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Pilot Tests of Multibore UF Membrane at Addur SWRO Desalination Plant, Bahrain
Desalination 203 (2007) 229–242
Pilot Tests of Multibore UF Membrane at Addur SWRO Desalination Plant, Bahrain Khalid Ahmed Bu-Rashid*,a, Wolfgang Czolkossb a
Ministry of Electricity and Water, Kingdom of Bahrain email: [email protected] b TAPROGGE GmbH, 58300 Wetter, Germany email: [email protected] Received 27 March 2006; accepted 15 April 2006
Abstract UF membranes with new Multibore design were tested in a pilot unit at the Addur SWRO Desalination Plant, Bahrain as pre-treatment for RO. The seawater at Addur is known for its extremely bad condition, mainly caused by industrial and residential waste disposals and high organic contents and bioactivity. Feed water for the pilot plant was taken from the Addur seawater supply line (intake 1.2 km offshore, chlorinated) directly, without making use of the existing media filtration. The existing UF/RO plant at Addur is suffering from heavy fouling and scaling problems. The pilot unit was started in March 2003. A first set of trials was done to test different operational modes and cleaning procedures under extreme fouling conditions. Based on these trials new modified UF modules were installed in March 2004 and operated until July 2005. During these 16 month of service the new Multibore UF membranes performed successfully. During summer month 2004 no membrane cleaning except normal backwash procedures with sodium hypo chloride was necessary for stable operation. The use of not chlorinated seawater as feed to the UF was tested in October 2004 and during the outage of the Addur plant February-April 2005. This water was taken by a newly installed 3’’ intake pipe from a location about 200 m offshore from a depth of 1.5 m only. This so called ‘‘raw beach water’’ turned out to be not suitable for UF filtration due to its extremely high fouling potential. The injection of flocculants into feedwater was tested to further improve the UF output and filtrate quality. The dosing of very small amounts of Fe proved to be beneficial. During summer 2005 the UF operation with Fe injection was stable with a flux of 70 l/m2h at a differential pressure around 250 mbar only. Filtrate SDI was always well below 3, even below 2 in July 2005. An additional RO pilot unit was installed in May 2004 to directly test the fouling potential of the filtrate from the UF pilot plant. In the RO pilot plant a permeator as it is used at the Addur SWRO plant was installed. The fouling of this permeator was remarkably lower compared to the fouling in the Addur *Corresponding author. Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21-25 May 2006 0011-9164/07/$– See front matter 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.04.010
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SWRO desalination plant. This proves the high quality of the UF filtrate in addition to the low SDI values. Because of the low consumption of cleaning chemicals, low differential pressure and no need for media filtration the operating costs for Multibore UF membranes would be remarkably lower compared to the operating costs of the existing arrangement at Addur. The pilot test under the extreme seawater conditions of Addur revealed that these new UF membranes are a very promising alternative for pre-treatment for SWRO desalination plants. Keywords: Ultra filtration; Silt Density Index; Multibore UF membrane
1. Introduction
desalination (B-9) leaving the first stage to improve the permeate quality. The membranes at Addur plant suffered very fast from flux decline primarily due to rapid biofouling, demanding extensive cleaning and frequent replacement of membranes within a short time of commissioning which impaired production capacity and availability for more than one decade. Initial experiments conducted in 1997 confirmed that the main problem rests in the design of the under rated conventional pre-treatment plant being inadequate to cope with the severe biofouling activity. Pilot tests in late 90’s gave strong indication that ultrafiltration membranes might be a suitable option to the problems faced in the pre-treatment plant, in
The Addur Desalination Plant is the only Seawater Reverse Osmosis Desalination Plant in the Kingdom of Bahrain, commissioned in 1990. The plant employs Dupont Permasep Hollow Fine Fibre membranes and the original design uses conventional sea water pre-treatment, see Fig. 1. In the pre-treatment section, pH is reduced via sulfuric acid and ferric chloride is dosed as coagulant. The coagulated flocks are filtered off in the dual media filter beds. Chlorination is performed by sodium hypochlorite solution, filtration through micron guard filters and dechlorination by sodium bisulfite. The RO membranes are arranged in two streams, sea water (B-10) followed by brackish water
HYPOCHLORITE GENERATIN UNIT
COAGULANT DOSING
ACID DOSING CL2 INJECTION
SODIUM BISULPHITE DOSING
COAGULANT AID DOSING CARTRIDG FILTER PUMP CARTRIDG BACKWASH FILTER PUMPS
DRAWBACK TANKS LIME
1ST R.O PASS
2
ND
PASS
2 DUAL MEDIA FILTER SE WATE SUPPL TRAVELLING PUM SCREENS
TRAS RAC
3
CLEARWEL
M ERT
ACID DOSING #
PRODUCT
4
AIR SCOURING BLOWER
2nd PASS BOOSTER PUMPS
1st PASS BOOSTER PUMPS
Fig. 1. Schematic process of the Addur as per the original design.
DECARBONATO PRODUCT PUMP
TO STORAGE TANKS
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combination with the existing pre-treatment or as stand alone UF. The plant rehabilitation program was implemented in 1999 using full UF scheme and limiting the dual media filtration to sand media without micron guard filtration. Soon, during 2000 and 2002 it became evident that the membranes suffered from biological fouling, analogues to the problems faced before the rehabilitation, requiring further refinement to the UF system. These shortfalls in the UF system affected the operation and reduced the production capacity of the plant associated with high production cost. Among the recommendations put forward in various papers was to employ pilot testing on the site where the real plant will be constructed for a minimum period of one year for proper assessment of the combined UF þ RO system. Experiments continued during 2002–2003 with an enhanced UF model of the present design under the existing contract ‘‘The production improvement works commenced mid 2005’’ employing different RO membrane design with successful results which was covered in separate paper [3]. During 2002, Taprogge approached the Ministry of Electricity and Water to permit the trial of UF membrane with Multibore design at Addur plant to assess the suitability of this new type of membranes for use in difficult seawater conditions. The initial operation of the pilot plant started in March 2003. This paper shall describe the different phases of the pilot trial and the performance parameters of the Multibore UF membranes using seawater directly from the intake without media filtration, in attempt to qualify the suitability of this type of UF system designs for use in SWRO plants processing difficult seawater with extremely high fouling potential. 2. The new multibore UF membrane New Multibore UF membranes (by inge AG, Germany) have already been well
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established for municipal and industrial drinking water applications treating sweet water from lakes, rivers or wells [1]. To prove the capability of these membranes for seawater applications they were tested in a pilot unit at the Ad Dur SWRO Desalination Plant, Bahrain as pre-treatment for RO. Addur is known to be the toughest place in the Gulf for desalination. The surrounding seawater is of high salinity and bioactivity. Industrial and residential waste disposals causes additional organic content. As a result the existing UF/RO plant at Addur is suffering from heavy fouling and scaling problems [2]. The Multibore UF membranes are hollow fibre membranes with 7 capillaries integrated into a single fibre, see Fig. 2. They are made of modified polyethersulfone (PESM). During the fabrication process of the fibres a skin with extremely fine pores is formed on the inner surface of the capillaries. This skin functions as the UF membrane with a molecular weight cut-off of 100.000–150.000 Dalton (pore diameter 20 nm). Flow direction during filtration is from inside of the capillaries to outside of the fibre. The foamy support structure surrounding the 7 integrated capillaries guarantees extraordinary stability and full membrane integrity during operation and cleaning. Inner diameter of the capillaries is 0.9 mm, outer diameter is 4.3 mm. The Multibore membranes are arranged in special modules containing 45 m2 of membrane area each. The modules are designed for vertical installation and are provided with feed connections at top and bottom. So dead end or cross flow filtration as well as backwash can be performed in 2 ways, allowing change of flow direction inside the capillaries. The pilot test should demonstrate the capability of the new Multibore UF membranes in treating seawater of extremely high fouling potential. Different operation parameters and pre-treatment should be tested and optimised
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Fig. 2. Multibore UF membrane and UF module Dizzer1 5000 with 45 m2 multibore UF membrane.
