- Email: [email protected]
Field evaluation of capillary UF technology as a pretreatment for large seawater RO systems
DESALINATION ELSEVIER
Desalination
147 (2002) 55-62 www.elsevier.com/locate/desal
Field evaluation of capillary UF technology as a pretreatment for large seawater RO systems P. Glueckstern”, M. Priel”, Mark Wilr”* “Mekorot Water Co., Tel Aviv, Israel “Hydranautics, Oceanside, California, USA Tel. +I (760) 901-2548; Fax +I (760) 901-2664; email: [email protected]
Received
1 February 2002; accepted 1 March 2002
Abstract RO seawater systems operating on a surface feed water, originating from an open intake source, require an extensive pretreatment process in order to control membrane fouling. Considerations of long-term membrane performance stability have lead to an initial design concept of operation of seawater RO systems at a low permeate flux rate and low permeate recovery. In recent years the nominal performance of composite seawater membrane elements has improved significantly, and in parallel new backwashable microfiltration and ultrafiltration capillary technologies have been introduced commercially. This new membrane technology can be utilized to treat seawater from a surface sources. Use of membrane capillary technology as a pretreatment step can improve quality of the surface feed water to a level comparable or better than the water quality from the well water sources. A better feed water quality enables more effective optimization of operating parameters in the RO systems. Both permeate flux and system recovery rate can be increased considerably without creating membrane fouling conditions. Operation of seawater systems at higher permeate flux and recovery rate results in improved economics of RO seawater desalting. Field evaluation of hybrid membrane systems consisting of UF membrane pretreatment unit and a RO seawater unit was conducted subsequently at two test sites. The first test site was at the Red Sea (Eilat site) and the second test site was on the Mediterranean (Ashdod site). The RO membranes were commercial seawater elements in spiral wound configuration. The UF equipment utilized capillary backwashable elements operated in dead end flow mode. For comparison, a second pilot system consisting of conventional pretreatment and an RO unit was operated in parallel at the above sites. The conventional pretreatment unit included in line flocculation followed by media filtration. The tests were conducted over a period of two years. Raw water quality reflected seasonal changes of composition and weather conditions. The performances of UF and RO equipment were evaluated over a wide range of operating parameters such as recovery and flux rate. Field results were used to project and compare the economics of the seawater RO desalting process using conventional and membrane pretreatment. This paper will describe the experimental procedure and results of parallel operation of an integrated membrane unit and a conventional RO sweater system. The economic analysis of both designs based on local site conditions, will be provided as well. Keywords:
Reverse osmosis; Ultrafiltration;
*Corresponding
Integrated membrane
systems
author.
Presented at the International /U/I. 7-12, 2002
Congress
on Membranes
and Membrane
Processes
00 I l-9 164/02/$See front matter 0 2002 Elsevier Science B.V. All rights reserved HI: SO0 I I-9 164(02)00576-3
(ICOM),
Toulouse,
France,
56
P. Glueckstern
et al. /Desalination
147 (2002) 55-62
1. Introduction The significant decrease of water cost produced by seawater RO systems, as observed in recently awarded water supply contracts, is not a result of a single technology breakthrough, but rather an effect of a trend of continuous improvement of all aspects of the seawater desalination process. Some of the improvements have been proven in field operation, while others were introduced recently and are still pending long term field verification. The membrane technology contribution to better process economics results from manufacturing membrane barrier polymer with higher salt rejection and packaging more membrane area into an element structure. Lower power consumption is a result of the higher efficiencies of process pumps, power recovery units and an optimization of the permeate recovery rate. Another cost saving factor is related to the ability of the membranes to operate at elevated feed temperatures. This enables the desalination of seawater effluent from the heat exchange outlet of power plants and eliminates the need for separate intake and outfall structures. The new process improvements, being introduced recently, include the utilization of a membrane pretreatment process that can reliably produce RO feed water of low suspended solids concentration. This pretreatment technology enables stable operation of the RO membranes at a higher permeate flux rate, therefore reducing the size of the RO unit [ 1,2]. 2. Permeate recovery rate The recovery rate determines average feed salinity and osmotic pressure in the RO system. Fig. 1 shows an example of the relationship between average feed salinity and osmotic pressure based on a Mediterranean seawater feed. It is obvious that with higher recovery rate the average feed salinity and the permeate salinity will increase. Also, at a higher recovery rate higher feed pressure will be required to maintain the design flux rate. The recovery rate has a major impact on the capacity, and therefore the cost, of all the
45
50 Recovery
55
60
65
ratio, %
Fig 1. Mediterranean feed water. Feed salinity 40,500 ppm TDS, osmotic pressure 28.8 bar.
RO process equipment components. The size of all process equipment which is determined according to feed or concentrate flow will decrease with increased recovery rate. This applies to the size of the feed water supply system and power consumption of intake pumps. The size of all pretreatment equipment; storage tank, booster pumps, filtration equipment and chemical dosing system is determined according to the feed flow. The same considerations apply to the sizing of the concentrate piping and the outfall facility.
