Microfiltration system as a pretreatment for RO units: Technical and economic assessment

r ¸ ,

DESALINATION ELSEVIER

Desalination 109 (1997) 165-175

Microfiltration system as a pretreatment for RO units: Technical and economic assessment S. Ebrahim*, S. Bou-Hamed, M. Abdel-Jawad, N. Burney Kuwait Institute for Scientific Research, PO Box 24885, 13109 Safat, Kuwait Tel. +965 4878124; Fax +965 4879238

Received 29 August 1996; accepted 18 November 1996

Abstract

Microfiltration (MF) seems to be a very attractive pretreatment technique that is increasingly being used in drinking water, seawater, and wastewater applications. Recently, it has been used as a pretreatment in reverse osmosis applications. This paper presents the results of a research project on the viability and the economics of MF as a pretreatment technique for a seawater reverse osmosis (SWRO) system. The overall performance of the MF unit indicates that the SDI of the surface seawater feed ranges between 0.26 and 3.10% with an average SDI value of 2.24. SDI of surface seawater feed is above 6.5%. Average filtrate flowrate is 3.20 m3/h. The optimal backwash interval is 10 min at variable feed flowrate. The MF unit is capable of reducing COD and BOD values and producing good quality water suitable as a feed for RO systems. The techno-economic study revealed that the total unit water costs produced by beachwell, MF and conventional surface pretreatment systems are 11.082, 12.264 and 28.153 ills/m3,t respectively. It is clear that the beachwell system is the most cost-effective among the three available techniques for seawater pretreatment. If for some reason the beachwell system is not technically feasible, then the MF system is the next most cost-effective system for seawater pretreatment. It has the added advantage of better water quality. Keywords: Membrane; Filtration; Treatment

t l 0 0 0 ills = US$3.33.

1. Introduction

Controlling membrane fouling has always been, and continues to be, a major problem in the seawater reverse osmosis (SWRO) desalination *Corresponding author.

process. Proper pretreatment of the feed water is the only available solution to avoid membrane fouling and, consequently, decrease the downtime of the RO system operation. One widely used treatment technique is the conventional pretreatment method, which controls and minimizes the plugging potential o f

0011-9164/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 1 1 - 9 1 6 4 ( 9 7 ) 0 0 0 6 2 - 3

166

S. Ebrahim et al. / Desalination 109 (1997) 165-175

the feedwater by coagulating, flocculating and filtering-out colloidal and suspended solids and hence reducing the silt density index (SDI) of the feedwater. This method, however, is cumbersome and costly. According to Abdel-Haleem et al. [1 ], the total running cost of the conventional pretreatment system at the Doha Reverse Osmosis Plant (DROP) in Kuwait is 22.97% of the total product water cost for a RO capacity of 4,546 m3/d. Moreover, filtrate quality and quantity produced by conventional pretreatment are not consistent [2]. Pretreatment represents a real problem in the SWRO process. There is a need for more economical and dependable alternative techniques capable of treating and removing colloidal matter, suspended solids and biofoulants from SWRO feedwater. The beachwell intake system and micro/ ultrafiltration (M/UF) are two promising techniques to replace conventional pretreatment and to overcome its drawbacks. Using naturally filtered seawater derived from a well drilled on the beach proved to be an acceptable alternative to conventional pretreatment; however, beachwell intake systems are site-dependent and susceptible to break-through [3]. Microfiltration (MF) and ultrafiltration (UF) seem to be very attractive pretreatment techniques that are increasingly being used in drinking water, seawater and wastewater applications. Several researchers have shown that MF and UF are capable of consistently reducing turbidities to <0.1 NTU, regardless of the influent turbidity level [4]. UF and MF have also been shown to be very effective for the removal of total coliform (TC) bacteria [5], Giardia [6,7] and Cryptosporidium [7,8]. UF is also very effective in removing viruses [6]. When MF was compared with conventional treatment at the Saratoga Water Treatment Plant, the costs of MF were comparable with conventional treatment [7]. Other advantages were: capital costs were competitive with the diatomaceous-earth (DE) filtration system; operation of the equipment was automatic and required minimal operator

