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Conventional pretreatment system for the Doha Reverse Osmosis Plant: Technical and economic assessment
DESALINATION Desalination 102 (1995) 179-187
ELSEVIER
Conventional pretreatment system for the Doha Reverse Osmosis Plant: Technical and economic assessment Sadeq H. Ebrahim*, M a h m o u d M. A b d e l - J a w a d and M. Safar
Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Tel.: 965-4836612, Fax: 965.4834712
,~
Abstracl
The conventional pretrea~ent method is widely used to treat surface seawater feed for the seawater reverse osmosis (SWRO) process; however, this method is cumbersome and costly and has many drawbacks that lead to higher product water cost by RO desalination technology. This paper ouOines the experience gained from 65 months of operation of a surface seawater conw:ntional pre:~reatment system at the Doha Reverse Osmosis Plant (DROP) in Kuwait. The paper descnbes'the pretreatment system used, problems and drawbacks encountered dining the course of operaaott, namely: instability of Silt Density index (SDI) value, high rate of chemical consumption, high ra~~. of water consumption for backwashing and high energy consumption, and gives an economic assessment for the pretreatment system. The economic analysis shows that the cost -for the conventional pretreaUnent system accounts for 26.7% of the total capital investment cost for a 4,546 m3/d twostage spiral-wound (SW) RO system, similar to the one used at DROP. A 30% s~ving in u:,it product water cost could be achieved if a beachv~ell intake pretreatment system is used instead ot~the conventional pretreatment system.
1.
Introduction
Successful long-term p e r f o r m a n c e o f a reverse osmosis (RO) s y s t e m depends on three factors: proper design, proper p r e t r e a t m e n t and p r o p e r o p e r a t i o n and maintenance o f ~ e RO system. All three factors are important, but proper pretreatment is the foundation for successful opera,~on of the RO plant. Pretreatment is done to minimize m e m b r a n e fouling, which ,;s the most serious problem encountered in the RO process. A drastic decline in RO performance
and serious damage to the membranes are the results or' membrane fouling that ultimately are reflecteo in l~gher product water cost. Ma~y types o f fcul~nts could occur on the surfaces o f RO m e m b r a n e s and can be broadly identified into the following: - Suspended solids - Cc.l!oids Metal oxides -~ Scales (i.e., CaCO3, CaSO4, BaSO4, SrSO~t~ " CaF2, SiO2) Biological slime - Organics Oil grease
*Corresponding author. 0011-9164/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0011.9 ! 64(95)00052-6
IS0
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
The format:ion of one or more of these types on the surfaces of the membrane is a function of the nature of the feedwater to the membrane and the nature and degree of pretreatment applied to the RO system; other factors include membrane material, permeator configuration, nature of operat2on and quality control [1]. In seawater desalination by RO, the first five types of foulants are most likely to occur, with calcium carbonate (CaCO3) as the dominant type of scale to form [2-4, 10]. In brackish water desalination by RO, calcium sulphate (CaSO4) scaling is widely encountered due to high content of sulphate ions in well waters [2]. Other researchers expei-ienced biological fouling due to sulphate-reducing bacteria (SRB), anaerobic bacteri:z and algae growth [5]. Biological fouling F (microorganisms) and organic fouling are the most dominant type of fouling in industrial and wastewater reclamation by RO [6-91. The conventioaal pretreatment method is widely applied in seawater reverse osmosis (SWRO) desalination processes to prevent membrane fouling. Feed seawater is pretreated by the conventional method to lower the silt density index (SDI), remove excessive turbidity or suspended solids, adjust and control the pH, inhibit or control the formation of scale, and disinfect or prevent slime growth and contamination of equipment. Type and size of the conventional pretreatment system varies depending on the nature of the feedwater to be treated and type of membrane used in the RO system. Cost of the conventional pretreatment system also depends on the type and size of the pretreatment system. At the Doha Reverse Osmosis Plant (DROP) in Kuwait, a conventional pretreatment system (common pretreatment) was usel:l to treat surface seawater feed to three dif.~erent g o lines. The average va,~es for ~ e main constituents of the surface seawater feed are presented in Table 1. Each of three lines was divided into two stages as follows:
Table 1 Averageseawaterqualityat Doha Parameter
Concentration*, mg/l
TDS at I80°C Total alkalinity, as CaCO 3 Carbonate Bicarbonate Carbon dioxide Sulphate Chloride Calcium Magve~ium Sodium Potassium Total i-on pH SDI value, %
47,000 175 14 185 14 3,400 24,000 570 1,700 !2,300 470 0.08 8.2 > 6.5
± ± ± ± ± ± ± ± ± ± ± ± ±
2,000 15 8 18 4 300 700 45 150 20 20 0.08 0.1
*Average concentrations of 12 samples collected monthly.
RO Line 1 First stage - Spiral wound, UOP-PA 1501 Second stage - Spiral wound, UOP-PA 8600 RO Line 2 First stage - Hollow fine fiber, DuPont B-10 Second stage - Hollow fine fiber, DuPont B-9 RO Line 3 First stage.
- Plate and frame, Em'o+ Schleicher & Schueli + FilmTec Secondstage - Spiral wound Hydtonautics804013 Each line had its own pretreatment system, in addition to the commgn prctreatment, and was designed to produce 1,000 m3/d of fresh water. The common pretreatment and Lines 1 and 2 were in operation from !984 until August 1990. Membranes of the first stage oi RO Line 1 were replaced in July 1989 by a new 8" spiral wound configurauon system from FilmTec (FT-30). Modvles of RO Line 2 were also replaced in December 1988 by the new version of the hollow fiber DuPont (B10T). With regard to RO Line 3, the plant was shutdown in March 1988 due to technical and
181
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
economic reasons related to the plate and frame configuration system. 2.
