M. Abdel-Jawad,

and A. AI-Hijji,

S. Ebrahim,

Desalination,

M. Al-

I IO (I 997)