Seawater desalination by reverse osmosis

Desalination, 52 (1985) 187-199 Elsevier Science Publishers B . V ., Amsterdam -Printed in The Netherlands

187

SEAWATER DESALINATION BY REVERSE OSMOSIS PLANT DESIGN, PERFORMANCE DATA, OPERATION AND MAINTENANCE (TANAJIB, ARABIAN GULF COAST)*

W . HEYDEN Preussag AG Bauwesen, Water Treatment Department, Heinrich-Hertz-Strasse 23, D-3005 Hemmingen 1 (F.R .G.) . Tel. (0511)4206-299 ; Telex 923523 PR WAS D

SUMMARY A systematic interpretation of critical combinations of feed water, pretreatment and membranes and adequate conclusions can avoid setbacks of reverse osmosis processes . Reputed systems manufacturers will find the right approach if their tasks will be changed from a limited responsibility - for design, construction, installation and commissioning only - to an overall responsibility which includes operation and maintenance. In 1982/83 Preussag received from Aramco in Saudi Arabia the order for the first temporary reverse osmosis seawater desalination plant at Tanajib, Arabian Gulf Coast, with a total net capacity of 600,000 gpd. The orders were placed on a turnkey basis including operation and maintenance . The highlights of the RO seawater desalination plant are : shallow beach wells, ultrafiltration, vacuum deaeration, high-pressure multi-stage centrifugal pumps with energy recovery turbine, single stage reverse osmosis and free programmable logic and process controller . During the first more than one and a half years of operation, product quantity and quality has always met the requirements of Aramco's specification . The excellent experiences until today with the first temporary Tanajib reverse osmosis seawater desalination plant permit the conclusion that RO technology can avoid major setbacks and may face the competition with the distillation processes with confidence .

INTRODUCTION It is not my intention to answer the most topical question : Is reverse osmosis winning the seawater desalination war against the distillation processes?

*Presented at the Symposium on Economics of Water Desalination Processes prepared by the Working Party on Fresh Water from the Sea of the European Federation of Chemical Engineering and Dechema, Bad Soden, 8-10 October 1984 . 0011-9164/85/$03 .30

9 1985 Elsevier Science Publishers B . V .



HIGH PRESSURE PUMPS WITH ENERGY RECOVERY

INTERMITTENT SODIUM BISULFITE

Fig. 1 . Simplified block-diagram.

BEACH WELL PUMPS FOR SEAWATER

NON-OXIDIZING DISINFECTANT

CARTRIDGE FILTERS TRATIO

REV RSE OSMOSIS

ULTRAF

STERILIZATION

CORRECTION OF PM VALUE

SEQUESTERING AGENT

SULFURIC ACID

PERMANENT SODIUM BISULFITE

1

0;

VACUUM OEAFRATOR

9

C7

k

L+1

x

m



SEAWATER DESALINATION BY RO - TANAJIB

1 89

My contribution should show that a systematic interpretation of critical combinations of feed water, pretreatment and membranes and adequate conclusions can avoid setbacks of RO processes . I am convinced that reputed systems manufacturers will find the right approach if their tasks are changed from a limited responsibility - for design, construction, installation and commissioning only - to an overall responsibility which includes operation and maintenance . We can demonstrate this by describing our fulfillment of an order received in summer 1982 from Aramco in Saudi Arabia - in the face of international competition - for the first temporary seawater desalination plant at Tanajib, Arabian Gulf Coast .