Description of the treatment system
common
pre-
At DROP, the chlorinated surface seawater is delivered to the common pretreatment from Doha East Power Station via a reinforced fiberglass pipe at a pressure of 2-3 bar (Fig. 1). If the chlorine content is below the required level, chlorine gas is added. The removal of suspended and colloidal particles from tne untreated water is carried out by fiocculation and filtration. The process of flocculation may be divided into two separate stages that merge into one another. -Destabilization (coagulation and flocculation) - Agglomeration Destabilization means charge compensation of the usually negative colloidal particles, thus leadi|tg to the formaCaon of microfiake~. This is achieved by mixing the untreated water with the fiocculant using a substantial amount of energy [11]. The flocculant, in this case FeCISO4 (ferric chloride sulphate), is added to the untreated water at the inlet to the destabilization tank. The eaergy required for mixing is applied by the agitator. Dwell time for the water is approximately 15 seconds at maximum flow [12]. The ideal pH value for destabilization and flocculation is set by adding H2SO4 (salphuric acid), also at the inlet to the destabilization tank. In the agglomeration stage, an appropriate amount of energy is used to combine the destabilized particles or so-called microflakes. At this stage, microfiakes are formed by a series of processes such as chemical transformations, electrokinetic effects and adsorption phenomena. Excess turbulence leads to a disintegt'ation of flakes that have already formed. These processes take place in the cascade configuration of tanks. To assist with agglomeration, the discharge of
tanks is provided with a dose of polyelectrolytic solution that has an interlacing effect on the flakes that have formed. The energy necessary for coagulation is applied by three agitators. To optimize the application of energy, the speed of the agitators in the agglomeration basins is adjustable. Fttrthermore, the last agitator, in the direction of flow, is fitted with a torque measuring instrument. The dwell time of the water in the agglomeration tanks is approximately lO vain at maximum flow. The pretreated water leaves the last tank via a pipe and is taken to the filter station. The purpose of filtration is to remove the flakes from tim water. This is achieved by passing the pretreated water through four parallel multi-layer filters. The filters are of the open gravity type. The mixture of water and flakes flows downstream through the filter material, where various types of filter mate~-ials are placed on top of one another. The ~ize of grains is finer in the direction of flow so that the particles to be filterea~ ,.~atwill be deposited uniformly over the e,~:J~ height of the filter so it is possible to av~id a large increase in pressure loss due tc surface filtration. The layer distribution in the filters is as follows: Grain size, mm Supporting layer Silica sand Hydroanthmcite
various 0.7-1.2 1.4-2.5
Filter height, m 0.3 1 0.7
The mixture of water and flakes is distributed evenly over the filter layer and leaves the filters via the pure-water discharge valves and the pure-water pipe. Flow through the filters is maintained constant by the two filter outlet controls. The filtering velocity is I0 n ~ at maximum flow. The filters can be backwashed using the filtered water and air supplied by a compressor. The time and sequence of water and air can be adjusted at will. The pretreated water, emerging from the multi-layer filters, is collected in the pure water pipe and fed to the intermediate tank before it is delivered to the
182
S.H. Ebrah ;met al. / Desalination 102 (1995) 179-187
Ferric c h l o r i d e sulphate dosing
Acid 4 dos-
Polyelectrolyte dosing
"7"
/
blain storage tank
/
Sea water fro
the
III L___.J
intak~ID.esta.bi£1zatlon tank {~hlorina- / l'ti~ n svstedl
,
,
'
,
Flocculation
'
'1 ~,
To ~ine
stage I
Filtrati0, n s t a g e [
[ ~
.....
/
/
~
--Air
To" line II =
line
. _ ~ __~
~e~t t a n ~
storage tank. Before entering the storage tank, chlorine gas can be added, if chlorine content is less than the setforth minimum level. The volume of filtrate being pumped is controlled at the intermediate tank. The filtrate in the storage tank is fed to the three RO lines. 3. Chemical dosing s y s t e m s
for
the RO lines
Be/ore the filtered water, coming from the CP, enters the first stage of each RO line, further treatment is carried out separately to obtain the quality feedwater specified by the manufacturers of the RO modules, The pretreatment systems for RO Lines 1, 2 and 3 consist of:
- T w o parallel activated carbon filters to ensure the complete removal of residual chlorine, and to act as a holding tank for the sodium hydrogen sulphite reaction with chlorine -Anti-sealant dosing system to prevent sulfate scaling - Acid dosing system to prevent carbonate scaling - T w o cartridge filters (micron filters) to filter out particles larger than 5 ttm. These two filters are located just before the highpressure pumps. 3.2. RO Line 2 -
-
3.1. RO Line 1 -
Sodium hydrogen sulphite dosing system to remove residual chlorine ill ',he feed
Fig. i. Schematic diagram for the common pretreatment system at DROP.
-
Acid dosing system to prevent carbonate scaling Polyeleetrolyte dosing system and three inline coagulation filters to further reduce tt,.~ silt density index (SDI) of the feed te ~ess than 3.0 Sodium hydrogen sulphite dosing system
183
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
-
to remove residual chlorine in the feed Three cartridge filters to r e m o v e particles larger than 5 ~tm. T w o filters are always in operation and the third is on standby basis.
~,~6 o
3.3. R O Line 3
• ~ SD! F i l t r a l e S~\ I xlll
- A c i d dosing system to prevent carbonate scaling -Anti-sealant d o s i n g s y s t e m to p r e v e n t sulfate scaling - Sodium h y d r o g e n dosing system to remove residual chlorine - T w o cartridge filters to filter out particles larger than 25 ttm.
4.
Operating evaluation
results
and
performance
":"
"
'
3
l,
Seawater feed flow r a t e , m 3 / h Pressure of seawater feed, bar Temperature of seawater feed, °(7 SDI of seawater feed, %/min pH of seawater feed SDI ~f seawater filtrate, %/min 2-,,Hof seawater filtrate Acid dosing (H2SO4) rate, mg/l FeCISO4 dosing, rate (Fe3+), mg/l Polyelectrolyt¢ dosing rate, mg/] C12 content in seawater feed, mg/l CI2 content in filter inlet, mg/1 CI2 content after storage tank, mg/]
493.11 2.06 26.24 6.47 g. 14 3.6 6.21 82.24 3.04 0.62 0.15 1.7 0.16
"
~
,
5
'"='-"
i
.
.
.
.
.
,s , , ,3 s, ~ i ~ s
37 .
Mom~
Fig. 2. Monthly average SDI and pH of filtrate from ~tugust 1984 to December 1989.