considering all seasonal and other changes of the water quality at Addur. 3. The pilot plant Fig. 3 shows the general arrangement of the pilot units at the Addur SWRO desalination plant. The feed water to the UF pilot plant is taken downstream of the intake screens but upstream of the sand filter (single media filter). So chlorination and screening is the only pretreatment of the seawater in front of the UF pilot plant. There is an automatic backwashable 200 m filter integrated in the pilot plant to protect membranes from debris and coarse particles. Fig. 4 shows the UF pilot plant. Up to 6 UF modules can be installed and operated automatically. Remote control by modem and
telephone line is possible, too. The rack is designed for a filtrate flow of 12 m3/h. Frequency controlled pumps allow operation at different flows and with less than 6 modules. Pneumatically driven valves allow the following operating modes:
dead end filtration from top or bottom backwash to top or bottom purge forward flush
Filtrate was collected in a tank to allow backwash using filtrate from this tank. Dosing pumps allowed to perform chemical enhanced backwash by injection of NaOCl-solution or other chemicals into the backwash flow.
TAPROGGE Pilot Plants Fe non chlorinated beachwater
Cl Seawater
Intake
1,2 km
Screens
SBS HCl UF Pilotplant
RO Pilotplant
UF Backwash
RO Reject
SMF* * Single media filter
Fig. 3. Arrangement of UF and RO pilot units.
UF
Ad Dur SWRO Desalination Plant
RO
Tank
Tanks
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233
Dupont permasep B10 RO module Multibore UF modules Dizzer5000
Fig. 4. UF and RO pilot plants in the intake building of Addur plant.
The pilot plant was equipped with all needed instrumentation for performance measurements and operational control. Regular laboratory analysis and SDI measurements were performed to monitor the quality of feed water and filtrate. The UF filtrate is meant to be used as feed to seawater reverse osmosis (SWRO). In this regard the SDI value is the indicator mostly used for the quality of the UF filtrate. However, SDI is known not to be a very reliable parameter for the fouling potential on RO membranes. For this reason an additional RO pilot plant with a permeator from spares of Addur SWRO Desalination Plant was installed, see Fig. 5. Filtrate from the UF pilot plant was used as feedwater to monitor the fouling behaviour of the RO membranes in addition to SDI measurements. 4. Factors affecting the UF performance Basic tests of different operational modes and different chemicals for cleaning of UF membranes and pre-treatment of feed water were being performed from March 2003 until February 2004.
New modified membranes were installed in March 2004 and operated until July 2005. During this time the operational mode of UF and pretreatment of feedwater was optimized. The following factors affected the performance of the UF: 4.1. Chlorination of feed water The feedwater to the Addur SWRO desalination plant was always chlorinated at the intake 1.2 km offshore to protect the intake piping and the media filter from biological growth (mussels, algae, bacteria). When this chlorination was shut off for a short time for maintenance or repair works, an immediate rise of UF permeability was observed. The lower permeability with chlorination can be explained by an increased content of dissolved organic carbon DOC due to the destruction of organisms by chlorination. Some DOC compounds (polysaccharides, building blocks, . . .) tend to block the membrane pores and require more intensive backwash to be removed. To test the performance of UF using non chlorinated water a 3’’ PVC pipe was laid into the sea as an alternative feed water source for the pilot plant, see Fig. 5. The intake point was about 200 m from the beach at a depth
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Addur plant intake 1.2 km offshore depth ~4 m
End of 3” pipe 200 m offshore depth ~1.5 m
3” pipe for non chlorinated water
Intake building with pilot plants
Fig. 5. Seawater intakes at Addur SWRO Desalination plant.
of about 1.5 m (according to tide), 0.3 m above the sandy ground. This water turned out to create heavy fouling on the UF membranes, requiring backwash every 8 minutes for stable operation. This extreme fouling potential is caused by high organic load of this water taken close from the surface, containing some effluent load, too. This result is contrary to the experience with non chlorinated water taken from a depth of about 4 m as described before. It was not possible to extend the intake pipe to deeper water, so the test using non chlorinated water was stopped. 4.2. Pre-treatment flocculants
of
feed
water
using
Laboratory tests to investigate the effect of flocculant dosing on UF Multibore membranes were performed at the IWW RheinischWestfa¨lisches Institut fu¨r Wasser, Mu¨lheim, Germany using original water from Addur intake. These tests clearly indicated a positive
effect on performance and cleaning of the membranes. As a consequence FeCl3 dosing was introduced to the Addur pilot tests as pre-treatment of UF feedwater. Attention has to be paid to to the dosing rate, to mixing at the injection point and to the reaction time allowed until reaching the membrane. If these conditions are well selected, a thin layer of iron hydroxide will build up on the membrane surface. Fig. 6 shows the surface of a membrane that was taken from the pilot plant at the end of the tests, before citric acid cleaning, for examination by electron microscope. The membrane surface was found to be covered with a thin brown layer that is rich in Fe as a result of the FeCl3 dosing as pre-treatment of the feedwater. Different waterborne particles are buried in these deposits. This ‘‘filtration cake’’ on the membrane surface acts as an additional prefilter. It protects the membrane from irreversible fouling by dissolved organic compounds (DOC), because these compounds get
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Fig. 6. Membrane surface with iron rich deposits from FeCl3 dosing and waterborne particles.
Vergrößerung = 2.00 KX
connected to the Fe-flocs and can be backwashed instead of blocking pores of the membrane. This effect also decreases the SDI value of the UF filtrate. If FeCl3-dosing rate and backwash procedures are optimised, the thickness and structure of this layer is maintained stable. The SDI of the filtrate is reduced without remarkable increase of differential pressure of the membrane. It was observed that the differential pressure even dropped down when FeCl3-dosing was started. This effect can be explained by reduction of pore blockages in the UF membrane due to the prefiltration effected by the filtration cake. In case the filtration cake has grown too thick it can be removed completely by normal backwash procedures with addition of NaOH and citric acid and extended soaking time. The injection of just 0.25 ppm Fe turned out to be sufficient to optimise the performance of UF. With this low dosing rate no sludge deposits at all built up in the pipes and valves between the injection point and the membrane, as it is sometimes observed at higher dosing rates (>1 ppm for normal flocculant treatment).