3. Permeate flux rate The design permeate flux rate also affect the economics of the RO process. Operation at a higher flux rate requires higher feed pressure, which results in higher power consumption. However, with a higher flux rate less membrane area is required, and therefore, a smaller number of membrane elements and pressure vessels are needed for the same output capacity. Flux rate indirectly affects the recovery rate. At a higher flux rate the apparent salt passage is lower, and therefore the RO system can be designed to operate at a higher recovery rate. An example of the typical relationship between flux rate, recovery rate and the resulting permeate salinity at feed water temperature of 20°C is shown in Fig. 2. The results in Fig. 2 were calculated for
I? Glueckstern
et al. /Desalination
500
40
45
50 Recovery
55
60
rate, %
Fig. 2. Projected permeate salinity for Mediterranean feed, 20°C at permeate flux rate range 8-l 1 gfd.
the feed water salinity of 40,500 ppm TDS (corresponding to Mediterranean seawater) using membrane elements of nominal permeate capacity of 22.3 m3/d (5900 gpd) and 99.7% salt rejection. For the given specific membrane flux and site condition of feed water salinity and temperature, the maximum flux rate is limited by the maximum pressure rating of the membrane elements and pressure vessels. Another design consideration is the fouling rate. With a higher flux rate, fouling rate increases. The usual range of average flux in seawater applications is 12-17 l/m2-h(7-10 gfd). The lower end of the range is for processing of highly fouling surface water. The higher limit is for a clean beach well water source. The seawater systems operating with membrane (UFA4F) pretreatment can be design to operate at the higher limit of the permeate flux range due to the very low concentration of suspended solids in the UF/ MF filtrate. 4. Energy requirement
57
147 (2002) 55-62
with increasing recovery. Fig. 3 shows the plot of the energy requirement vs. recovery rate. The energy usage includes electricity consumed by intake pumps, the pretreatment system and the high pressure feed pumps. The minimum energy consumption value is achieved at about 50-55% recovery rate and varies with feed water salinity. In addition to the energy, chemical consumption and capital cost also varies with the recovery rate. Fig. 4 shows a plot of the combined contribution to the water cost of these three components. Because chemicals consumption and capital cost decreases with increasing recovery rate, the minimum value of the water cost shifts to a higher recovery rate as compared to the energy vs. recovery plot in Fig. 3.
I;~~~
5 z z I-
3.501~ ~~~ ~~_~~~~~_~__ _ ~~~ _~ ~~~ ~_ ~~~~1 40
45
50
55
60
65
70
rate, %
Recovery
Fig. 3. Energy requirement at flux rate of 13.5 l/mz-h (8gfd).
-Mediterra.
~~
and water cost
The energy requirement is directly related to the feed pressure and the feed water flow which is a function of system recovery rate. Higher recovery rate requires higher feed pressure to overcome the increasing average osmotic pressure. However, the feed flow rate decreases
40
45
50 Recovery
55
60
65
70
rate, %
Fig. 4. Selected water cost components: energy, chemicals, capital.
58
P Glueckstern et al. /Desalination
5. Field testing program The field testing program had two main objectives: a) compare performance of RO seawater systems operating on the surface seawater treated with conventional pretreatment and with capillary UF membrane technology, and b) evaluate if a seawater RO system can reliably operate at higher flux rate and recovery due to improvement of feed water quality obtained from UF pretreatment. As discussed previously, ability to operate RO seawater unit at higher flux and recovery rate enables optimization of the RO process and reduction of water cost. The field test was conducted at two sites. Initially the pilot operation was conducted on a test site in the gulf of Eilat on the Red Sea Shore from September 1998 till October 1999. The Red Sea water is relatively clean with low concentration of suspended solids and low level of biological activity. The second stage of the test was conducted at Ashdod, on the Mediterranean, from February 2000 till October 2000. The Mediterranean seawater quality fluctuates widely with extended periods of high concentration of suspended solids and high organic load. The water quality data and the test results are summarized in Table 1. The major indicators of raw water quality for RO applications; turbidity and SDI, differed significantly at the two locations. The Read Sea water had turbidity in the range of 0.2-1.1 and SD1 in the range of 2.3-6.0. The turbidity of Mediterranean was in the range of l-10 and SD1 values were consistently above 6.5. Also, the maximum value of bacterial count in the Mediterranean was twice as high as the one measured in the Red Sea water. 6. Pilot units configuration At each site, two pilot units were operated in parallel. One pilot unit represented a conventional process of solid reduction: settling and coagulation step followed by granular media pressure filters and cartridge filtration. Due to lower concentration of suspended solids in the Red Sea, as compared
147 (2002) 55-62
to Mediterranean, only a single sand filtration step was used at Eilat site. For the Mediterranean water a two-stage media filtration was applied. The second pilot unit represented an advanced membrane integrated system. The feed water pretreatment consisted of a mechanical strainer followed by a UF backwashable capillary membrane unit. The UF module contained 25 m* of membrane area in the form of 0.8 mm ID capillary fibers, made of polyether sulfone polymer.The MWCO of the fiber material is 150,000 Dalton. At the outlet of both pretreatment systems, the treated feed water was processed by RO units equipped with commercial spiral wound polyamide membrane elements. 7. Operating