S
l
'-', •,
. I
Average
..,
•
During the period from A u g u s t 1984 until D e c e m b e r 1989, the C P had been operating with availability o f m o r e than 97%. The a v e r a g e m o n t h l y parameters are s h o w n ~n Figs. 2--4 and the mean values are prese[~*,~d in Table 2.
Paramet.er
"'"W
'
: ~3 i7 ,-I " ~ . ~
R
Table 2 Mean parameters for seawater feed 8 n d filtrate for the period from August 1984 to December 1989
"" , •
•
s
li~i I l l i I ~ i~
I 1%
"" ,,,
$
9
,•
,
T
,..=
13 17 21 25 29 33 37 41 45 49 53 $7 61 65 Mouths
Fig. 3. Monthly average FeClSO4 and pelyelecm~lyte dosing rates from August 1984 to December 1989.
=t _:.,o !I
it
!t
;+
t!
V
7
7
i
10
$
9
13 17 21 25 29 33 .~7 41 45 49 53 $7 61 65 Mouths
Fig. 4. Momhly average seawater temperature from August 1984 to December 1989.
184
S.H. Ebrahitn et al. /Desalination 102 (1995) 17o-187
Mosl of the time, the system was successfully controlled to give the designed quantity of filtrate with SDI value less than 4.0 and mean SDI value of 3.6 (Fig. 2); buL in some cases, it failed to produce acceptable quality and the required qua~tity of filtered seawater. The causes for these failures can be attributed to several factors, mainly: clogging of dual media filters, effect of pH, dosing rate of FeCISO4, dosing rate of polyelectrolyte, energy input, and climatic conditions (i.e., temperature, dust storm, wind).
4.1. Clogging of dual media filters Due to occasional increase in marine biological activities, the ~u~faces of the dual media filters become completely clogged causing a deterioration in the quality and quantity of filtered water. Coi~sequently, the backwash frequency had to be increased to twice daily instead of once a day. There was also an increase in the time required to reduce the water level in the filter to 30% before the start of the backwash sequence, which was 2 rain air followed by 8 min water and blow-down, then 2 rain air followed by 8 win water. To overcome this problem, the backwash program was modified to 3 min backwash with water, to break down the clogging layer on the surface of the filter, followed by backwashing twice with flow of air and water for 2 min and 6 rain, respectively.
.4..3. Dosing rate of FeCIS04 and polyelectrolyte To improve the SDI of the filtrate, the ~e +3 dosed at an average rate of 4-5 mg/1 for the first 27 months of operation (Fig, 3). The addition of cationic polyelectrolyte (0.3-0.5 mg/l) in the CP system from February 1986 (Fig. 3) along with the reduction of the filtrate pH to 6.0, enabled the system to produce SDI value of less than 4.0 with the reduction in Fe +3 dosing rate to 2.5-2 mg/l. This optimization, greatly redveed the quantities of FeCISO4 consumed and, eventually, reduced the cost of the filtrate produced from the CP.
4.4. Energy input Flocculation is achieved by mixing the untreated water with the fiocculant using a substantial amount of energy. For the CP system al DROP, FeCISO4 is used as flocculant, whereas the energy is supplied through the mixer of the destabilization tank (Fig. 1). Failure of the mixer caused an increase in the SDI value that was evident during the course of operation. Hence, the SDl~was remarkably improved during 1987 as a result of mechanical modification of the destabilization tank's mixer. New gear pinions were installed to this motor that allowed reverse rotation of the mixer against the natural water flow. It is believed that the new modification resulted in better distribution/mixing of the added flocculant.
4.2. Effect of pH 4. 5. Effect of climate During the first 19 months of operation, the filtrate pH value was set to be between 6.5 to 6.8, as recommended by the designer of the CP; however, it was found through laboratory tests that reducing the pq value to 6.0 e.~hances the fiocculation process, and reduce quantities of Fe +3 dosed to achieve a SDI value of less than 4.0.
Kuwait's climate has a noticeable effect on the filtrate quality at DROP. High turbidity caused by dust storms increases the SDI of the filtrate. The water quality of the filtrate in the 500 m 3 storage tank was affected by dust entering through the vent during sand storms. The system was modified to draw the filter~
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
feed directly from the dual media filters during bad weather. Also, strong winds associated with low tides tend to increase the silt content in the intake seawater, and eventually increase the SDI of the filtrate. Moreover, increase in marine biological activities, in summer, increases the potential for clogging tile dual media filters. Therefore, higher SDI values are observed during summer (April to September) when high temperatures are associated with strong winds and dust. During the reported period (August 1984 December 1989), a total of 21,828,201 m 3 of filtrate had been produced by the CP. Only 70% was used as feed for RO lines, while the remaining 30% was used to backwash the multi-layer filters and to reorganize and resettle the filter media (infiltration), and very small quantity was wasted due to overflow. During the last three years of operation, only 65.54% of the filtrate was utilized for RO lines while the remaining 34.46% was used for backwash, infiltration and overflow (Table 3). The average chemical and energy consumption of the CP, for the last tl~e,years of operation, is shown in Table ~t. Based on actual cost of chemicals, the total cost for these chemicals per m 3 of filtrate produced is KD 0.0142 (US $ = KD 0 303). The total energy input to the produced filtrate is 0.23 kWh/m3. Based on 0.026 KD/kWh cost of energy in Kuwait, the total energy cost per m3 of filtrate produced is KD 0.006. Based on product water cost breakdown calculated from data compiled at DROP for RO systems of different capacities and module configurations, the con cent~onal pretreatment running cost constitutes 21 to 25% of the total product water running cost. This percentage can be further reduced by 410%, in addition to a significant reduction in energy consumption, if beachwell or micro/ultrafiltration pretreatment methods are used instead of the conventional pretreatment method [14].