attention; finished water quality was independent of raw water quality; it was compact and easy to install; and residual solids were minimized by elimination of coagulation chemicals or flocculation aids. Taniguchi et al. [9] indicated that the SDI value of the treated seawater with MF is kept below 4 constantly, irrespective of the extent of the pollution of the raw seawater. It is expected that MF will compete strongly with conventional pretreatment in terms of reliability and total operating costs. It is consequently believed that MF will be more widely applied to RO feedwater pretreatment. The main aspects of cost reduction are expected to include: 1. Reduction in capital cost: • No standby capacity • Reduction in the size of chemical dosing systems • Elimination of fine filters in the RO system 2. Reduction in operating costs: • Less membrane replacement cost (due to lengthened membrane useful life) • Less chemical consumption cost (no chemicals are needed for disinfection, coagulation and dechlorination) • Elimination of cartridge filters cost (no cartridge filters are needed before the RO step) • Less RO system down-time • Less maintenance cost for the high pressure pump and the measuring instruments • Less labor cost (less manpower is needed to operate the conventional pretreatment system and to clean the membranes and maintain the system) This paper presents the results of a research project to investigate the viability and the economics of MF as a pretreatment technique for a SWRO system. The project is part of a joint research program between Ministry of Electricity and Water of Kuwait (MEW) and Kuwait

167

S. Ebrahim et al. / Desalination 109 (1997) 165-175

process valves are fitted with pneumatic actuators. The actuators are operated by pressurized air. A programmable logic controller (PLC) mounted in the control cabinet, controls the pilot solenoid valves and pump operation. The PLC also monitors various control switches and other inputs and illuminates the appropriate indicator lamps during machine operation. Chlorinated raw surface seawater is fed to a break tank after passing through a coarse strainer, and injected with 2.5 ppm of sodium bisulphite (NaHSO3) to remove any traces of chlorine. From the break tank, the feed stream is fed to the shell side of two MF modules where direct flow filtration takes place. The filtrate exits the modules via top and bottom filtrate ports. The difference in pressure between the outside and inside of the fibers is known as the transmembrane pressure (TMP). Minimizing TMP increases the efficiency of the backwashing process since built up particulate matter is easier to remove from the surface of the membrane. The machine is equipped with an automatic air (pressurized air) backwashing system. The purpose of backwashing is to remove particle

Institute for Scientific Research (KISR). Preliminary results were presented at the IDA Conference in Abu Dhabi [10]. This paper describes the equipment, experimental set-up, data analysis and technical and economic assessment.

2. Description of the microfiltration unit The MF (MEMCOR 20M10 CMF) unit is a skid-mounted MF machine designed to remove impurities larger than 0.2 # m from feedwater. It is designed to operate in the direct filtration mode. The machine consists of filtration modules, circulation pump, associated valving, pipework, instrumentation and a control system, all mounted in a stainless steel frame. Fig. 1 shows a pipe and instrument (P&I) diagram of the unit. The unit has two filtration modules made from polypropylene; each has a nominal membrane filtration area of 10 m 2, supplying a total nominal filtration area of 20 m 2. A feed pump drives the raw seawater into the filtration modules, either from the break tank or from an external tank. The F. EXHAUST EXT. RECIRC. PROI UCT

TO SDI UNIT "

[.

LEGEND: LS : Level Control MV : Manual Valve ~IVR : Non-Return Valve PV : Pneumatic Valve SP : S a m p l e Point

P~

rR

AIR

b.,4

PVI

FEED

TAN sP

:== LS2

m|

F

STRAINER

FEED PUMP

Old'IN B.W. DRAIN

Fig. 1. Pipe and instrument diagram for the mierofiltration unit.