parameters
7.1. The conventional pretreatment The reduction of suspended solids was achieved by initial settling combined with coagulation using ferric salt followed by media filtration. The retention time in settling tank was 60 min. Dosing rate of ferric coagulant was 0.3-0.7 ppm. The filtration velocity of sand filters was 6.5-7.2 m/h. Filter backwash was applied every 100 h with air scouring. Due to the high bacterial count in seawater at Ashdod, a continuous chlorination at the level of 1.2 ppm was applied. No continuous chlorination was applied at Eilat. 7.2. Ultrafiltration pretreatment The feed water to the UF unit was filtrated with 50 urn screen filter. The capillary UF module, down stream of the strainer, operated at the flux rate of 60-l 20 l/m2-h. To mitigate the membrane fouling rate, ferric coagulant was added to the UF feed at the rate of 0.3 ppm. The UF membrane was backwashed with filtrate at the intervals of 15-30 min. During the backwash step a free chlorine (in the form of hypochloride) was added to the filtrate at the level of 20 ppm. The presence of hypochloride during backwash resulted in an
t? Glueckstern et al. /Desalination
59
147 (2002) 55-62
Table 1 Research summary
( MEDITERRANEAN SEA TDS, ppm Temperature, Turbidity,
~~ “C
NTU
SDl(15)
__:*_~/
,_~
18.7 53
SO_Specific Flux, l/m*h RO Recovery,
%
TV a= 6.5
-~
7,500 - 15,000
Solids, ppm
Total Bacteria Count,-cfu/ml
- Y-_T~~.L
-15130
l-10 _-
Total Particles Count Suspended
~.
~-~ r-z _,
_~40,500 =r~
~~
~~ -Pal_ 2-14
I
800 - 3@66
~~ ._~~
60
I? Glueckstern et al. /Desalination
increase of water pH and formation of carbonate scale on the membranes, which reduced permeability. The carbonate scale was removed and membrane permeability restored by conducting low pH (approximately pH = 2) backwash, twice a day. To remove other foulants, the UF membrane was cleaned using subsequent soaking and flushing with NaOH and citric acid. 7.3. RO units The membranes in two parallel RO units operated at a flux rate of 8.2-l 1.2 gfd (13.9-19 l/m2h) and recovery rate of 45-55%. The salinity of the seawater at the Red Sea site was 42,000 ppm TDS and at the Mediterranean site was about 40,500 ppm TDS. The feed water temperature range was 2030°C (Red Sea) and 15-30°C (Mediterranean). At the Ashdod site the seawater was taken from the outlet of the heat rejection unit of the power station part of the time. During summer months the temperature of this stream reached as high as to 39°C. 8. Operating
results of pilot units
During periods of good to average quality of raw seawater, both the UF and the conventional pretreatment produced feed water of good quality. The SD1 was in the range of 0.8-3.8 and turbidity in the range of 0.1-0.2 NTU. On the average, the UF pretreatment produced feed water with lower SD1 and turbidity than the conventional pretreatment system. On the other hand the conventional pretreatment had smaller water loses during the backwash step, resulting in higher recovery rate than the water recovery of the UF unit (Table 1). However, the most significant difference in performance of two pretreatment systems was observed during periods of stormy weather and high concentration of suspended solids in the seawater. During such conditions, the UF pretreatment performed significantly better than the conventional one. With an increased load of suspended solids in the sweater, the frequency of media filters
147 (2002) 5562
backwashing had to be increased. Eventually SD1 of the effluent was to high to be processed by the RO membranes and the RO unit had to be shut down until the water quality improved. The quality of feed water produced by the UF system, operating in parallel with the conventional system, affected was very little by the fluctuation of the seawater quality. The higher solids load in the feed water to the UF unit could be compensated by operation in partial cross flow mode. The performance of RO units on feed water treated by UF and conventional pretreatment was very similar in respect to flux stability. One problem observed during operation of the RO at high recovery rate (55%) was an increased pressure drop. The pressure drop increase has been traced to the formation of iron containing scale in the RO elements at high recovery rate. Most likely the iron originated from the flocculant used in the pretreatment system. This problem has been resolved by intermittent operation of the RO unit at low pH. During the operation of the RO unit with feed water received from the heat rejection unit of the power station, it was recorded that permeate flow decreased and pressure drop and salt passage increased beyond what was observed in operation with feed water obtained directly from the intake. This contradicts the experience of another seawater pilot unit pilot of similar configuration operated at Tampa, Florida [3]. At Tampa, the seawater was also obtained from the heat reject unit of the power station with a maximum temperature of 40°C. Stable membrane performance was observed during pilot operation at this temperature. This difference in performance at similar site conditions indicates that additional characterization of feed water quality is required at Ashdod to determine fouling constituents, which may be specific to this site. 9. Economics of the membrane
pretreatment.