185
The capital cost of the conventional pretreatment system represents a significant share of the capital investment cost for a twostage RO system. Based on actual cost data obtained from the manufacturer of the common pretreatment system, the cost for the CP accounts for 26.7% of the total capital investment cost for a 4,546 m3/d two-stage spiral-wound (SW) RO system (Table 5). This percentage can be significantly reduced if other pretreatment methods, p.amely, beachwell intake" system or micro/ ultrafiltration pretreatment system, that have less machinery and equipment, are used. A comparative study was carried out at the Kuwait Institute "for Scientific Research (KISR) to compare the cost of desalinated water by RO using the beachw¢ll intake system and conventional surface se~.water pretreatment system. It was concluded that the unit water cost using the beachwell system ,is 183~96 flls/m3 compared with 263.28 fils,'m3 using the convention',0 pretreatment system [15]. This means that mere is a saving of 30% in unit product water cost if a beachwell intake syst¢m is used instead of the conventional pretreatment system.
5. Conclusion The conw~ntionai pretreatment system used at DROP /has succeeded in providing feedwater fo~l"the RO lines with SDI less than 4.0. The avallability of this system was more than 97°/, Oaring the operated period; however, this system was very difficult to control. Several factors obstruct the continuous full utilization of this system. Among these factors are: instability of SDI value, high rate el chemical consumption (i.e., acid, flocculant and flocculant-aid), frequent backwashing and high rate of water consumption for this purpose, dangerous hazard resulting from acid storing and handling, and relative high operating cost resulting from excess consumption of
186
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
Table 3 Common pretreatment: Flow balance 1987 RO Lines, m 3 Overflow, m 3 Infiitration, m 3 Backwash, m 3 Total filtrate, m 3
1988
3,372,092 160,566 674,012 340,638 4,547,308
2,518,2;~72 603,757 466,623 276,378 3,865,130
1989
Total"
%
1.641,033 690,624 406,948 340,595 3,079,200
7,531,497 145,494 1,547.583 957,611 11,491,638
65.54 12.66 13.47 8.33 100
Table 4 Chemicals and energy consumptions for common pretreatment
Chemicals consumption, mg/l H2SO4 FeCISO4 Chlorine Polyelectrolyte Energy consumption, kWldm 3 Energy input to filtrate through the intake "<,fDoha Energy input to filtrate produced Total energy input to filtrate prodqced
Building and construction Convent. pretreatment system Machinery and equipment RO membranes Other assets Total cost
KD
%
203,537 464,488 778,796 275,310 17,270 1,739,401
1988
1989
Average
84 3 1.83 0.57
82.17 2.59 1.98 0.4
74.55 1.95 2.02 0.65
80.24 2.51 1.94 0.54
0. I O. 113 O. 213
Table 5 Capital investment cost of seawater desalination for 4,546 m3/d two-stooge spiral wound RO system Item I
1987
11.7 26.7 44.78 15.83 0.99 100
O. 1 0.12 0.224
References
[1|
[3] c h e m i c a l s , filtrate a n d e n e r g y . Consequently, alternative seawater pretreatment are extensive investigation. Among ultrafiitration and beachwell gaining more attention and B e a c h w e l l i n t a k e s e e m s to b e
methods for now under these, micro/ intake are R&D work. more viable,
O. 1 O. 13 0.23
t e c h n i c a l l y a n d e c o n o m i c a l l y ; a l t h o u g h it is site d e p e n d e n t [15]. M i c r o / u l t r a f i l t r a t i o n is also e x p e c t e d to b e c o m p e t i l i v e , t e c l m i c a l l y and economically, compared with the c o n v e n t i o n a l p r e t r e a t m e n t p r o c e s s [ 15-171.
[2]
Source: Abdelhalim et al. [13].
O. 1 O. 151 O. 251
[4] [5]
N.R.G. Walton, R e pretreatment injection - A little chemical control and management, Desalination 82 (1991) 281-300. Permasep Engineering Manual (PEM), E.I. DuPont de Nemours & Co., 1982. T.K. Osta and L.M. Bakheet, Pretreatment system in reverse osmosis plant, Desalination 63 (1987) 71-80. B. Ericsson and B. Hallmans, Membrane filtration as a pretreatment method, Desalination 82 (1991) 249-266. S.R. Ahmed, M.S. Alansari and T. Kannari, Biological fouling and control at Ras Ahu Jarjur R O Plant - A n e w approach. Desalination 74 (1989) 69-84.
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
[6]
H. Ridgway, C. Justice, C. Whittaker, D. Argo and B. Olson, Bio-film fouling of RO membranes - Its nature and effect on treatment of water reuse, J. AWWA (1984). [7] D.G. Argo and M. Rigby, Evaluation of membrane processes and their role in wastewater reclamation, Prepared for Office of Water Research and Technology, Vol. 3A, Washington DC, November 1981. [8] B. Winfield, A study of the factors affecting the rate of fouling of reverse osmosis membranes treating secondary sewage effluent, Water Research 13 (1977). [9] G. Belfort, Ptetreatment and cleaning of hyperfiltratio,~ (reverse osmosis) membranes in municipal wastewater renovation, Desalination 21 (1977). [10] H. Winters, Control of organic fouling at two seawater reverse osmosis plants, Desalination 66 (1987) 319-325. [111 J. Cohen and S. Hannah, Water Quality and Treatment, 3rd Ed.. American Water Work Association (AWWA), Chapter 3, McGraw Hill, 1971.
187
[12] Krupp Operating Manual of Kuwait Seawater R e Pilot Plant, Common Pretreatment, Part I, Section 1, Krupp Industrietechnik. [13] M. Abdelhalim and M. Abdel-Jawad, Economic put6ntial~ of seawater desalination by revetse osmosis, Desalination o4 (1988) 65-82. [14] M.M. Abde!halim and M. Abdel-Jawad, Feasibility of reducing desalted water cost by reverse osmosis, Proceedings of 1990 Biennial Conference of National Water Supply Improvement Association (NWSIA), Florida, Vol. 1, August 1990, 295-318. [15] Z. Qamiyah, Beachweil Seawater Intake, Kuwait Institute for Scientific Research (KISR), Report No. 3152, 1989. [16] G.B. Tanny and R. D'Agostino, Membrane microfiltration as a pretrea~ment for seawater reverse osmosis, Preceeding of the 7th International SymposbJm on Fresh Water from the Sea, Vol. 2, 1980, 307-317. [17] V. Mavrov, I. Dobreversky and B. Boney, Ultrafiltration as a pretreaunent method for membrane desalination by reverse osmosis and electrodialysis, Synthetic Polymeric Membranes (1987) 263-267.