10 µm
235
Hochsp. = 5.00 kV Signal A = SE2 Datum : 9 Sep 2005 Arbeitsabstand = 8 mm Photo Nr. = 1677 Zeit : 12:30:34
4.3. Seasonal changes of seawater The incoming seawater temperature at the Addur plant ranges from 16–20 C in winter to 30–36 C in summer. The seawater is very rich in its organic content and the bioactivity is especially high during early and late summer when water temperatures are around 30 C. SDI of feedwater is often >19 during these times, whereas SDI is in the range 15–18 during winter months. The chlorine dosage at the intake must be raised to around 2 ppm in summer, whereas around 1 ppm is sufficient during winter month to avoid bioactivity in intake lines and media filter. Consequently the fouling tendency on the UF membranes is higher during summer month requiring more intensive backwash procedures to maintain their performance. 4.4. Other factors Sometimes during the test period peaks of feed water turbidity and SDI were noticed. Some of these peaks were caused by sandstorms, others could not be explained. Some peaks have been connected with high humic acid
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concentrations, which indicate an effluent fume causing this peak. Some of these peaks did not effect the UF performance while some reduced the permeability of the membrane temporarily, but recovered by normal backwash. However, these temporary changes of feed water reflect normal operating conditions at the Addur plant. The UF membranes have to cope with this situation.
NaOCl (50 ppm as free Cl2, 20min soaking) every 2–3 h. The daily net filtrate production of each module (without filtrate needed for backwash) was 46 m3/d, resulting in a net flux of 42 l/m2h. Heavy fouling of the UF membranes occurred when non chlorinated seawater taken from the beach was used as feed for a test as explained in chapter 3.1. As a countermeasure the backwash frequency and the chlorine concentration for CEB was increased. Finally 2 extended CEB’s using NaOH (pH12) and citric acid (1%) with several hours soaking regained the full membrane performance. It has to be pointed out that this was the first extra cleaning procedure needed after 5½ month operation during summer 2004. No special detergent or complicated cleaning procedure
5. Operation and performance of UF pilot plant During the summer month of 2004 stable operation at a flux of 70l/m2h was achieved. Fig. 7 shows the permeability and the differential pressure across the membrane of this period. Membranes were kept clean by backwash every 17–20 min and by chemical enhanced backwash (CEB) with addition of
UF Permeability and Transmembrane Pressure July–December 2004 700 Extended CEB with NaOH and citric acid 18./19.12.04
Extended CEB with NaOH and NaOCl
Permeability [l/m2 hbar] TMP [mbar]
600
Raw “beachwater” as UF feed 24.10.–1.11, 14.–24.11.
500
400 Permeability at 25°C
300
Differential pressure
200
100 Flux 60 l/m2h
Flux 70 l/m2h 0 9.7
19.7
29.7
8.8
18.8
28.8
7.9
17.9
27.9
7.10
Date 2004
Fig. 7. Membrane performance during summer 2004.
17.10 27.10
6.11
16.11 26.11
6.12
16.12
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was necessary. This shows the high fouling resistance and reliability of the Multibore membrane. During summer 2005 pre-treatment of feed water by FeCl3 dosing was applied to improve membrane performance and filtrate quality, as described in chapter 3.2. Fig. 8 shows the permeability and the differential pressure across the membrane for the period June to July. On June 5 a CEB with 1% citric acid, 6 h soaking time was performed regaining the full membrane performance of the ‘‘as-new’’-condition. The permeability became stable at a range of 200–300 l/m2hbar compared to 300–500 l/m2h
bar during summer 2004 without Fe dosing. The decrease of permeability was caused by the filtration cake building up on the membrane surface which increases the differential pressure across the membrane. On the other hand this filtration cake improved the quality of the filtrate (see chapter 5) and made backwash easier, improving the net production. Backwash was performed every 20 min, CEB with addition of NaOCl every 3 h. Backwash procedures have been optimised testing different flows during June. Shorter backwash with increased flow turned
Permeability and Differential Pressure of UF Membranes 5.6.-29.7.05 Optimisation of Fe dosing and Backwash 900
CEB with 1% Citric Acid 6 hrs soaking
CEB with 1% Citric Acid, 6hrs soaking, after NaOH-CEB
800
CEB with NaOH (pH12) soaking over night
Permeability [l/m2hbar]
700 Filtration time increased from 17 to 20 min => stable operation continues
600 Fe dosing switched off 25.6 18:00–26.6. 10:00 => drop in permeability
500
CEB with 2 hrs soaking during module inspektion
400 Test to optimize backwash flow
300 200
drop in Cl concentratin for CEB Reduction of BW time from 60 to 45s
100
BW flow increased from 200 to 215 l/m2h 0.25 ppm Fe
0.25 ppm Fe
0.35 ppm Fe
Δp [mbar]
0 400 300 200 Transmembrane Differential Pressure of UF 100 0 5.6
10.6
15.6
20.6
25.6
30.6
Fig. 8. Membrane performance during summer 2005.
5.7
10.7
15.7
20.7
25.7
30.7
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out to be more effective and was applied in July. Operation of the membrane was stable at a resulting net filtrate production of 47,8 l/m2h. Table 1 shows the membrane performance with and without flocculant dosing, compared with the data obtained for spiral wound membranes [3] for the specified duration. 6. Quality of UF filtrate The quality of the UF filtrate can be judged by the measured Silt Density Index SDI and by the fouling of the RO module fed with the UF filtrate. SDI of pilot plant feed water and filtrate was measured manually 1–2 times a day. The fouling of the permeator in the RO pilot unit was monitored by the rising pressure loss from feed inlet to reject outlet. 6.1. SDI Fig. 9 shows the SDI of feed water and filtrate for the complete test period from March 2004 until end of test in July 2005. Following common practice at Addur the SDI was measured by collecting 500 ml after 15 min for filtrate (SDI15) and by collecting 200 ml after 5 min for feed water (SDI5). By definition of SDI5 used for feed water the maximum value is 20. This means, all values over 18 already indicate very high fouling potential. During summer SDI values at Addur were around 19 with peak values
over 19.5 indicating extreme fouling potential. SDI peaks of feed water often caused higher SDI values of filtrate as well. It can be seen that filtrate SDI was lower during summer 2005 with Fe dosing compared to summer 2004 without Fe dosing. This indicates the positive effect of this pre-treatment on filtrate quality. It has to be pointed out, that these low SDI values have been reached without sand filtration upstream of the UF, which is used for the Addur plant, see Fig. 3. 6.2. Fouling of RO membrane The rise in pressure loss over the RO permeator indicating fouling formation was very slow. Fig. 10 (top) shows the development of pressure loss for the permeator of the RO pilot plant and for the actual plant during first year of operation. Operation time of the permeator until required cleaning was remarkably longer than in the actual Addur Plant. Fig. 10 (bottom) shows the operation of the Addur plant permeators during the first year of service in 1990/ 1991. About 8 cleanings were performed during first operation year, in the year 2004 about 40 cleanings were necessary. As per the pilot study of the Multibore UF, only 4 cleanings were needed during the first year of operation, confirming the high quality of this type of UF systems.