The results of an economic evaluation of a UF pretreatment system designed to produce 40,000 m’/d of filtrate as a feed to 20,000 m3/d
P: Glueckstern et al. /Desalination
RO seawater system are summarized in Table 2. The specific investment, including site and utilities is estimated to be $112.5-$137.5/m3-d. The total filtrate cost, including investment and operating cost, is calculated to be in the range of $0.048-$0.057/m’. The cost of UF equipment is estimated to be $62.5-$87.5/m3-d. It is evident from the above UF equipment cost numbers that the membrane pretreatment is more expensive than the conventional pretreatment. The cost of conventional pretreatment equipment, which
147 (2002) 55-62
61
includes clarifier and two stage filtration is approximately $40-$45/m3-d. Assuming that the cost of site and utilities is similar in both cases, the equipment cost difference will be in the range of $22.5-$42.5/m7-d. At the interest rate of 6.5% and a 25 year plant life, this difference translates to an additional contribution to RO permeate cost of $O.Ol-$0.02/m3. The additional cost can be justified for sites where the raw water is of very poor quality and extensive conventional pretreatment is required. The membrane pretreatment
Table 2. Integrated UF/RO seawater system design. RO system capacity 20,000 m/d, UF system capacity 40,000 m/d
UF - design parameters Design net filtrate, m*/d Designed flux rate, l/m*-h Module membrane area, m* Number of UF modules Number of UF trains UF trains footprint, m UF system recovery rate, % Feed pressure, kPa Filtrate pressure, kPa Backwash pressure, kPa Backwash frequency, min Backwash step duration, min Integrity test frequency, days Integrity test duration, min Cleaning frequency, days Cleaning procedure duration, min
Cost data 40,000 94 46 450 5 12.0x14.5 90 220 20 200 25 1 30 45 60 240
UF equipment cost, $ Site and utilities Total Specific cost, (UF equipment only) $/m3-d Specific cost (total), $/m3-d
Feed pumps, average kW 105 Backwash pumps, average kW 25 Valves and other misc., average kW 10 Total average power, kW 140 Specific power, kWh/m3 0.09
Ferric chloride Chlorine Citric acid NaOH
Continuous Every backwash Every 60 d Every 60 d
Usage, kg/d 13.3 1.42 12.5 3.12
Plant life, y Interest rate, % Plant load factor, % Power, $kWh Chlorine, $/kg Sodium hydroxide (ZS%), $/kg Citric acid, $/kg Sulfuric acid (98%), $/kg Ferric chloride (40%) $/kg
25 6.5 95 0.05 0.55 0.13 2.75 0.44 0.33
UF filtrate cost Investment cost, $/m3
0.0265-0.0354
Operating cost
UF process chemicals consumption Frequency
112.5-137.5
Economic parameters
UF process power consumption
Chemical
2,500,000-4,000,000 2,000,000 4,500,000-6,000,000 62.5-87.5
Usa e, L? g/m 0.330 0.036 0.312 0.078
Power cost, $/m3 Chemicals usage, $/m3 Maintenance (including membrane replacement), $/m3 Total operating cost, $/m3
0.0045 0.0020 0.0150
Total UF filtrate cost, $/m3
0.048-0.057
0.0215
62
l? Glueckstern
et al. /Desalination
also improves reliability of the process, enabling operation of the RO unit independently of the raw water quality fluctuations. 10. Conclusions The desalted water cost in seawater systems is affected by the performance of the RO membranes, the operating parameters and the system configuration. The increase in salt rejection of the current RO membranes provides additional flexibility for process cost optimization through the ability to design and operate the RO system at higher recovery rate. The water cost considerations indicate that in seawater RO systems the optimum recovery rate, with current membranes, is in the range of 50-60%. The recovery rate value corresponding to the optimum water cost depends on feed water salinity and power rate. The field test results of UF membrane pretreatment, tested at two different sites confirm that the membrane pretreatment is a reliable technology capable of providing consistently good quality feed water for RO seawater system independently of the raw
147 (2002) 55-62
water quality fluctuation. The cost of membrane pretreatment is higher than the cost of conventional pretreatment. The application of membrane pretreatment in seawater RO desalting systems is feasible for sites which require very extensive conventional pretreatment or where wide fluctuation of raw water quality can be expected. Acknowledgements Authors acknowledge financial support of this project provided by The Middle East Desalination Research Center.
References M. Wilf and K. Klinko, Performance of commercial seawater membranes, Desalination, 96 (1994) 456478. VI l? Glueckstem and M. Priel, Advanced concepts of large seawater desalination systems in Israel, Desalination, 119 (1998) 33-45. 131 Results of operation of pilot unit at Tampa site, Hydranautics, internal memorandum.