ELSEVIER
Conventional pretreatment system for the Doha Reverse Osmosis Plant: Technical and economic assessment Sadeq H. Ebrahim*, M a h m o u d M. A b d e l - J a w a d and M. Safar
Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait Tel.: 965-4836612, Fax: 965.4834712
,~
Abstracl
The conventional pretrea~ent method is widely used to treat surface seawater feed for the seawater reverse osmosis (SWRO) process; however, this method is cumbersome and costly and has many drawbacks that lead to higher product water cost by RO desalination technology. This paper ouOines the experience gained from 65 months of operation of a surface seawater conw:ntional pre:~reatment system at the Doha Reverse Osmosis Plant (DROP) in Kuwait. The paper descnbes'the pretreatment system used, problems and drawbacks encountered dining the course of operaaott, namely: instability of Silt Density index (SDI) value, high rate of chemical consumption, high ra~~. of water consumption for backwashing and high energy consumption, and gives an economic assessment for the pretreatment system. The economic analysis shows that the cost -for the conventional pretreaUnent system accounts for 26.7% of the total capital investment cost for a 4,546 m3/d twostage spiral-wound (SW) RO system, similar to the one used at DROP. A 30% s~ving in u:,it product water cost could be achieved if a beachv~ell intake pretreatment system is used instead ot~the conventional pretreatment system.
1.
Introduction
Successful long-term p e r f o r m a n c e o f a reverse osmosis (RO) s y s t e m depends on three factors: proper design, proper p r e t r e a t m e n t and p r o p e r o p e r a t i o n and maintenance o f ~ e RO system. All three factors are important, but proper pretreatment is the foundation for successful opera,~on of the RO plant. Pretreatment is done to minimize m e m b r a n e fouling, which ,;s the most serious problem encountered in the RO process. A drastic decline in RO performance
and serious damage to the membranes are the results or' membrane fouling that ultimately are reflecteo in l~gher product water cost. Ma~y types o f fcul~nts could occur on the surfaces o f RO m e m b r a n e s and can be broadly identified into the following: - Suspended solids - Cc.l!oids Metal oxides -~ Scales (i.e., CaCO3, CaSO4, BaSO4, SrSO~t~ " CaF2, SiO2) Biological slime - Organics Oil grease
*Corresponding author. 0011-9164/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0011.9 ! 64(95)00052-6
IS0
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
The format:ion of one or more of these types on the surfaces of the membrane is a function of the nature of the feedwater to the membrane and the nature and degree of pretreatment applied to the RO system; other factors include membrane material, permeator configuration, nature of operat2on and quality control [1]. In seawater desalination by RO, the first five types of foulants are most likely to occur, with calcium carbonate (CaCO3) as the dominant type of scale to form [2-4, 10]. In brackish water desalination by RO, calcium sulphate (CaSO4) scaling is widely encountered due to high content of sulphate ions in well waters [2]. Other researchers expei-ienced biological fouling due to sulphate-reducing bacteria (SRB), anaerobic bacteri:z and algae growth [5]. Biological fouling F (microorganisms) and organic fouling are the most dominant type of fouling in industrial and wastewater reclamation by RO [6-91. The conventioaal pretreatment method is widely applied in seawater reverse osmosis (SWRO) desalination processes to prevent membrane fouling. Feed seawater is pretreated by the conventional method to lower the silt density index (SDI), remove excessive turbidity or suspended solids, adjust and control the pH, inhibit or control the formation of scale, and disinfect or prevent slime growth and contamination of equipment. Type and size of the conventional pretreatment system varies depending on the nature of the feedwater to be treated and type of membrane used in the RO system. Cost of the conventional pretreatment system also depends on the type and size of the pretreatment system. At the Doha Reverse Osmosis Plant (DROP) in Kuwait, a conventional pretreatment system (common pretreatment) was usel:l to treat surface seawater feed to three dif.~erent g o lines. The average va,~es for ~ e main constituents of the surface seawater feed are presented in Table 1. Each of three lines was divided into two stages as follows:
Table 1 Averageseawaterqualityat Doha Parameter
Concentration*, mg/l
TDS at I80°C Total alkalinity, as CaCO 3 Carbonate Bicarbonate Carbon dioxide Sulphate Chloride Calcium Magve~ium Sodium Potassium Total i-on pH SDI value, %
47,000 175 14 185 14 3,400 24,000 570 1,700 !2,300 470 0.08 8.2 > 6.5
± ± ± ± ± ± ± ± ± ± ± ± ±
2,000 15 8 18 4 300 700 45 150 20 20 0.08 0.1
*Average concentrations of 12 samples collected monthly.
RO Line 1 First stage - Spiral wound, UOP-PA 1501 Second stage - Spiral wound, UOP-PA 8600 RO Line 2 First stage - Hollow fine fiber, DuPont B-10 Second stage - Hollow fine fiber, DuPont B-9 RO Line 3 First stage.
- Plate and frame, Em'o+ Schleicher & Schueli + FilmTec Secondstage - Spiral wound Hydtonautics804013 Each line had its own pretreatment system, in addition to the commgn prctreatment, and was designed to produce 1,000 m3/d of fresh water. The common pretreatment and Lines 1 and 2 were in operation from !984 until August 1990. Membranes of the first stage oi RO Line 1 were replaced in July 1989 by a new 8" spiral wound configurauon system from FilmTec (FT-30). Modvles of RO Line 2 were also replaced in December 1988 by the new version of the hollow fiber DuPont (B10T). With regard to RO Line 3, the plant was shutdown in March 1988 due to technical and
181
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
economic reasons related to the plate and frame configuration system. 2.