Table 1 UF membrane performance Multibore UF
Flux [l/m2h] net production [m3/m2d] TMP [mbar] SDI
Addur plant [3]
summer 2004 no flocculant dosing
summer 2005 with flocculant dosing
2000–2004
70 1.01 100–200 2–3.5
70 1.15 200–250 1–3
40 (20–60) <0.7 (est.) 300–2000 2000: 1–3.5 2001: 2–4 2002–04: 3–6
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18
SDI
SDI Feed
16
14
5 Addition of ~0.3ppm Fe to feedwater 4
SDI
3
SDI UF-Filtrate 2
1 raw “beachwater” as feed water from normal intake as feed 0 1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Date 2004/2005
Fig. 9. SDI of feed water and filtrate of the UF pilot plant.
7. Resulting benefits membranes at Addur
of
multibore
UF
7.1. Consumption of chemicals In [3] the total cost for chemicals for operation of existing spiral wound UF modules at Addur are given for the years 2001–2003. Specific cost is more than
0.013US$ per m3 UF filtrate. Use of Multibore membranes and their optimised operation as demonstrated in summer 2005 allow substantial reduction of chemical consumption. Specific costs will be reduced by 75% down to about 0.003US$ per m3 UF filtrate. The consumption of chemicals for cleaning of RO membranes will be reduced as well by the
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600 Taprogge RO pilot unit differential pressure only 4 cleanings/year required to keep dp < 300 kPa Autumn
Cleaning
Summer
Summer
Cleaning
400
Cleaning
Cleaning
Pressure Loss Delta p [kPa]
500
300
200 Winter 100 Phase 1 29.05.–20.09.2004
Phase 2 22.09.–24.10.04
Phase 4a Phase 4b Phase 3 28.10.–25.12.2004 27.12.04–21.02.05 21.06.–26.07.05
0 0
1000
2000
4000
3000
5000
6000
7000
8000
9000
600 Ad Dur Train A differential pressure within the first operating year taken from: Burashid, Hussain in Desalination 165 (2004) 11–22, 8 cleanings required during 1st operating year, dp up to 500 kPa. (In 2004 cleaning needed every 7–10 days)
Pressure Loss Delta p [kPa]
500
Summer
400 Winter
300 Summer 200 Start June 1990
100
Permeator: Dupont Permasep B10 6835T in 2004 cleaning needed every 7–10 days
0 0
1000
2000
3000
4000
5000
6000
7000
8000
9000
Running Hours
Fig. 10. Comparison of differential pressure over RO module and required cleaning intervals in pilot plant and Addur plant.
K.A. Bu-Rashid, W. Czolkoss / Desalination 203 (2007) 229–242
extension of cleaning intervals due to improved quality of UF filtrate, see chapter 5. Beside cost saving this also means a substantial reduction of chemicals discharged into the Arabian Gulf. The impact of effluents from desalination plants into the seawater is a growing concern [4]. 7.2. Energy consumption The energy consumption of UF depends on the required trans membrane pressure TMP of the modules for filtration and backwash. The total power consumption for 1 m3 UF filtrate from Multibore membranes is around 0,05 kWh only due to their low TMP. The TMP of the spiral wound UF modules currently used at Addur is more than 1 bar compared to 0.25 bar with Multibore membranes. This indicates that the power consumption for UF can be reduced by 75% compared to the existing plant. 7.3. Pretreatment of feedwater At the existing plant at Addur single media filtration is arranged upstream the UF for pre-treatment of feed water. This media filter requires large footprint area and causes high costs for investment and maintenance. The Multibore membranes require 200 m pre-filtration only to protect the modules from debris and coarse particles. A fully automatic backwashable fine filter could replace the single media filter of the existing plant. This means another reduction of operating cost. 7.4. Membrane reliability Operating experience from other applications already proved the high reliability and integrity of the Multibore UF membranes. During the tests in Addur no fibre breakage occurred. Full membrane performance could always be
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regained by backwash procedures, even after repeated use of non chlorinated water taken from the beach causing extreme membrane fouling. As a result membrane replacement cost will be low using Multibore UF membranes. 7.5. Membrane performance During the tests the operation mode for the Multibore UF membranes was optimised for filtrate output and filtrate quality. The UF pilot plant was operated stable at a flux of 70 l/m2h, which is an excellent value for the seawater conditions at Addur. Existing UF of the Addur plant is operated at a flux around 40 l/m2h (20–60 l/m2h [3]). The higher performance of the Multibore UF modules compared to the existing spiral wound UF modules (see Table 1) reduces the number of modules required. Multibore UF modules can be arranged vertically in racks with compact design for small footprint and short connecting piping, as shown in Fig. 11. This minimises investment cost, short connecting piping helps to minimise operating and maintenance costs. 7.6. Fouling of RO membranes The SDI of the pilot plants UF filtrate was well below 3 with flocculant dosing, even below 2 at the end of the test in summer 2005, while SDI of the Addur plant UF filtrate was up to 6 in 2004 [3]. Fouling tendency of the pilot plant RO permeator was remarkably lower using UF filtrate from Multibore UF membranes compared to Addur plant experience, see Fig. 10. 8. Conclusion Addur is known to provide the toughest seawater in the Arabian Gulf for desalination due to its extremely high fouling potential. Effective pre-treatment under all seasonal changing conditions is essential to prevent fouling of the RO membranes and maintain their performance. Addur SWRO
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Filtrate
ca.2200 mm
Feed, Discharge
1960 + 370 mm
1960 mm
1640 mm
Fig. 11. Compact rack for up to 50 Multibore UF modules.