[II
Desalination
147 (2002) 55-62 www.elsevier.com/locate/desal
Field evaluation of capillary UF technology as a pretreatment for large seawater RO systems P. Glueckstern”, M. Priel”, Mark Wilr”* “Mekorot Water Co., Tel Aviv, Israel “Hydranautics, Oceanside, California, USA Tel. +I (760) 901-2548; Fax +I (760) 901-2664; email: [email protected]
Received
1 February 2002; accepted 1 March 2002
Abstract RO seawater systems operating on a surface feed water, originating from an open intake source, require an extensive pretreatment process in order to control membrane fouling. Considerations of long-term membrane performance stability have lead to an initial design concept of operation of seawater RO systems at a low permeate flux rate and low permeate recovery. In recent years the nominal performance of composite seawater membrane elements has improved significantly, and in parallel new backwashable microfiltration and ultrafiltration capillary technologies have been introduced commercially. This new membrane technology can be utilized to treat seawater from a surface sources. Use of membrane capillary technology as a pretreatment step can improve quality of the surface feed water to a level comparable or better than the water quality from the well water sources. A better feed water quality enables more effective optimization of operating parameters in the RO systems. Both permeate flux and system recovery rate can be increased considerably without creating membrane fouling conditions. Operation of seawater systems at higher permeate flux and recovery rate results in improved economics of RO seawater desalting. Field evaluation of hybrid membrane systems consisting of UF membrane pretreatment unit and a RO seawater unit was conducted subsequently at two test sites. The first test site was at the Red Sea (Eilat site) and the second test site was on the Mediterranean (Ashdod site). The RO membranes were commercial seawater elements in spiral wound configuration. The UF equipment utilized capillary backwashable elements operated in dead end flow mode. For comparison, a second pilot system consisting of conventional pretreatment and an RO unit was operated in parallel at the above sites. The conventional pretreatment unit included in line flocculation followed by media filtration. The tests were conducted over a period of two years. Raw water quality reflected seasonal changes of composition and weather conditions. The performances of UF and RO equipment were evaluated over a wide range of operating parameters such as recovery and flux rate. Field results were used to project and compare the economics of the seawater RO desalting process using conventional and membrane pretreatment. This paper will describe the experimental procedure and results of parallel operation of an integrated membrane unit and a conventional RO sweater system. The economic analysis of both designs based on local site conditions, will be provided as well. Keywords:
Reverse osmosis; Ultrafiltration;
*Corresponding
Integrated membrane
systems
author.
Presented at the International /U/I. 7-12, 2002
Congress
on Membranes
and Membrane
Processes
00 I l-9 164/02/$See front matter 0 2002 Elsevier Science B.V. All rights reserved HI: SO0 I I-9 164(02)00576-3
(ICOM),
Toulouse,
France,
56
P. Glueckstern
et al. /Desalination
147 (2002) 55-62
1. Introduction The significant decrease of water cost produced by seawater RO systems, as observed in recently awarded water supply contracts, is not a result of a single technology breakthrough, but rather an effect of a trend of continuous improvement of all aspects of the seawater desalination process. Some of the improvements have been proven in field operation, while others were introduced recently and are still pending long term field verification. The membrane technology contribution to better process economics results from manufacturing membrane barrier polymer with higher salt rejection and packaging more membrane area into an element structure. Lower power consumption is a result of the higher efficiencies of process pumps, power recovery units and an optimization of the permeate recovery rate. Another cost saving factor is related to the ability of the membranes to operate at elevated feed temperatures. This enables the desalination of seawater effluent from the heat exchange outlet of power plants and eliminates the need for separate intake and outfall structures. The new process improvements, being introduced recently, include the utilization of a membrane pretreatment process that can reliably produce RO feed water of low suspended solids concentration. This pretreatment technology enables stable operation of the RO membranes at a higher permeate flux rate, therefore reducing the size of the RO unit [ 1,2]. 2. Permeate recovery rate The recovery rate determines average feed salinity and osmotic pressure in the RO system. Fig. 1 shows an example of the relationship between average feed salinity and osmotic pressure based on a Mediterranean seawater feed. It is obvious that with higher recovery rate the average feed salinity and the permeate salinity will increase. Also, at a higher recovery rate higher feed pressure will be required to maintain the design flux rate. The recovery rate has a major impact on the capacity, and therefore the cost, of all the
45
50 Recovery
55
60
65
ratio, %
Fig 1. Mediterranean feed water. Feed salinity 40,500 ppm TDS, osmotic pressure 28.8 bar.
RO process equipment components. The size of all process equipment which is determined according to feed or concentrate flow will decrease with increased recovery rate. This applies to the size of the feed water supply system and power consumption of intake pumps. The size of all pretreatment equipment; storage tank, booster pumps, filtration equipment and chemical dosing system is determined according to the feed flow. The same considerations apply to the sizing of the concentrate piping and the outfall facility.