Description of the treatment system
common
pre-
At DROP, the chlorinated surface seawater is delivered to the common pretreatment from Doha East Power Station via a reinforced fiberglass pipe at a pressure of 2-3 bar (Fig. 1). If the chlorine content is below the required level, chlorine gas is added. The removal of suspended and colloidal particles from tne untreated water is carried out by fiocculation and filtration. The process of flocculation may be divided into two separate stages that merge into one another. -Destabilization (coagulation and flocculation) - Agglomeration Destabilization means charge compensation of the usually negative colloidal particles, thus leadi|tg to the formaCaon of microfiake~. This is achieved by mixing the untreated water with the fiocculant using a substantial amount of energy [11]. The flocculant, in this case FeCISO4 (ferric chloride sulphate), is added to the untreated water at the inlet to the destabilization tank. The eaergy required for mixing is applied by the agitator. Dwell time for the water is approximately 15 seconds at maximum flow [12]. The ideal pH value for destabilization and flocculation is set by adding H2SO4 (salphuric acid), also at the inlet to the destabilization tank. In the agglomeration stage, an appropriate amount of energy is used to combine the destabilized particles or so-called microflakes. At this stage, microfiakes are formed by a series of processes such as chemical transformations, electrokinetic effects and adsorption phenomena. Excess turbulence leads to a disintegt'ation of flakes that have already formed. These processes take place in the cascade configuration of tanks. To assist with agglomeration, the discharge of
tanks is provided with a dose of polyelectrolytic solution that has an interlacing effect on the flakes that have formed. The energy necessary for coagulation is applied by three agitators. To optimize the application of energy, the speed of the agitators in the agglomeration basins is adjustable. Fttrthermore, the last agitator, in the direction of flow, is fitted with a torque measuring instrument. The dwell time of the water in the agglomeration tanks is approximately lO vain at maximum flow. The pretreated water leaves the last tank via a pipe and is taken to the filter station. The purpose of filtration is to remove the flakes from tim water. This is achieved by passing the pretreated water through four parallel multi-layer filters. The filters are of the open gravity type. The mixture of water and flakes flows downstream through the filter material, where various types of filter mate~-ials are placed on top of one another. The ~ize of grains is finer in the direction of flow so that the particles to be filterea~ ,.~atwill be deposited uniformly over the e,~:J~ height of the filter so it is possible to av~id a large increase in pressure loss due tc surface filtration. The layer distribution in the filters is as follows: Grain size, mm Supporting layer Silica sand Hydroanthmcite
various 0.7-1.2 1.4-2.5
Filter height, m 0.3 1 0.7
The mixture of water and flakes is distributed evenly over the filter layer and leaves the filters via the pure-water discharge valves and the pure-water pipe. Flow through the filters is maintained constant by the two filter outlet controls. The filtering velocity is I0 n ~ at maximum flow. The filters can be backwashed using the filtered water and air supplied by a compressor. The time and sequence of water and air can be adjusted at will. The pretreated water, emerging from the multi-layer filters, is collected in the pure water pipe and fed to the intermediate tank before it is delivered to the
182
S.H. Ebrah ;met al. / Desalination 102 (1995) 179-187
Ferric c h l o r i d e sulphate dosing
Acid 4 dos-
Polyelectrolyte dosing
"7"
/
blain storage tank
/
Sea water fro
the
III L___.J
intak~ID.esta.bi£1zatlon tank {~hlorina- / l'ti~ n svstedl
,
,
'
,
Flocculation
'
'1 ~,
To ~ine
stage I
Filtrati0, n s t a g e [
[ ~
.....
/
/
~
--Air
To" line II =
line
. _ ~ __~
~e~t t a n ~
storage tank. Before entering the storage tank, chlorine gas can be added, if chlorine content is less than the setforth minimum level. The volume of filtrate being pumped is controlled at the intermediate tank. The filtrate in the storage tank is fed to the three RO lines. 3. Chemical dosing s y s t e m s
for
the RO lines
Be/ore the filtered water, coming from the CP, enters the first stage of each RO line, further treatment is carried out separately to obtain the quality feedwater specified by the manufacturers of the RO modules, The pretreatment systems for RO Lines 1, 2 and 3 consist of:
- T w o parallel activated carbon filters to ensure the complete removal of residual chlorine, and to act as a holding tank for the sodium hydrogen sulphite reaction with chlorine -Anti-sealant dosing system to prevent sulfate scaling - Acid dosing system to prevent carbonate scaling - T w o cartridge filters (micron filters) to filter out particles larger than 5 ttm. These two filters are located just before the highpressure pumps. 3.2. RO Line 2 -
-
3.1. RO Line 1 -
Sodium hydrogen sulphite dosing system to remove residual chlorine ill ',he feed
Fig. i. Schematic diagram for the common pretreatment system at DROP.
-
Acid dosing system to prevent carbonate scaling Polyeleetrolyte dosing system and three inline coagulation filters to further reduce tt,.~ silt density index (SDI) of the feed te ~ess than 3.0 Sodium hydrogen sulphite dosing system
183
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
-
to remove residual chlorine in the feed Three cartridge filters to r e m o v e particles larger than 5 ~tm. T w o filters are always in operation and the third is on standby basis.
~,~6 o
3.3. R O Line 3
• ~ SD! F i l t r a l e S~\ I xlll
- A c i d dosing system to prevent carbonate scaling -Anti-sealant d o s i n g s y s t e m to p r e v e n t sulfate scaling - Sodium h y d r o g e n dosing system to remove residual chlorine - T w o cartridge filters to filter out particles larger than 25 ttm.
4.
Operating evaluation
results
and
performance
":"
"
'
3
l,
Seawater feed flow r a t e , m 3 / h Pressure of seawater feed, bar Temperature of seawater feed, °(7 SDI of seawater feed, %/min pH of seawater feed SDI ~f seawater filtrate, %/min 2-,,Hof seawater filtrate Acid dosing (H2SO4) rate, mg/l FeCISO4 dosing, rate (Fe3+), mg/l Polyelectrolyt¢ dosing rate, mg/] C12 content in seawater feed, mg/l CI2 content in filter inlet, mg/1 CI2 content after storage tank, mg/]
493.11 2.06 26.24 6.47 g. 14 3.6 6.21 82.24 3.04 0.62 0.15 1.7 0.16
"
~
,
5
'"='-"
i
.
.
.
.
.
,s , , ,3 s, ~ i ~ s
37 .
Mom~
Fig. 2. Monthly average SDI and pH of filtrate from ~tugust 1984 to December 1989.