168

S. Ebrahim et al. / Desalination 109 (1997) 165-I 75

buildup on the surface of the membranes. Backwashing frequency can be set at time intervals ranging from 1 to 60 min. It is controlled during normal operation by the PLC and occurs at the set time intervals. Backwash cycles can also be initiated manually during the filtration mode. During the backwash cycle, high pressure air is fed into the modules via an air inlet valve located near the top filtrate port. This high pressure air is forced into the fiber lumens and passes through the membrane pores to the shell side of the module (cross flow made). The high pressure air expands the fibers and dislodges the accumulated materials on the outside of the fibers. Feedwater sweeps through the module from the bottom to top and carries the dislodged particles out of the modules to the backwash outlet. The unit is designed to allow a chemical clean-in-place (CIP) process. CIP eliminates the need to dismantle components to clean them. The CIP cycle uses a cross-flow filtration mode to reduce the TMP to almost zero. Cross flow refers to flow through the module from bottom to top, inside the fibers of the module and back to the break tank. The unit is cleaned at least every 2-3 weeks or when the TMP reaches 140 kPa. The standard chemicals used for cleaning are 1% NaOH + 1% Memclean EXA2C solution. During the chemical cleaning process, the machine runs for 5 min in cross flow with the filtrate valve open allowing some chemicals to pass through the membrane pores. After 5 min the filtrate valve closes and the machine continues to recycle in cross flow for a further 30 min. Once this is completed, the machine goes into overnight soak. This process allows all particles to be cleaned and restores the membrane to its operation condition. A buffer tank of 250 1 is installed at the filtrate side of the system to maintain a steady flow of micro filtered water to a SDI system during a 15 min reading time.

3. Performance assessment

The first test of the MF unit started in a direct flow mode at a feed and filtrate flowrate of about 5 m3/h, a backwash frequency of 40 min and SDI value ranging between 2 and 3%. Figs. 2-4 show the operation parameters against running hours for flowrates, SDI and temperature, respectively. The filtrate flowrate dropped significantly after only 516 h of operation to less than the design value of 3 m3/h, and reached 1 n~/h after about 1700 h of operation and could not be restored even when the backwash frequency was reduced to 10 min. Several attempts were made to clean the modules using chemicals recommended by the manufacturer (MEMCLEAN). All these attempts, however, failed to restore the productivity as shown in Fig. 2 and indicated by CIP results. These membranes were dismantled and examined by the manufacturer. It was concluded that the membranes showed signs of irreversible fouling. There are several factors that could be responsible for the fouling: first, the higher feed flowrate than recommended for the module associated with large backwash interval at the start of trial tests; second, the marine organisms and calcite debris entering the unit. To overcome this problem, chlorine was added, and a residual of 0.5 ppm in the feed stream was maintained and then removed by adding sodium bisulphite. Moreover, an additional strainer was installed to prevent marine organisms from entering the unit. This last modification reduced the biofouling potential of the feed water. The flux value was successfully retained as the original value (3 ma/h), using membrane cleaning chemicals as shown in Table 1. The surface seawater at the Doha-Kuwait site has a SD| value of over 6.5%.With this type of water, the MF cleaning cycle was found to be every 170 h, for a duration of half an hour. The 170 h cleaning cycle followed by a backwash sequence of 10 min resulted in retaining the original design of the MF membrane flux, as shown in Fig. 2.

169

S. Ebrahim et al. / Desalination 109 (1997) 165-175

.i

.,w ..w

Table 1 Effect of chemical cleaning on the performance of microfiltration membranes (first test)

t'

5

LEGEND:

~,

~"

~i~;.T.~,° "

,~_

3

CIP

--t/._tlIK

.

111I.

1

Chemical

Quantity Flowrate (m3/h)

3 ]=

Before After Before After Memclean, I 0.55 +NaOH, kg 1.5 Flocclean 411, kg 2.5

500

1000

|500

~

2r'-a{~

3000

3500

-:L e•

•

, I'lL-

.t.

l t. ? 0

¢t.$~

.,..,:

i

i

i

~

5oo

lOOO

1500

2ooo

o

•

• :

.

t

~

3

129

46

2.3

3

193

49

Note: Backwash intervals were maintained at 10 min after each cleaning.

Fig. 2. Feed and filtrate flowrates with chemical cleaning application vs time (first test).

L- ;

1.9

~000

Running hours

,~4

TMP (kPa)

I

3~0

I

3~

t

I

4OOO

450O

cleaning cycles, which might have resulted in membrane fouling. The SDI value for the filtrate was excellent during the operation period. Most o f the time, it ranged between 0.24 and 3% with an average value of 2.22%, except for a few readings of values higher than 4.0%. (Fig. 3). The SDI value for the incoming feed water from the Doha Site, Kuwait, was constantly higher than 6.5%.

4. Final optimization

Running hours

A new set of polypropylene membranes with a total area of 30 m 2 was installed for f'mal testing and performance assessment. Table 2 presents average operational parameters for the MF unit after 4000 h (final test). Figs. 5-7 show MF flow charts with average chemical characteristics, filtrate flowrate and the SDI values for the final tests, respectively.