3. Permeate flux rate The design permeate flux rate also affect the economics of the RO process. Operation at a higher flux rate requires higher feed pressure, which results in higher power consumption. However, with a higher flux rate less membrane area is required, and therefore, a smaller number of membrane elements and pressure vessels are needed for the same output capacity. Flux rate indirectly affects the recovery rate. At a higher flux rate the apparent salt passage is lower, and therefore the RO system can be designed to operate at a higher recovery rate. An example of the typical relationship between flux rate, recovery rate and the resulting permeate salinity at feed water temperature of 20°C is shown in Fig. 2. The results in Fig. 2 were calculated for
I? Glueckstern
et al. /Desalination
500
40
45
50 Recovery
55
60
rate, %
Fig. 2. Projected permeate salinity for Mediterranean feed, 20°C at permeate flux rate range 8-l 1 gfd.
the feed water salinity of 40,500 ppm TDS (corresponding to Mediterranean seawater) using membrane elements of nominal permeate capacity of 22.3 m3/d (5900 gpd) and 99.7% salt rejection. For the given specific membrane flux and site condition of feed water salinity and temperature, the maximum flux rate is limited by the maximum pressure rating of the membrane elements and pressure vessels. Another design consideration is the fouling rate. With a higher flux rate, fouling rate increases. The usual range of average flux in seawater applications is 12-17 l/m2-h(7-10 gfd). The lower end of the range is for processing of highly fouling surface water. The higher limit is for a clean beach well water source. The seawater systems operating with membrane (UFA4F) pretreatment can be design to operate at the higher limit of the permeate flux range due to the very low concentration of suspended solids in the UF/ MF filtrate. 4. Energy requirement
57
147 (2002) 55-62
with increasing recovery. Fig. 3 shows the plot of the energy requirement vs. recovery rate. The energy usage includes electricity consumed by intake pumps, the pretreatment system and the high pressure feed pumps. The minimum energy consumption value is achieved at about 50-55% recovery rate and varies with feed water salinity. In addition to the energy, chemical consumption and capital cost also varies with the recovery rate. Fig. 4 shows a plot of the combined contribution to the water cost of these three components. Because chemicals consumption and capital cost decreases with increasing recovery rate, the minimum value of the water cost shifts to a higher recovery rate as compared to the energy vs. recovery plot in Fig. 3.
I;~~~
5 z z I-
3.501~ ~~~ ~~_~~~~~_~__ _ ~~~ _~ ~~~ ~_ ~~~~1 40
45
50
55
60
65
70
rate, %
Recovery
Fig. 3. Energy requirement at flux rate of 13.5 l/mz-h (8gfd).
-Mediterra.
~~
and water cost
The energy requirement is directly related to the feed pressure and the feed water flow which is a function of system recovery rate. Higher recovery rate requires higher feed pressure to overcome the increasing average osmotic pressure. However, the feed flow rate decreases
40
45
50 Recovery
55
60
65
70
rate, %
Fig. 4. Selected water cost components: energy, chemicals, capital.
58
P Glueckstern et al. /Desalination
5. Field testing program The field testing program had two main objectives: a) compare performance of RO seawater systems operating on the surface seawater treated with conventional pretreatment and with capillary UF membrane technology, and b) evaluate if a seawater RO system can reliably operate at higher flux rate and recovery due to improvement of feed water quality obtained from UF pretreatment. As discussed previously, ability to operate RO seawater unit at higher flux and recovery rate enables optimization of the RO process and reduction of water cost. The field test was conducted at two sites. Initially the pilot operation was conducted on a test site in the gulf of Eilat on the Red Sea Shore from September 1998 till October 1999. The Red Sea water is relatively clean with low concentration of suspended solids and low level of biological activity. The second stage of the test was conducted at Ashdod, on the Mediterranean, from February 2000 till October 2000. The Mediterranean seawater quality fluctuates widely with extended periods of high concentration of suspended solids and high organic load. The water quality data and the test results are summarized in Table 1. The major indicators of raw water quality for RO applications; turbidity and SDI, differed significantly at the two locations. The Read Sea water had turbidity in the range of 0.2-1.1 and SD1 in the range of 2.3-6.0. The turbidity of Mediterranean was in the range of l-10 and SD1 values were consistently above 6.5. Also, the maximum value of bacterial count in the Mediterranean was twice as high as the one measured in the Red Sea water. 6. Pilot units configuration At each site, two pilot units were operated in parallel. One pilot unit represented a conventional process of solid reduction: settling and coagulation step followed by granular media pressure filters and cartridge filtration. Due to lower concentration of suspended solids in the Red Sea, as compared
147 (2002) 55-62
to Mediterranean, only a single sand filtration step was used at Eilat site. For the Mediterranean water a two-stage media filtration was applied. The second pilot unit represented an advanced membrane integrated system. The feed water pretreatment consisted of a mechanical strainer followed by a UF backwashable capillary membrane unit. The UF module contained 25 m* of membrane area in the form of 0.8 mm ID capillary fibers, made of polyether sulfone polymer.The MWCO of the fiber material is 150,000 Dalton. At the outlet of both pretreatment systems, the treated feed water was processed by RO units equipped with commercial spiral wound polyamide membrane elements. 7. Operating