S
l
'-', •,
. I
Average
..,
•
During the period from A u g u s t 1984 until D e c e m b e r 1989, the C P had been operating with availability o f m o r e than 97%. The a v e r a g e m o n t h l y parameters are s h o w n ~n Figs. 2--4 and the mean values are prese[~*,~d in Table 2.
Paramet.er
"'"W
'
: ~3 i7 ,-I " ~ . ~
R
Table 2 Mean parameters for seawater feed 8 n d filtrate for the period from August 1984 to December 1989
"" , •
•
s
li~i I l l i I ~ i~
I 1%
"" ,,,
$
9
,•
,
T
,..=
13 17 21 25 29 33 37 41 45 49 53 $7 61 65 Mouths
Fig. 3. Monthly average FeClSO4 and pelyelecm~lyte dosing rates from August 1984 to December 1989.
=t _:.,o !I
it
!t
;+
t!
V
7
7
i
10
$
9
13 17 21 25 29 33 .~7 41 45 49 53 $7 61 65 Mouths
Fig. 4. Momhly average seawater temperature from August 1984 to December 1989.
184
S.H. Ebrahitn et al. /Desalination 102 (1995) 17o-187
Mosl of the time, the system was successfully controlled to give the designed quantity of filtrate with SDI value less than 4.0 and mean SDI value of 3.6 (Fig. 2); buL in some cases, it failed to produce acceptable quality and the required qua~tity of filtered seawater. The causes for these failures can be attributed to several factors, mainly: clogging of dual media filters, effect of pH, dosing rate of FeCISO4, dosing rate of polyelectrolyte, energy input, and climatic conditions (i.e., temperature, dust storm, wind).
4.1. Clogging of dual media filters Due to occasional increase in marine biological activities, the ~u~faces of the dual media filters become completely clogged causing a deterioration in the quality and quantity of filtered water. Coi~sequently, the backwash frequency had to be increased to twice daily instead of once a day. There was also an increase in the time required to reduce the water level in the filter to 30% before the start of the backwash sequence, which was 2 rain air followed by 8 min water and blow-down, then 2 rain air followed by 8 win water. To overcome this problem, the backwash program was modified to 3 min backwash with water, to break down the clogging layer on the surface of the filter, followed by backwashing twice with flow of air and water for 2 min and 6 rain, respectively.
.4..3. Dosing rate of FeCIS04 and polyelectrolyte To improve the SDI of the filtrate, the ~e +3 dosed at an average rate of 4-5 mg/1 for the first 27 months of operation (Fig, 3). The addition of cationic polyelectrolyte (0.3-0.5 mg/l) in the CP system from February 1986 (Fig. 3) along with the reduction of the filtrate pH to 6.0, enabled the system to produce SDI value of less than 4.0 with the reduction in Fe +3 dosing rate to 2.5-2 mg/l. This optimization, greatly redveed the quantities of FeCISO4 consumed and, eventually, reduced the cost of the filtrate produced from the CP.
4.4. Energy input Flocculation is achieved by mixing the untreated water with the fiocculant using a substantial amount of energy. For the CP system al DROP, FeCISO4 is used as flocculant, whereas the energy is supplied through the mixer of the destabilization tank (Fig. 1). Failure of the mixer caused an increase in the SDI value that was evident during the course of operation. Hence, the SDl~was remarkably improved during 1987 as a result of mechanical modification of the destabilization tank's mixer. New gear pinions were installed to this motor that allowed reverse rotation of the mixer against the natural water flow. It is believed that the new modification resulted in better distribution/mixing of the added flocculant.
4.2. Effect of pH 4. 5. Effect of climate During the first 19 months of operation, the filtrate pH value was set to be between 6.5 to 6.8, as recommended by the designer of the CP; however, it was found through laboratory tests that reducing the pq value to 6.0 e.~hances the fiocculation process, and reduce quantities of Fe +3 dosed to achieve a SDI value of less than 4.0.
Kuwait's climate has a noticeable effect on the filtrate quality at DROP. High turbidity caused by dust storms increases the SDI of the filtrate. The water quality of the filtrate in the 500 m 3 storage tank was affected by dust entering through the vent during sand storms. The system was modified to draw the filter~
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
feed directly from the dual media filters during bad weather. Also, strong winds associated with low tides tend to increase the silt content in the intake seawater, and eventually increase the SDI of the filtrate. Moreover, increase in marine biological activities, in summer, increases the potential for clogging tile dual media filters. Therefore, higher SDI values are observed during summer (April to September) when high temperatures are associated with strong winds and dust. During the reported period (August 1984 December 1989), a total of 21,828,201 m 3 of filtrate had been produced by the CP. Only 70% was used as feed for RO lines, while the remaining 30% was used to backwash the multi-layer filters and to reorganize and resettle the filter media (infiltration), and very small quantity was wasted due to overflow. During the last three years of operation, only 65.54% of the filtrate was utilized for RO lines while the remaining 34.46% was used for backwash, infiltration and overflow (Table 3). The average chemical and energy consumption of the CP, for the last tl~e,years of operation, is shown in Table ~t. Based on actual cost of chemicals, the total cost for these chemicals per m 3 of filtrate produced is KD 0.0142 (US $ = KD 0 303). The total energy input to the produced filtrate is 0.23 kWh/m3. Based on 0.026 KD/kWh cost of energy in Kuwait, the total energy cost per m3 of filtrate produced is KD 0.006. Based on product water cost breakdown calculated from data compiled at DROP for RO systems of different capacities and module configurations, the con cent~onal pretreatment running cost constitutes 21 to 25% of the total product water running cost. This percentage can be further reduced by 410%, in addition to a significant reduction in energy consumption, if beachwell or micro/ultrafiltration pretreatment methods are used instead of the conventional pretreatment method [14].
185
The capital cost of the conventional pretreatment system represents a significant share of the capital investment cost for a twostage RO system. Based on actual cost data obtained from the manufacturer of the common pretreatment system, the cost for the CP accounts for 26.7% of the total capital investment cost for a 4,546 m3/d two-stage spiral-wound (SW) RO system (Table 5). This percentage can be significantly reduced if other pretreatment methods, p.amely, beachwell intake" system or micro/ ultrafiltration pretreatment system, that have less machinery and equipment, are used. A comparative study was carried out at the Kuwait Institute "for Scientific Research (KISR) to compare the cost of desalinated water by RO using the beachw¢ll intake system and conventional surface se~.water pretreatment system. It was concluded that the unit water cost using the beachwell system ,is 183~96 flls/m3 compared with 263.28 fils,'m3 using the convention',0 pretreatment system [15]. This means that mere is a saving of 30% in unit product water cost if a beachwell intake syst¢m is used instead of the conventional pretreatment system.