Fig. 3. Silt Density Index vs time (first test).

,

,

,,;,

=

,,;,

=

~

_'

,

,==

Table 2 Operational parameters for the microflltration unit (final test)

Running h o u m

Fig. 4. Seawater temperature vs time. The gradual increase in the temperature of the feed water was not reflected in high productivity (Figs. 2 and 4). The reason can be attributed to decline in the productivity due to irregular

Parameter

Average

Feed flow, m3/h Filtrate flow, m3/h Feed SDI, % Filtrate SDI, % Feed temperature, °C Filtrate velocity, m/s

3.38 3.20 >6.5 2.24 24.65 0.37

17 0

S. Ebrahim et al. / Desalination 109 (1997) 165-175

A v s r a g e Values [8.W. FEEOl

!SDI TDS Na" K* Ca" Mg" AI~'" IFe~ 3F =04 S04 Sr*" 3a~ 3u ~ -ICO3 302 3OD BOD Tot.cont,

Average Values (Filtrate)

>6 39884 ppm 12794 ppm 387 ppm 471 ppm 1504 ppm 1.35 ppm 0.053 ppm 21873 ppm 0.23 ppm 2167 ppm 7.28 ppm 0.17 ppm 0.025 ppm 143 ppm 1.88 ppm 454 ppm 6 ppm 857 ppm 8.3

pH DRAIN

Raclrclate line

<

~ ~:-'.~4-..,

SDI TDS NaK" Ca" Mg ~ Aim Fe" Cl PO4

2.23 " 40929ppm 13284ppm 437 ppm 514 ppm 1539 ppm 1.0 ppm 0.025 ppm 22128ppm 0.056 ppm

S~

3275 ppm

Sr Ba.~

7.3 ppm 0.19 ppm 0.025 ppm

T

IIHc°3

142.5 ppm

•, ~

IICO2

2.0 pprn

T

IIc ° °

443 ppm

IleOO

3

tlTot.conL

1062 ppm

ItpH

63

I _.J

'~

II Unit

ppm

B.W.Draln

Total Running Hours : Total ProductJon : Ave.Productivity : Total power consumption: Energy consumption :

: MICRORLTRATION(M10)

!lNew Membrane Installed(26/711995)

4603.10 14253.47 m ~ 3.10 m3/h 5612.00 kWh

0.394

kWh/m =

Fig. 5. Microfiltration data analysis flow chart (final test).

40 50

3.5

..:_.

.

.:

3.0

4.0 35

25

30

20

•

-11,

Ot

.•

15

2O

~lk4BJ~'

"lb ~Z-e41E'

.~

~

•

]1~4[

r :

15 10

05

O.5 O0 450O

I 5OOO

i

t

,

5500

6000

6500

I

]

I

I

7000

7500

8000

8500

Running

hours

Fig. 6, Filtrate flowrate vs time (final test).

I ~00

r 9500

10000

0

5000

5500

6000

65~O 7000 7500 Running hours

80(~3

8500

9OOO

95O0

Fig. 7. Values of the Silt Density Index vs time (final test).

S. Ebrahim et al. / Desalination 109 (1997) 165-175

Three major operational parameters that affect the flux were monitored and evaluated. These are: • Quality of the feed - - Any change in the feed water salinity and quality affects the viscosity and density of the feed solution and, thus, will have an effect on the flux and productivity. This was investigated by feeding the MF unit with beachwell water (BW), with a conductivity of about 15,000 #S/cm, SDI below 1% and a temperature of 27°C, instead of surface seawater of SDI over 6.5%. The result was an improvement in productivity (Fig. 2). This effect was also noticed when this type of water was fed to the RO system [3]. • Feed temperature - - Temperature has a very profound effect on the polypropylene MF membranes. The membrane manufacturing process had the effect of temperature on the membrane flux as shown in Table 3. It can be seen that the change in the flux is about a 2.4% increase/decrease per one degree Celsius. In practical terms, the design of the MF unit using polypropylene membranes should account for 23% extra flux required during the winter in Kuwait (seawater temperature ranges between 10 and 35°C during the winter and summer, respectively). During the testing period, a variation in the temperature of surface seawater ranged between 16 and 33 °C (Fig. 4). An experiment was conducted to investigate the effect of temperature on the productivity of Table 3 Membrane flux vs temperature (as given by the manufacturers)