Desalination plant uses single media filtration SMF and spiral wound UF as pre-treatment for RO. This arrangement suffered from many problems in the past and could not provide the required performance. As an alternative pre-treatment new Multibore UF membranes were successfully tested at this location. As per the analytical results obtained from the test trials, this type of membranes could be used at Addur SWRO desalination plant without the need for single media filtration. The Multibore UF membranes provide different benefits such as lower consumption of chemicals and energy. This reduces operational cost and is beneficial to the environment. SMF is not required anymore when using Multibore UF, which reduces maintenance costs at Addur. The higher performance and smaller footprint required for UF and the renunciation of a media filter reduces investment cost for new projects. During the tests the operation mode for the Multibore UF membranes was optimised for filtrate output and filtrate quality. Fouling tendency of the pilot plant RO permeator was remarkably lower using UF filtrate from
Multibore UF membranes compared to Addur plant experience. The pilot testing at Addur proved, that Multibore UF membranes with their high performance, effectiveness and reliability are a very promising alternative for pretreatment for SWRO desalination plants even at toughest seawater conditions at the Arabian Gulf. References [1] P. Berg, R. Winkler, J. Wunram and M. Schmidt, Hydrodynamic optimisation of UF Multibore Membranes and Modules, Case Studies, Membranes in Drinking and Industrial Water Production, University of L’Aquila Italy, November 15–17, 2004 [2] Khalid Burashid and Ali Redha Hussain, SWRO plant operation and maintenance experience: Addur desalination plant operation assessment. Desalination 165 (2004) 11–22. [3] Khalid Burashid, Ahmed Hashim, T. Kannari, K. Tada and H. Iwahori, UF Membrane Performance Experience at Addur; Expectations, Reality and Prospects. IDA World Congress, Singapore, 2005 [4] Ahmed Hashim and Muneer Hajjaj, Impact of desalination plants fluid effluents on the integrity of seawater, with the Arabian Gulf in perspective. Desalination 182 (2005) 373–393
Pilot Tests of Multibore UF Membrane at Addur SWRO Desalination Plant, Bahrain Khalid Ahmed Bu-Rashid*,a, Wolfgang Czolkossb a
Ministry of Electricity and Water, Kingdom of Bahrain email: [email protected] b TAPROGGE GmbH, 58300 Wetter, Germany email: [email protected] Received 27 March 2006; accepted 15 April 2006
Abstract UF membranes with new Multibore design were tested in a pilot unit at the Addur SWRO Desalination Plant, Bahrain as pre-treatment for RO. The seawater at Addur is known for its extremely bad condition, mainly caused by industrial and residential waste disposals and high organic contents and bioactivity. Feed water for the pilot plant was taken from the Addur seawater supply line (intake 1.2 km offshore, chlorinated) directly, without making use of the existing media filtration. The existing UF/RO plant at Addur is suffering from heavy fouling and scaling problems. The pilot unit was started in March 2003. A first set of trials was done to test different operational modes and cleaning procedures under extreme fouling conditions. Based on these trials new modified UF modules were installed in March 2004 and operated until July 2005. During these 16 month of service the new Multibore UF membranes performed successfully. During summer month 2004 no membrane cleaning except normal backwash procedures with sodium hypo chloride was necessary for stable operation. The use of not chlorinated seawater as feed to the UF was tested in October 2004 and during the outage of the Addur plant February-April 2005. This water was taken by a newly installed 3’’ intake pipe from a location about 200 m offshore from a depth of 1.5 m only. This so called ‘‘raw beach water’’ turned out to be not suitable for UF filtration due to its extremely high fouling potential. The injection of flocculants into feedwater was tested to further improve the UF output and filtrate quality. The dosing of very small amounts of Fe proved to be beneficial. During summer 2005 the UF operation with Fe injection was stable with a flux of 70 l/m2h at a differential pressure around 250 mbar only. Filtrate SDI was always well below 3, even below 2 in July 2005. An additional RO pilot unit was installed in May 2004 to directly test the fouling potential of the filtrate from the UF pilot plant. In the RO pilot plant a permeator as it is used at the Addur SWRO plant was installed. The fouling of this permeator was remarkably lower compared to the fouling in the Addur *Corresponding author. Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21-25 May 2006 0011-9164/07/$– See front matter 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.04.010
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SWRO desalination plant. This proves the high quality of the UF filtrate in addition to the low SDI values. Because of the low consumption of cleaning chemicals, low differential pressure and no need for media filtration the operating costs for Multibore UF membranes would be remarkably lower compared to the operating costs of the existing arrangement at Addur. The pilot test under the extreme seawater conditions of Addur revealed that these new UF membranes are a very promising alternative for pre-treatment for SWRO desalination plants. Keywords: Ultra filtration; Silt Density Index; Multibore UF membrane
1. Introduction
desalination (B-9) leaving the first stage to improve the permeate quality. The membranes at Addur plant suffered very fast from flux decline primarily due to rapid biofouling, demanding extensive cleaning and frequent replacement of membranes within a short time of commissioning which impaired production capacity and availability for more than one decade. Initial experiments conducted in 1997 confirmed that the main problem rests in the design of the under rated conventional pre-treatment plant being inadequate to cope with the severe biofouling activity. Pilot tests in late 90’s gave strong indication that ultrafiltration membranes might be a suitable option to the problems faced in the pre-treatment plant, in
The Addur Desalination Plant is the only Seawater Reverse Osmosis Desalination Plant in the Kingdom of Bahrain, commissioned in 1990. The plant employs Dupont Permasep Hollow Fine Fibre membranes and the original design uses conventional sea water pre-treatment, see Fig. 1. In the pre-treatment section, pH is reduced via sulfuric acid and ferric chloride is dosed as coagulant. The coagulated flocks are filtered off in the dual media filter beds. Chlorination is performed by sodium hypochlorite solution, filtration through micron guard filters and dechlorination by sodium bisulfite. The RO membranes are arranged in two streams, sea water (B-10) followed by brackish water
HYPOCHLORITE GENERATIN UNIT
COAGULANT DOSING
ACID DOSING CL2 INJECTION
SODIUM BISULPHITE DOSING
COAGULANT AID DOSING CARTRIDG FILTER PUMP CARTRIDG BACKWASH FILTER PUMPS
DRAWBACK TANKS LIME
1ST R.O PASS
2
ND
PASS
2 DUAL MEDIA FILTER SE WATE SUPPL TRAVELLING PUM SCREENS
TRAS RAC
3
CLEARWEL
M ERT
ACID DOSING #
PRODUCT
4
AIR SCOURING BLOWER
2nd PASS BOOSTER PUMPS
1st PASS BOOSTER PUMPS
Fig. 1. Schematic process of the Addur as per the original design.
DECARBONATO PRODUCT PUMP
TO STORAGE TANKS
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combination with the existing pre-treatment or as stand alone UF. The plant rehabilitation program was implemented in 1999 using full UF scheme and limiting the dual media filtration to sand media without micron guard filtration. Soon, during 2000 and 2002 it became evident that the membranes suffered from biological fouling, analogues to the problems faced before the rehabilitation, requiring further refinement to the UF system. These shortfalls in the UF system affected the operation and reduced the production capacity of the plant associated with high production cost. Among the recommendations put forward in various papers was to employ pilot testing on the site where the real plant will be constructed for a minimum period of one year for proper assessment of the combined UF þ RO system. Experiments continued during 2002–2003 with an enhanced UF model of the present design under the existing contract ‘‘The production improvement works commenced mid 2005’’ employing different RO membrane design with successful results which was covered in separate paper [3]. During 2002, Taprogge approached the Ministry of Electricity and Water to permit the trial of UF membrane with Multibore design at Addur plant to assess the suitability of this new type of membranes for use in difficult seawater conditions. The initial operation of the pilot plant started in March 2003. This paper shall describe the different phases of the pilot trial and the performance parameters of the Multibore UF membranes using seawater directly from the intake without media filtration, in attempt to qualify the suitability of this type of UF system designs for use in SWRO plants processing difficult seawater with extremely high fouling potential. 2. The new multibore UF membrane New Multibore UF membranes (by inge AG, Germany) have already been well
231
established for municipal and industrial drinking water applications treating sweet water from lakes, rivers or wells [1]. To prove the capability of these membranes for seawater applications they were tested in a pilot unit at the Ad Dur SWRO Desalination Plant, Bahrain as pre-treatment for RO. Addur is known to be the toughest place in the Gulf for desalination. The surrounding seawater is of high salinity and bioactivity. Industrial and residential waste disposals causes additional organic content. As a result the existing UF/RO plant at Addur is suffering from heavy fouling and scaling problems [2]. The Multibore UF membranes are hollow fibre membranes with 7 capillaries integrated into a single fibre, see Fig. 2. They are made of modified polyethersulfone (PESM). During the fabrication process of the fibres a skin with extremely fine pores is formed on the inner surface of the capillaries. This skin functions as the UF membrane with a molecular weight cut-off of 100.000–150.000 Dalton (pore diameter 20 nm). Flow direction during filtration is from inside of the capillaries to outside of the fibre. The foamy support structure surrounding the 7 integrated capillaries guarantees extraordinary stability and full membrane integrity during operation and cleaning. Inner diameter of the capillaries is 0.9 mm, outer diameter is 4.3 mm. The Multibore membranes are arranged in special modules containing 45 m2 of membrane area each. The modules are designed for vertical installation and are provided with feed connections at top and bottom. So dead end or cross flow filtration as well as backwash can be performed in 2 ways, allowing change of flow direction inside the capillaries. The pilot test should demonstrate the capability of the new Multibore UF membranes in treating seawater of extremely high fouling potential. Different operation parameters and pre-treatment should be tested and optimised
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Fig. 2. Multibore UF membrane and UF module Dizzer1 5000 with 45 m2 multibore UF membrane.