5. Conclusion The conw~ntionai pretreatment system used at DROP /has succeeded in providing feedwater fo~l"the RO lines with SDI less than 4.0. The avallability of this system was more than 97°/, Oaring the operated period; however, this system was very difficult to control. Several factors obstruct the continuous full utilization of this system. Among these factors are: instability of SDI value, high rate el chemical consumption (i.e., acid, flocculant and flocculant-aid), frequent backwashing and high rate of water consumption for this purpose, dangerous hazard resulting from acid storing and handling, and relative high operating cost resulting from excess consumption of
186
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
Table 3 Common pretreatment: Flow balance 1987 RO Lines, m 3 Overflow, m 3 Infiitration, m 3 Backwash, m 3 Total filtrate, m 3
1988
3,372,092 160,566 674,012 340,638 4,547,308
2,518,2;~72 603,757 466,623 276,378 3,865,130
1989
Total"
%
1.641,033 690,624 406,948 340,595 3,079,200
7,531,497 145,494 1,547.583 957,611 11,491,638
65.54 12.66 13.47 8.33 100
Table 4 Chemicals and energy consumptions for common pretreatment
Chemicals consumption, mg/l H2SO4 FeCISO4 Chlorine Polyelectrolyte Energy consumption, kWldm 3 Energy input to filtrate through the intake "<,fDoha Energy input to filtrate produced Total energy input to filtrate prodqced
Building and construction Convent. pretreatment system Machinery and equipment RO membranes Other assets Total cost
KD
%
203,537 464,488 778,796 275,310 17,270 1,739,401
1988
1989
Average
84 3 1.83 0.57
82.17 2.59 1.98 0.4
74.55 1.95 2.02 0.65
80.24 2.51 1.94 0.54
0. I O. 113 O. 213
Table 5 Capital investment cost of seawater desalination for 4,546 m3/d two-stooge spiral wound RO system Item I
1987
11.7 26.7 44.78 15.83 0.99 100
O. 1 0.12 0.224
References
[1|
[3] c h e m i c a l s , filtrate a n d e n e r g y . Consequently, alternative seawater pretreatment are extensive investigation. Among ultrafiitration and beachwell gaining more attention and B e a c h w e l l i n t a k e s e e m s to b e
methods for now under these, micro/ intake are R&D work. more viable,
O. 1 O. 13 0.23
t e c h n i c a l l y a n d e c o n o m i c a l l y ; a l t h o u g h it is site d e p e n d e n t [15]. M i c r o / u l t r a f i l t r a t i o n is also e x p e c t e d to b e c o m p e t i l i v e , t e c l m i c a l l y and economically, compared with the c o n v e n t i o n a l p r e t r e a t m e n t p r o c e s s [ 15-171.
[2]
Source: Abdelhalim et al. [13].
O. 1 O. 151 O. 251
[4] [5]
N.R.G. Walton, R e pretreatment injection - A little chemical control and management, Desalination 82 (1991) 281-300. Permasep Engineering Manual (PEM), E.I. DuPont de Nemours & Co., 1982. T.K. Osta and L.M. Bakheet, Pretreatment system in reverse osmosis plant, Desalination 63 (1987) 71-80. B. Ericsson and B. Hallmans, Membrane filtration as a pretreatment method, Desalination 82 (1991) 249-266. S.R. Ahmed, M.S. Alansari and T. Kannari, Biological fouling and control at Ras Ahu Jarjur R O Plant - A n e w approach. Desalination 74 (1989) 69-84.
S.H. Ebrahim et al. /Desalination 102 (1995) 179-187
[6]
H. Ridgway, C. Justice, C. Whittaker, D. Argo and B. Olson, Bio-film fouling of RO membranes - Its nature and effect on treatment of water reuse, J. AWWA (1984). [7] D.G. Argo and M. Rigby, Evaluation of membrane processes and their role in wastewater reclamation, Prepared for Office of Water Research and Technology, Vol. 3A, Washington DC, November 1981. [8] B. Winfield, A study of the factors affecting the rate of fouling of reverse osmosis membranes treating secondary sewage effluent, Water Research 13 (1977). [9] G. Belfort, Ptetreatment and cleaning of hyperfiltratio,~ (reverse osmosis) membranes in municipal wastewater renovation, Desalination 21 (1977). [10] H. Winters, Control of organic fouling at two seawater reverse osmosis plants, Desalination 66 (1987) 319-325. [111 J. Cohen and S. Hannah, Water Quality and Treatment, 3rd Ed.. American Water Work Association (AWWA), Chapter 3, McGraw Hill, 1971.
187
[12] Krupp Operating Manual of Kuwait Seawater R e Pilot Plant, Common Pretreatment, Part I, Section 1, Krupp Industrietechnik. [13] M. Abdelhalim and M. Abdel-Jawad, Economic put6ntial~ of seawater desalination by revetse osmosis, Desalination o4 (1988) 65-82. [14] M.M. Abde!halim and M. Abdel-Jawad, Feasibility of reducing desalted water cost by reverse osmosis, Proceedings of 1990 Biennial Conference of National Water Supply Improvement Association (NWSIA), Florida, Vol. 1, August 1990, 295-318. [15] Z. Qamiyah, Beachweil Seawater Intake, Kuwait Institute for Scientific Research (KISR), Report No. 3152, 1989. [16] G.B. Tanny and R. D'Agostino, Membrane microfiltration as a pretrea~ment for seawater reverse osmosis, Preceeding of the 7th International SymposbJm on Fresh Water from the Sea, Vol. 2, 1980, 307-317. [17] V. Mavrov, I. Dobreversky and B. Boney, Ultrafiltration as a pretreaunent method for membrane desalination by reverse osmosis and electrodialysis, Synthetic Polymeric Membranes (1987) 263-267.