Temperature (° C)

Relative flux (%)

Backwash interval (%)

5 10 15 20 25 30 35 40

66 77 88 100 112 125 139 t53

151 130 114 100 89 80 72 65

171

4O

3.5 t X

~

. too

3o

25

=¢ 6O

20

rate

40

I

lol

8650

~

8700

'

875O

8800

8850

8900

8950

Running houm

Fig. 8. Filtrate flowrate and TMP vs running hours at

18 °C (final test). 40-

35

lop

30

25 .60

20

• 15

10 8900

o ~ 8950

I 9000

I ~50

Filtrate flow rate TMP

; 9100

I 9150

J 9200

20 9250

Running hours

Fig. 9. Filtrate flowrate and TMP vs running hours at 28°C (final test). the MF unit. The unit was fed with seawater at different temperatures. Using feed water at 18 °C, the flux declined from 3.9 to 2.6 m3/h during 250 h, whereas TMP increased from about 50 kPa to about 120 kPa (Fig. 8). At 28°C, it took 340 h to decrease the flux to 3.0 m3/h for the same rise in TMP (i.e., from 54 to 120 kPa) (Fig. 9). It can be noticed from Figs. 8 and 9 that the productivity of the MF membranes increased with the increase in water temperature. On average, it increased by 0.085 m3/h/deg (=2 m3/d/deg). It can also be noticed that an increase in the feed water temperature led to an increase in the duration of the running hours without membrane cleaning. With a 10°C increase in the feed water temperature, this period was extended by 1.5 times.

] 72

S. Ebrahim et al. / Desalination 109 (1997) 165-175 4.1

4.1

4 . ~ :3.9

0

1

~

3.8 3.7 ~"

3.1

3.5

~EE 3.0

13.o

[

--O--Variable Flow J] " ' A m Constant Flow /

3.0 , : : : : : ; : : ; ; ; : , : ~ I I

2.5 2.0 1.5

2.5

~ 3.5 E 3.4 ~ 3.3 E :3.2 3.1 3 3,6

E 3.5 ~= 3,4

33"3.2

4.0

3.5

3.9

~ :~.7 ~ 3.6

E"

4.0

4

~o 2.0 E 1.5 E

10

I --
0.5

4.5

Backwash Setting 10 Minutes 4.5

4.0 ~b~

4.0 3.5

3.5

3.0

3.0

2.5

2.5

2.0

2.0

1.5 [ ~--O--Variable ConstantFlow Flow j}

O.O, ', ;;;;;;;;PI(~IIIIIIJ~I

IJIJ

, 4.0

•

3.5

:3.0 2.5

.~

2.0

2.0

1.5

1.5 1.0

1.0

0.5

0.5

0.0

0.0

I ~ h _ Constant Flow 1 [ ~ V a r i a b l e Flow ~

Running Hours

Backwash Setting 20 Minutes

Backwash Setting 15 Minutes 4.0

, 4.0

3.5 3.0 2.5

t2°

~o 2.0 E • 1.s _~ 1.0

0.5 0.0 o4

I

I

I

I

I

P

I

I

t

I

I

Running Hours

Backwash Setting 25 M i n u t e s Fig. 10. Effect of backwash interval on flux (final test).

I

1.5

Running Hours

A

0.5

Running Hours

Backwash Setting 5 Minutes

0.1"0 5

1.o ~.