considering all seasonal and other changes of the water quality at Addur. 3. The pilot plant Fig. 3 shows the general arrangement of the pilot units at the Addur SWRO desalination plant. The feed water to the UF pilot plant is taken downstream of the intake screens but upstream of the sand filter (single media filter). So chlorination and screening is the only pretreatment of the seawater in front of the UF pilot plant. There is an automatic backwashable 200 m filter integrated in the pilot plant to protect membranes from debris and coarse particles. Fig. 4 shows the UF pilot plant. Up to 6 UF modules can be installed and operated automatically. Remote control by modem and
telephone line is possible, too. The rack is designed for a filtrate flow of 12 m3/h. Frequency controlled pumps allow operation at different flows and with less than 6 modules. Pneumatically driven valves allow the following operating modes:
dead end filtration from top or bottom backwash to top or bottom purge forward flush
Filtrate was collected in a tank to allow backwash using filtrate from this tank. Dosing pumps allowed to perform chemical enhanced backwash by injection of NaOCl-solution or other chemicals into the backwash flow.
TAPROGGE Pilot Plants Fe non chlorinated beachwater
Cl Seawater
Intake
1,2 km
Screens
SBS HCl UF Pilotplant
RO Pilotplant
UF Backwash
RO Reject
SMF* * Single media filter
Fig. 3. Arrangement of UF and RO pilot units.
UF
Ad Dur SWRO Desalination Plant
RO
Tank
Tanks
K.A. Bu-Rashid, W. Czolkoss / Desalination 203 (2007) 229–242
233
Dupont permasep B10 RO module Multibore UF modules Dizzer5000
Fig. 4. UF and RO pilot plants in the intake building of Addur plant.
The pilot plant was equipped with all needed instrumentation for performance measurements and operational control. Regular laboratory analysis and SDI measurements were performed to monitor the quality of feed water and filtrate. The UF filtrate is meant to be used as feed to seawater reverse osmosis (SWRO). In this regard the SDI value is the indicator mostly used for the quality of the UF filtrate. However, SDI is known not to be a very reliable parameter for the fouling potential on RO membranes. For this reason an additional RO pilot plant with a permeator from spares of Addur SWRO Desalination Plant was installed, see Fig. 5. Filtrate from the UF pilot plant was used as feedwater to monitor the fouling behaviour of the RO membranes in addition to SDI measurements. 4. Factors affecting the UF performance Basic tests of different operational modes and different chemicals for cleaning of UF membranes and pre-treatment of feed water were being performed from March 2003 until February 2004.
New modified membranes were installed in March 2004 and operated until July 2005. During this time the operational mode of UF and pretreatment of feedwater was optimized. The following factors affected the performance of the UF: 4.1. Chlorination of feed water The feedwater to the Addur SWRO desalination plant was always chlorinated at the intake 1.2 km offshore to protect the intake piping and the media filter from biological growth (mussels, algae, bacteria). When this chlorination was shut off for a short time for maintenance or repair works, an immediate rise of UF permeability was observed. The lower permeability with chlorination can be explained by an increased content of dissolved organic carbon DOC due to the destruction of organisms by chlorination. Some DOC compounds (polysaccharides, building blocks, . . .) tend to block the membrane pores and require more intensive backwash to be removed. To test the performance of UF using non chlorinated water a 3’’ PVC pipe was laid into the sea as an alternative feed water source for the pilot plant, see Fig. 5. The intake point was about 200 m from the beach at a depth
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Addur plant intake 1.2 km offshore depth ~4 m
End of 3” pipe 200 m offshore depth ~1.5 m
3” pipe for non chlorinated water
Intake building with pilot plants
Fig. 5. Seawater intakes at Addur SWRO Desalination plant.
of about 1.5 m (according to tide), 0.3 m above the sandy ground. This water turned out to create heavy fouling on the UF membranes, requiring backwash every 8 minutes for stable operation. This extreme fouling potential is caused by high organic load of this water taken close from the surface, containing some effluent load, too. This result is contrary to the experience with non chlorinated water taken from a depth of about 4 m as described before. It was not possible to extend the intake pipe to deeper water, so the test using non chlorinated water was stopped. 4.2. Pre-treatment flocculants
of
feed
water
using
Laboratory tests to investigate the effect of flocculant dosing on UF Multibore membranes were performed at the IWW RheinischWestfa¨lisches Institut fu¨r Wasser, Mu¨lheim, Germany using original water from Addur intake. These tests clearly indicated a positive
effect on performance and cleaning of the membranes. As a consequence FeCl3 dosing was introduced to the Addur pilot tests as pre-treatment of UF feedwater. Attention has to be paid to to the dosing rate, to mixing at the injection point and to the reaction time allowed until reaching the membrane. If these conditions are well selected, a thin layer of iron hydroxide will build up on the membrane surface. Fig. 6 shows the surface of a membrane that was taken from the pilot plant at the end of the tests, before citric acid cleaning, for examination by electron microscope. The membrane surface was found to be covered with a thin brown layer that is rich in Fe as a result of the FeCl3 dosing as pre-treatment of the feedwater. Different waterborne particles are buried in these deposits. This ‘‘filtration cake’’ on the membrane surface acts as an additional prefilter. It protects the membrane from irreversible fouling by dissolved organic compounds (DOC), because these compounds get
K.A. Bu-Rashid, W. Czolkoss / Desalination 203 (2007) 229–242
Fig. 6. Membrane surface with iron rich deposits from FeCl3 dosing and waterborne particles.
Vergrößerung = 2.00 KX
connected to the Fe-flocs and can be backwashed instead of blocking pores of the membrane. This effect also decreases the SDI value of the UF filtrate. If FeCl3-dosing rate and backwash procedures are optimised, the thickness and structure of this layer is maintained stable. The SDI of the filtrate is reduced without remarkable increase of differential pressure of the membrane. It was observed that the differential pressure even dropped down when FeCl3-dosing was started. This effect can be explained by reduction of pore blockages in the UF membrane due to the prefiltration effected by the filtration cake. In case the filtration cake has grown too thick it can be removed completely by normal backwash procedures with addition of NaOH and citric acid and extended soaking time. The injection of just 0.25 ppm Fe turned out to be sufficient to optimise the performance of UF. With this low dosing rate no sludge deposits at all built up in the pipes and valves between the injection point and the membrane, as it is sometimes observed at higher dosing rates (>1 ppm for normal flocculant treatment).