parameters
7.1. The conventional pretreatment The reduction of suspended solids was achieved by initial settling combined with coagulation using ferric salt followed by media filtration. The retention time in settling tank was 60 min. Dosing rate of ferric coagulant was 0.3-0.7 ppm. The filtration velocity of sand filters was 6.5-7.2 m/h. Filter backwash was applied every 100 h with air scouring. Due to the high bacterial count in seawater at Ashdod, a continuous chlorination at the level of 1.2 ppm was applied. No continuous chlorination was applied at Eilat. 7.2. Ultrafiltration pretreatment The feed water to the UF unit was filtrated with 50 urn screen filter. The capillary UF module, down stream of the strainer, operated at the flux rate of 60-l 20 l/m2-h. To mitigate the membrane fouling rate, ferric coagulant was added to the UF feed at the rate of 0.3 ppm. The UF membrane was backwashed with filtrate at the intervals of 15-30 min. During the backwash step a free chlorine (in the form of hypochloride) was added to the filtrate at the level of 20 ppm. The presence of hypochloride during backwash resulted in an
t? Glueckstern et al. /Desalination
59
147 (2002) 55-62
Table 1 Research summary
( MEDITERRANEAN SEA TDS, ppm Temperature, Turbidity,
~~ “C
NTU
SDl(15)
__:*_~/
,_~
18.7 53
SO_Specific Flux, l/m*h RO Recovery,
%
TV a= 6.5
-~
7,500 - 15,000
Solids, ppm
Total Bacteria Count,-cfu/ml
- Y-_T~~.L
-15130
l-10 _-
Total Particles Count Suspended
~.
~-~ r-z _,
_~40,500 =r~
~~
~~ -Pal_ 2-14
I
800 - 3@66
~~ ._~~
60
I? Glueckstern et al. /Desalination
increase of water pH and formation of carbonate scale on the membranes, which reduced permeability. The carbonate scale was removed and membrane permeability restored by conducting low pH (approximately pH = 2) backwash, twice a day. To remove other foulants, the UF membrane was cleaned using subsequent soaking and flushing with NaOH and citric acid. 7.3. RO units The membranes in two parallel RO units operated at a flux rate of 8.2-l 1.2 gfd (13.9-19 l/m2h) and recovery rate of 45-55%. The salinity of the seawater at the Red Sea site was 42,000 ppm TDS and at the Mediterranean site was about 40,500 ppm TDS. The feed water temperature range was 2030°C (Red Sea) and 15-30°C (Mediterranean). At the Ashdod site the seawater was taken from the outlet of the heat rejection unit of the power station part of the time. During summer months the temperature of this stream reached as high as to 39°C. 8. Operating
results of pilot units
During periods of good to average quality of raw seawater, both the UF and the conventional pretreatment produced feed water of good quality. The SD1 was in the range of 0.8-3.8 and turbidity in the range of 0.1-0.2 NTU. On the average, the UF pretreatment produced feed water with lower SD1 and turbidity than the conventional pretreatment system. On the other hand the conventional pretreatment had smaller water loses during the backwash step, resulting in higher recovery rate than the water recovery of the UF unit (Table 1). However, the most significant difference in performance of two pretreatment systems was observed during periods of stormy weather and high concentration of suspended solids in the seawater. During such conditions, the UF pretreatment performed significantly better than the conventional one. With an increased load of suspended solids in the sweater, the frequency of media filters
147 (2002) 5562
backwashing had to be increased. Eventually SD1 of the effluent was to high to be processed by the RO membranes and the RO unit had to be shut down until the water quality improved. The quality of feed water produced by the UF system, operating in parallel with the conventional system, affected was very little by the fluctuation of the seawater quality. The higher solids load in the feed water to the UF unit could be compensated by operation in partial cross flow mode. The performance of RO units on feed water treated by UF and conventional pretreatment was very similar in respect to flux stability. One problem observed during operation of the RO at high recovery rate (55%) was an increased pressure drop. The pressure drop increase has been traced to the formation of iron containing scale in the RO elements at high recovery rate. Most likely the iron originated from the flocculant used in the pretreatment system. This problem has been resolved by intermittent operation of the RO unit at low pH. During the operation of the RO unit with feed water received from the heat rejection unit of the power station, it was recorded that permeate flow decreased and pressure drop and salt passage increased beyond what was observed in operation with feed water obtained directly from the intake. This contradicts the experience of another seawater pilot unit pilot of similar configuration operated at Tampa, Florida [3]. At Tampa, the seawater was also obtained from the heat reject unit of the power station with a maximum temperature of 40°C. Stable membrane performance was observed during pilot operation at this temperature. This difference in performance at similar site conditions indicates that additional characterization of feed water quality is required at Ashdod to determine fouling constituents, which may be specific to this site. 9. Economics of the membrane
pretreatment.