0.0 , ', : : : : ; : : : ; ; : ; : i ~ ~ : : , 0.0

Running Hours

I

I

I

I

~ ~ ,"r ~

1.0 0.5

173

S. Ebrahim et al. / Desalination 109 (1997) 165-175

• Backwash interval - - To improve productivity of the MF unit, regular backwash of the membranes must be carried out to dislodge and remove foulants from their surfaces. To investigate the effect of the backwash on the membrane flux and productivity, a set of experiments was performed. In these tests, variation in the backwash sequence was made by setting it to 5, 10, 15, 20 and 25 min while maintaining all other variables constant including the feed flowrate at 3.8 m3/h. The longer the backwash interval, the higher risk of fouling. During these tests, the flux is kept constant by adjusting the feed valve continuously while the TMP increased until it reached 98 kPa. At this point, the flux could not be kept constant any more. Therefore, when TMP reached 120 kPa, the unit was stopped and chemical cleaning of the MF membrane was carried out. It is concluded that the optimal membrane backwash interval is every 10 min of operation (Fig. 10). This setting is found to be the best for the MF system using surface seawater from Doha, Kuwait. In optimizing the operational parameters of the MF system, constant and variable flowrates of the feed were tested. The comparison was based on the time required to reach TMP of 120 kPa. It can be seen from Fig. 10 that using the variable feed flowrate technique resulted in a longer time before reaching the TMP value of 120 kPa.

This means that at a constant feed flow rate, the rate of membrane fouling is faster than at a variable flowrate; hence, more frequent chemical cleaning to restore the membrane performance is needed when a constant feed flowrate is adapted.

4. Chemical cleaning A number of cleaning tests were carried out to establish a suitable procedure for membrane cleaning. Table 4 presents the effect of chemical cleaning on the performance of the MF membranes. It can be seen that the results of the tests in retrieving flowrate and TMP are comparable with the basic values of these parameters. Hence, any of these chemicals can be used to clean the HF polypropylene MF membranes. During this study, 1% Memclean EX-A2 and 1% caustic soda (NaOH) at pH 11.0, followed by 0.5% citric acid, were used to remove organic substances and microbiological foulants. The average values for COD and BOD were reduced slightly by the MF unit (Fig. 5). The total count increased in the filtrate due to the dechlorination of the feedwater. In conclusion, the overall performance of the MF unit indicates that the SDI of the filtrate ranged, most of the time, between 0.26 and 3.10% with an average SDI value of 2.24

Table 4 Effect of chemical cleaning on the performance of microfiltration membranes Chemical

Memclean, 1 + NaOH, kg Memclean, 1 + NaOH, kg + Citric acid, kg Flocclean 411, kg

Quantity

1 1 l 1 0.5 2.5

Frequency

Flowrate (m3/h)

TMP (kPa)

Before

After

Before

After

2.3

3.8

125

56

When TMP reaches 125 kPa

2.3

4.0

125

48

When TMP reaches 125 kPa

2.3

3.8

125

52

When TMP reaches 125 kPa

Note: Backwash intervals, after cleaning 10 min.

174

S. Ebrahim et al. / Desalination 109 (1997) 165-175

(Table 2 and Fig. 7). The SDI of surface seawater feed is above 6.5%. The average filtrate flowrate is 3.20 m3/h (Table 2 and Fig 6). The optimal backwash interval is 10 min at a variable feed flowrate. The MF unit is capable of reducing COD and BOD values, and producing good quality water suitable as a feed for RO systems.

5. T e c h n o - e c o n o m i c assessment

A techno-economic assessment study was carried out to assess the MF system (MFS) against two other pretreatment systems, the i.e., beachwell system (BWS) and conventional surface system (CSS). Detailed technical information regarding BWS and CSS is outlined in [2] and [3]. The study revealed that the total unit water costs produced by BWS, MFS and CSS are 11.082, 12.264 and 28.153 fils/m 3, respectively (1000 fils=US $3.33) [11]. The beachwell system gives substantial cost savings and is the most

cost-effective among the three available techniques for seawater pretreatment (Table 5). If for some reason the beachwell system is technically not feasible, such as appropriate locations in sufficient number are not available, the MF system, which is found to be almost equally costeffective as the beachwell system, is the next cost-effective system for seawater pretreatment. The MFS has the added advantage of better water quality.

6. Conclusions

1. The overall performance of the MF system revealed that it is capable of treating surface seawater and producing good quality water, on a continuous basis, suitable to feed RO system without further treatment (average SDI value is 2.24% and optimal backwash frequency is every 10 min). 2. The techno-economic evaluation revealed that the total unit water cost produced by beach-

Table 5 Comparison of unit cost of seawater pretreatment by alternative systems for 1995 (plant capacity 27,276 m3/d) Cost component

Unit cost (ill/m3)* Conventional surface

A. Capital cost (depreciation) Feed pumps Chlorine dosing plant Structure - - intake Structure - - outfall Pretreatment plant Beachwell Microfiltration plant B. Operating cost Electricity Chemicals Filters

10.365 0.119 0.015 0.387 0.387 9.457

Total unit cost (A+B) (ills/m3)* *1000 ills -- US $3.33.