10 µm
235
Hochsp. = 5.00 kV Signal A = SE2 Datum : 9 Sep 2005 Arbeitsabstand = 8 mm Photo Nr. = 1677 Zeit : 12:30:34
4.3. Seasonal changes of seawater The incoming seawater temperature at the Addur plant ranges from 16–20 C in winter to 30–36 C in summer. The seawater is very rich in its organic content and the bioactivity is especially high during early and late summer when water temperatures are around 30 C. SDI of feedwater is often >19 during these times, whereas SDI is in the range 15–18 during winter months. The chlorine dosage at the intake must be raised to around 2 ppm in summer, whereas around 1 ppm is sufficient during winter month to avoid bioactivity in intake lines and media filter. Consequently the fouling tendency on the UF membranes is higher during summer month requiring more intensive backwash procedures to maintain their performance. 4.4. Other factors Sometimes during the test period peaks of feed water turbidity and SDI were noticed. Some of these peaks were caused by sandstorms, others could not be explained. Some peaks have been connected with high humic acid
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concentrations, which indicate an effluent fume causing this peak. Some of these peaks did not effect the UF performance while some reduced the permeability of the membrane temporarily, but recovered by normal backwash. However, these temporary changes of feed water reflect normal operating conditions at the Addur plant. The UF membranes have to cope with this situation.
NaOCl (50 ppm as free Cl2, 20min soaking) every 2–3 h. The daily net filtrate production of each module (without filtrate needed for backwash) was 46 m3/d, resulting in a net flux of 42 l/m2h. Heavy fouling of the UF membranes occurred when non chlorinated seawater taken from the beach was used as feed for a test as explained in chapter 3.1. As a countermeasure the backwash frequency and the chlorine concentration for CEB was increased. Finally 2 extended CEB’s using NaOH (pH12) and citric acid (1%) with several hours soaking regained the full membrane performance. It has to be pointed out that this was the first extra cleaning procedure needed after 5½ month operation during summer 2004. No special detergent or complicated cleaning procedure
5. Operation and performance of UF pilot plant During the summer month of 2004 stable operation at a flux of 70l/m2h was achieved. Fig. 7 shows the permeability and the differential pressure across the membrane of this period. Membranes were kept clean by backwash every 17–20 min and by chemical enhanced backwash (CEB) with addition of
UF Permeability and Transmembrane Pressure July–December 2004 700 Extended CEB with NaOH and citric acid 18./19.12.04
Extended CEB with NaOH and NaOCl
Permeability [l/m2 hbar] TMP [mbar]
600
Raw “beachwater” as UF feed 24.10.–1.11, 14.–24.11.
500
400 Permeability at 25°C
300
Differential pressure
200
100 Flux 60 l/m2h
Flux 70 l/m2h 0 9.7
19.7
29.7
8.8
18.8
28.8
7.9
17.9
27.9
7.10
Date 2004
Fig. 7. Membrane performance during summer 2004.
17.10 27.10
6.11
16.11 26.11
6.12
16.12
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was necessary. This shows the high fouling resistance and reliability of the Multibore membrane. During summer 2005 pre-treatment of feed water by FeCl3 dosing was applied to improve membrane performance and filtrate quality, as described in chapter 3.2. Fig. 8 shows the permeability and the differential pressure across the membrane for the period June to July. On June 5 a CEB with 1% citric acid, 6 h soaking time was performed regaining the full membrane performance of the ‘‘as-new’’-condition. The permeability became stable at a range of 200–300 l/m2hbar compared to 300–500 l/m2h
bar during summer 2004 without Fe dosing. The decrease of permeability was caused by the filtration cake building up on the membrane surface which increases the differential pressure across the membrane. On the other hand this filtration cake improved the quality of the filtrate (see chapter 5) and made backwash easier, improving the net production. Backwash was performed every 20 min, CEB with addition of NaOCl every 3 h. Backwash procedures have been optimised testing different flows during June. Shorter backwash with increased flow turned
Permeability and Differential Pressure of UF Membranes 5.6.-29.7.05 Optimisation of Fe dosing and Backwash 900
CEB with 1% Citric Acid 6 hrs soaking
CEB with 1% Citric Acid, 6hrs soaking, after NaOH-CEB
800
CEB with NaOH (pH12) soaking over night
Permeability [l/m2hbar]
700 Filtration time increased from 17 to 20 min => stable operation continues
600 Fe dosing switched off 25.6 18:00–26.6. 10:00 => drop in permeability
500
CEB with 2 hrs soaking during module inspektion
400 Test to optimize backwash flow
300 200
drop in Cl concentratin for CEB Reduction of BW time from 60 to 45s
100
BW flow increased from 200 to 215 l/m2h 0.25 ppm Fe
0.25 ppm Fe
0.35 ppm Fe
Δp [mbar]
0 400 300 200 Transmembrane Differential Pressure of UF 100 0 5.6
10.6
15.6
20.6
25.6
30.6
Fig. 8. Membrane performance during summer 2005.
5.7
10.7
15.7
20.7
25.7
30.7
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out to be more effective and was applied in July. Operation of the membrane was stable at a resulting net filtrate production of 47,8 l/m2h. Table 1 shows the membrane performance with and without flocculant dosing, compared with the data obtained for spiral wound membranes [3] for the specified duration. 6. Quality of UF filtrate The quality of the UF filtrate can be judged by the measured Silt Density Index SDI and by the fouling of the RO module fed with the UF filtrate. SDI of pilot plant feed water and filtrate was measured manually 1–2 times a day. The fouling of the permeator in the RO pilot unit was monitored by the rising pressure loss from feed inlet to reject outlet. 6.1. SDI Fig. 9 shows the SDI of feed water and filtrate for the complete test period from March 2004 until end of test in July 2005. Following common practice at Addur the SDI was measured by collecting 500 ml after 15 min for filtrate (SDI15) and by collecting 200 ml after 5 min for feed water (SDI5). By definition of SDI5 used for feed water the maximum value is 20. This means, all values over 18 already indicate very high fouling potential. During summer SDI values at Addur were around 19 with peak values
over 19.5 indicating extreme fouling potential. SDI peaks of feed water often caused higher SDI values of filtrate as well. It can be seen that filtrate SDI was lower during summer 2005 with Fe dosing compared to summer 2004 without Fe dosing. This indicates the positive effect of this pre-treatment on filtrate quality. It has to be pointed out, that these low SDI values have been reached without sand filtration upstream of the UF, which is used for the Addur plant, see Fig. 3. 6.2. Fouling of RO membrane The rise in pressure loss over the RO permeator indicating fouling formation was very slow. Fig. 10 (top) shows the development of pressure loss for the permeator of the RO pilot plant and for the actual plant during first year of operation. Operation time of the permeator until required cleaning was remarkably longer than in the actual Addur Plant. Fig. 10 (bottom) shows the operation of the Addur plant permeators during the first year of service in 1990/ 1991. About 8 cleanings were performed during first operation year, in the year 2004 about 40 cleanings were necessary. As per the pilot study of the Multibore UF, only 4 cleanings were needed during the first year of operation, confirming the high quality of this type of UF systems.