The results of an economic evaluation of a UF pretreatment system designed to produce 40,000 m’/d of filtrate as a feed to 20,000 m3/d
P: Glueckstern et al. /Desalination
RO seawater system are summarized in Table 2. The specific investment, including site and utilities is estimated to be $112.5-$137.5/m3-d. The total filtrate cost, including investment and operating cost, is calculated to be in the range of $0.048-$0.057/m’. The cost of UF equipment is estimated to be $62.5-$87.5/m3-d. It is evident from the above UF equipment cost numbers that the membrane pretreatment is more expensive than the conventional pretreatment. The cost of conventional pretreatment equipment, which
147 (2002) 55-62
61
includes clarifier and two stage filtration is approximately $40-$45/m3-d. Assuming that the cost of site and utilities is similar in both cases, the equipment cost difference will be in the range of $22.5-$42.5/m7-d. At the interest rate of 6.5% and a 25 year plant life, this difference translates to an additional contribution to RO permeate cost of $O.Ol-$0.02/m3. The additional cost can be justified for sites where the raw water is of very poor quality and extensive conventional pretreatment is required. The membrane pretreatment
Table 2. Integrated UF/RO seawater system design. RO system capacity 20,000 m/d, UF system capacity 40,000 m/d
UF - design parameters Design net filtrate, m*/d Designed flux rate, l/m*-h Module membrane area, m* Number of UF modules Number of UF trains UF trains footprint, m UF system recovery rate, % Feed pressure, kPa Filtrate pressure, kPa Backwash pressure, kPa Backwash frequency, min Backwash step duration, min Integrity test frequency, days Integrity test duration, min Cleaning frequency, days Cleaning procedure duration, min
Cost data 40,000 94 46 450 5 12.0x14.5 90 220 20 200 25 1 30 45 60 240
UF equipment cost, $ Site and utilities Total Specific cost, (UF equipment only) $/m3-d Specific cost (total), $/m3-d
Feed pumps, average kW 105 Backwash pumps, average kW 25 Valves and other misc., average kW 10 Total average power, kW 140 Specific power, kWh/m3 0.09
Ferric chloride Chlorine Citric acid NaOH
Continuous Every backwash Every 60 d Every 60 d
Usage, kg/d 13.3 1.42 12.5 3.12
Plant life, y Interest rate, % Plant load factor, % Power, $kWh Chlorine, $/kg Sodium hydroxide (ZS%), $/kg Citric acid, $/kg Sulfuric acid (98%), $/kg Ferric chloride (40%) $/kg
25 6.5 95 0.05 0.55 0.13 2.75 0.44 0.33
UF filtrate cost Investment cost, $/m3
0.0265-0.0354
Operating cost
UF process chemicals consumption Frequency
112.5-137.5
Economic parameters
UF process power consumption
Chemical
2,500,000-4,000,000 2,000,000 4,500,000-6,000,000 62.5-87.5
Usa e, L? g/m 0.330 0.036 0.312 0.078
Power cost, $/m3 Chemicals usage, $/m3 Maintenance (including membrane replacement), $/m3 Total operating cost, $/m3
0.0045 0.0020 0.0150
Total UF filtrate cost, $/m3
0.048-0.057
0.0215
62
l? Glueckstern
et al. /Desalination
also improves reliability of the process, enabling operation of the RO unit independently of the raw water quality fluctuations. 10. Conclusions The desalted water cost in seawater systems is affected by the performance of the RO membranes, the operating parameters and the system configuration. The increase in salt rejection of the current RO membranes provides additional flexibility for process cost optimization through the ability to design and operate the RO system at higher recovery rate. The water cost considerations indicate that in seawater RO systems the optimum recovery rate, with current membranes, is in the range of 50-60%. The recovery rate value corresponding to the optimum water cost depends on feed water salinity and power rate. The field test results of UF membrane pretreatment, tested at two different sites confirm that the membrane pretreatment is a reliable technology capable of providing consistently good quality feed water for RO seawater system independently of the raw
147 (2002) 55-62
water quality fluctuation. The cost of membrane pretreatment is higher than the cost of conventional pretreatment. The application of membrane pretreatment in seawater RO desalting systems is feasible for sites which require very extensive conventional pretreatment or where wide fluctuation of raw water quality can be expected. Acknowledgements Authors acknowledge financial support of this project provided by The Middle East Desalination Research Center.
References M. Wilf and K. Klinko, Performance of commercial seawater membranes, Desalination, 96 (1994) 456478. VI l? Glueckstem and M. Priel, Advanced concepts of large seawater desalination systems in Israel, Desalination, 119 (1998) 33-45. 131 Results of operation of pilot unit at Tampa site, Hydranautics, internal memorandum.
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