Beachwell 2.917 --

Microfiltration 3.537 0.119

0.387 -2.530

0.387 0.387

17.788 9.648 4.662 3.478

-8.165 6.426 -1.739

2.644 8.727 7.090 1.637

28.153

I 1.082

12.264

S. Ebrahim et al. / Desalination 109 (1997) 165-175

well, MF and conventional surface systems are 11.082, 12.264 and 28.153 fils/m3, respectively. 3. It is clear that the conventional surface seawater treatment is the highest in cost. In addition, it is the most complicated method of seawater treatment. Hence, it should be avoided for surface seawater treatment. 4. MF and beachwell systems are comparable in cost (12.264 and 11.082 fils/m3). However, the MF produces seawater feed of better quality than beachwell (seawater feed of less COD and BOD) which could offset the marginal cost advantage of the beachwell system.

7. Recommendations 1. For designing new commercial seawater RO desalination plants, a beachwell intake system should be considered as a first priority. If this system is technically not feasible, a MF system should be considered as an alternative pretreatment method. 2. A conventional surface pretreatment system is not recommended due to its economics and operating drawbacks. It can be considered as a pretreatment for a seawater RO plant only if beachwell and MF systems are not feasible. 3. In selecting a seawater pretreatment system for RO, MF or beachwell should be evaluated for biological content prior to the final selection of the system. 4. More studies should be carried out to assess the viability and economic feasibility of using a MF system to treat different kinds of water (brackish water, wastewater, etc.).

Acknowledgments The authors wish to acknowledge the financial contribution and the technical support of the Ministry of Electricity and Water (MEW), and

17 5

the assistance of their colleagues in the Water Desalination Department (WDD) of the Water Resources Division at the Kuwait Institute for Scientific Research (KISR).

References [1] M.M. Abdel-Haleem, J.S. Dahdah and M. AbdelJawad, Comprehensivecost and economic analysis of seawater desalination by reverse osmosis systems in Kuwait. Kuwait Institute for Scientific Research, Report No. KISR2878, 1988. [2] S. Ebrahim, M. Abel-Jawad and M. Safar, Proceedings, IDA, Biennial Conference, Palm Beach, Florida, 1 (1994) 149. [3] M. Abdel-Jawad and S. Ebrahim, Desalination, 99 (1994) 57. [4] S.S. Adham, J.G. Jacangelo and J.M. Laine, J. Amer. Water Works Assoc., 87(3) (1995) 62. [5] K.S. Heneghan and M.M. Clark, Surface water treatment by combined ultrafiltration/PACadsorbtion/coagulation for a removal of natural organics, turbidity and bacteria. Presented at the American Water WorksAssociationMembrane Technology in the Water Industry Conference, Orlando, Florida, 1991. [6] J.G. Jacangelo, E.M. Aieta, K.E. Cams, E.W. Cummings and J. Mallevialle, J. Amer. Water Works Assoc., 83 (1991) 9. [7] R.S. Yoo, D.R. Brown, R.J. Pardini and G.D. Bentson, J. Amer. Water Works Assoc., 87(3) (1995) 38. [8] S.S. Adham, J.G. Jacangelo and J.M. Laine, Effect of membrane type on the removal of Cryptosporidium parvum, Giardia muris, and MS2 virus. Proc., Amer. Water Works Assoc. Annual Conference, New York, 1991. [9] Y. Taniguchi, K. Ohta, T. Okabe, M. Hirai and T. Goto, Proceedings, IDA World Congress on Desalination and Water Sciences, Abu Dhabi, 2 (1995) 135. [10] S. Ebrahim, S. Bou-Hamad, M. Safar and A. AlSairafi, Proceedings, IDA World Congress on Desalination and Water Sciences, Abu-Dhabi, 7 (1995) 229. [11] S. Ebrahim, S. Bou-Hamad, M. Abdel-Jawad, N. Burney and M. Safar, Surface seawater pretreatment by microfiltration. Kuwait Institute for Scientific Research. Final Report, June 1996.