Table 1 UF membrane performance Multibore UF
Flux [l/m2h] net production [m3/m2d] TMP [mbar] SDI
Addur plant [3]
summer 2004 no flocculant dosing
summer 2005 with flocculant dosing
2000–2004
70 1.01 100–200 2–3.5
70 1.15 200–250 1–3
40 (20–60) <0.7 (est.) 300–2000 2000: 1–3.5 2001: 2–4 2002–04: 3–6
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K.A. Bu-Rashid, W. Czolkoss / Desalination 203 (2007) 229–242 20
18
SDI
SDI Feed
16
14
5 Addition of ~0.3ppm Fe to feedwater 4
SDI
3
SDI UF-Filtrate 2
1 raw “beachwater” as feed water from normal intake as feed 0 1.3
1.4
1.5
1.6
1.7
1.8
1.9
1.10
1.11
1.12
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Date 2004/2005
Fig. 9. SDI of feed water and filtrate of the UF pilot plant.
7. Resulting benefits membranes at Addur
of
multibore
UF
7.1. Consumption of chemicals In [3] the total cost for chemicals for operation of existing spiral wound UF modules at Addur are given for the years 2001–2003. Specific cost is more than
0.013US$ per m3 UF filtrate. Use of Multibore membranes and their optimised operation as demonstrated in summer 2005 allow substantial reduction of chemical consumption. Specific costs will be reduced by 75% down to about 0.003US$ per m3 UF filtrate. The consumption of chemicals for cleaning of RO membranes will be reduced as well by the
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600 Taprogge RO pilot unit differential pressure only 4 cleanings/year required to keep dp < 300 kPa Autumn
Cleaning
Summer
Summer
Cleaning
400
Cleaning
Cleaning
Pressure Loss Delta p [kPa]
500
300
200 Winter 100 Phase 1 29.05.–20.09.2004
Phase 2 22.09.–24.10.04
Phase 4a Phase 4b Phase 3 28.10.–25.12.2004 27.12.04–21.02.05 21.06.–26.07.05
0 0
1000
2000
4000
3000
5000
6000
7000
8000
9000
600 Ad Dur Train A differential pressure within the first operating year taken from: Burashid, Hussain in Desalination 165 (2004) 11–22, 8 cleanings required during 1st operating year, dp up to 500 kPa. (In 2004 cleaning needed every 7–10 days)
Pressure Loss Delta p [kPa]
500
Summer
400 Winter
300 Summer 200 Start June 1990
100
Permeator: Dupont Permasep B10 6835T in 2004 cleaning needed every 7–10 days
0 0
1000
2000
3000
4000
5000
6000
7000
8000
9000
Running Hours
Fig. 10. Comparison of differential pressure over RO module and required cleaning intervals in pilot plant and Addur plant.
K.A. Bu-Rashid, W. Czolkoss / Desalination 203 (2007) 229–242
extension of cleaning intervals due to improved quality of UF filtrate, see chapter 5. Beside cost saving this also means a substantial reduction of chemicals discharged into the Arabian Gulf. The impact of effluents from desalination plants into the seawater is a growing concern [4]. 7.2. Energy consumption The energy consumption of UF depends on the required trans membrane pressure TMP of the modules for filtration and backwash. The total power consumption for 1 m3 UF filtrate from Multibore membranes is around 0,05 kWh only due to their low TMP. The TMP of the spiral wound UF modules currently used at Addur is more than 1 bar compared to 0.25 bar with Multibore membranes. This indicates that the power consumption for UF can be reduced by 75% compared to the existing plant. 7.3. Pretreatment of feedwater At the existing plant at Addur single media filtration is arranged upstream the UF for pre-treatment of feed water. This media filter requires large footprint area and causes high costs for investment and maintenance. The Multibore membranes require 200 m pre-filtration only to protect the modules from debris and coarse particles. A fully automatic backwashable fine filter could replace the single media filter of the existing plant. This means another reduction of operating cost. 7.4. Membrane reliability Operating experience from other applications already proved the high reliability and integrity of the Multibore UF membranes. During the tests in Addur no fibre breakage occurred. Full membrane performance could always be
241
regained by backwash procedures, even after repeated use of non chlorinated water taken from the beach causing extreme membrane fouling. As a result membrane replacement cost will be low using Multibore UF membranes. 7.5. Membrane performance During the tests the operation mode for the Multibore UF membranes was optimised for filtrate output and filtrate quality. The UF pilot plant was operated stable at a flux of 70 l/m2h, which is an excellent value for the seawater conditions at Addur. Existing UF of the Addur plant is operated at a flux around 40 l/m2h (20–60 l/m2h [3]). The higher performance of the Multibore UF modules compared to the existing spiral wound UF modules (see Table 1) reduces the number of modules required. Multibore UF modules can be arranged vertically in racks with compact design for small footprint and short connecting piping, as shown in Fig. 11. This minimises investment cost, short connecting piping helps to minimise operating and maintenance costs. 7.6. Fouling of RO membranes The SDI of the pilot plants UF filtrate was well below 3 with flocculant dosing, even below 2 at the end of the test in summer 2005, while SDI of the Addur plant UF filtrate was up to 6 in 2004 [3]. Fouling tendency of the pilot plant RO permeator was remarkably lower using UF filtrate from Multibore UF membranes compared to Addur plant experience, see Fig. 10. 8. Conclusion Addur is known to provide the toughest seawater in the Arabian Gulf for desalination due to its extremely high fouling potential. Effective pre-treatment under all seasonal changing conditions is essential to prevent fouling of the RO membranes and maintain their performance. Addur SWRO
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Filtrate
ca.2200 mm
Feed, Discharge
1960 + 370 mm
1960 mm
1640 mm
Fig. 11. Compact rack for up to 50 Multibore UF modules.
Desalination plant uses single media filtration SMF and spiral wound UF as pre-treatment for RO. This arrangement suffered from many problems in the past and could not provide the required performance. As an alternative pre-treatment new Multibore UF membranes were successfully tested at this location. As per the analytical results obtained from the test trials, this type of membranes could be used at Addur SWRO desalination plant without the need for single media filtration. The Multibore UF membranes provide different benefits such as lower consumption of chemicals and energy. This reduces operational cost and is beneficial to the environment. SMF is not required anymore when using Multibore UF, which reduces maintenance costs at Addur. The higher performance and smaller footprint required for UF and the renunciation of a media filter reduces investment cost for new projects. During the tests the operation mode for the Multibore UF membranes was optimised for filtrate output and filtrate quality. Fouling tendency of the pilot plant RO permeator was remarkably lower using UF filtrate from
Multibore UF membranes compared to Addur plant experience. The pilot testing at Addur proved, that Multibore UF membranes with their high performance, effectiveness and reliability are a very promising alternative for pretreatment for SWRO desalination plants even at toughest seawater conditions at the Arabian Gulf. References [1] P. Berg, R. Winkler, J. Wunram and M. Schmidt, Hydrodynamic optimisation of UF Multibore Membranes and Modules, Case Studies, Membranes in Drinking and Industrial Water Production, University of L’Aquila Italy, November 15–17, 2004 [2] Khalid Burashid and Ali Redha Hussain, SWRO plant operation and maintenance experience: Addur desalination plant operation assessment. Desalination 165 (2004) 11–22. [3] Khalid Burashid, Ahmed Hashim, T. Kannari, K. Tada and H. Iwahori, UF Membrane Performance Experience at Addur; Expectations, Reality and Prospects. IDA World Congress, Singapore, 2005 [4] Ahmed Hashim and Muneer Hajjaj, Impact of desalination plants fluid effluents on the integrity of seawater, with the Arabian Gulf in perspective. Desalination 182 (2005) 373–393