PLANT ORDER AND DESIGN

The initial order was placed for a plant of two lines with a total net capacity of 400,000 gpd (approx . 1,500 m'/d) . After successful start-up of the plant in spring 1983, we received an additional order for a third line with a net capacity of 200,000 gpd . Both orders were placed on a turnkey basis including operation and maintenance . Aramco emphasized their indispensable need for drinking water by very tough penalties for failures in delivery time, quantities and qualities . The very short delivery time specified (7 months including installation and commissioning) led us to decide on more or less full preassembly of the plant with most of it containerized . Full responsibility for quantities and qualities led us to a process combination according to the simplified block diagram in Fig . 1 with the following highlights : (1) Shallow beach wells (instead of open seawater intake) each with a capacity of approx. 50 m'/h . Advantages : No expensive off-shore installation ; excellent mechanical cleaning of the seawater by subterranean filtration ; reduction of marine growth and microorganism problems ; avoidance of any flocculation, sedimentation and media-filtration and good balance of seawater temperature and salinity . Disadvantages : Complex pumping installation and insufficient knowledge about the influence on raw water quality by subterranean flow which, however, can sufficiently be clarified by drilling of representative test-wells . The test-wells with respective observation holes also permit the determination of the depression cone for fixing the distances between the beach wells (Fig . 2) . After having drilled a test well and observation holes we made several pumping tests and determined the resulting dynamic water levels when steady-state conditions had been reached .



190

W .HEYDEN OBSERVATION

HOLES

P, /.C /A

.( .

P /4 .AV /.V

r/.!Y/.( /No

SURFACE /xm / /,$/.NiJicY/,tW/AW RSTIOL AAUP_ LEV.C DYNAMIC WATER LEVEL

I TEST WELL

X

IMPERMEABLE UNIT



R

Fig . 2 . Determination of permeability Kf from pumping test results . Kf = Q x In (rk Jr; )/,r(y k ' - y,') ; k > i, where Kf = permeability (m/s), Q = pumping rate (m'/s), rl,rk = distances of observation holes from test well (m), y,, yk = water levels above impermeable unit inside observation holes (m), H = static water level above impermeable unit (m), R = radius of depression cone (m) .

Design : See Fig. 3 . Principle performance of one shallow beach well : depth : approx . 13-16 m ; diameter : approx . 1 .2 m ; two different coaxial gravel packs . comprising modules from Desalination Systems, U .S .A ., with non-cellulosic membranes and polysulfone support . Advantages : Continuous and easy operation ; excellent removal of small particles including colloidal matter (particularly oily pollutants) ; no breakthrough ; good protection of down-stream installed RO membranes ; no chemicals required during operation ; easy disinfection by chemical shock treatment and small space requirement . Disadvantages : Additional costs ; higher raw water demand because of OF recovery rate of approx . 85% and higher energy consumption because of differential pressure to overcome . Design : 54 8-in . elements type U 350 G 50 per line with a pore size of approx . 20 to 40 A and a net capacity of approx . 53 m 3 /d each at 7 bar and

(2) Ultrafiltration



SEAWATER DESALINATION BY RO - TANAJIB

191

25°C (design basis: 6 bar and 20°C), 9 pressure vessels, product to be taken at both ends of each pressure vessel, once a month preventive cleaning with citric acid at pH 4 .

~ ~\l I

WELL HEAD

CONCRETE SLAB

No

WA

WE

ROOM

FINI

TO CONCRETE COLLECTION RING MAIN

DUNE SA D

i

00 41,

STATIC WATER LEVEL

11

III

IIIA

III RISER PIPE ON 100

III II

OUTER GRAVEL PACK (0,7-1 .2 mm) INNER GRAVEL PACK (2 -3 mm) SUBMERSIBLE PUMP

I ®® 300 ®® 200 m m

Fig. 3 . Shallow beach well design, Tanajib .



192

W. HEYDEN

(3) Vacuum deaeration for the removal of oxygen . Advantages : Avoidance of destruction of the RO membranes due to oxidation ; reduction of stainless steel corrosion problems to a minimum permitting concessions by selecting stainless steel qualities and saving of SBS (sodium bisulfite) . Disadvantages : Additional costs ; additional space requirement and additional energy consumption . Design : Residual oxygen : 0 .2 ppm ; specific energy consumption : 0 .2 kWh/m3 .

(4) High-pressure multi-stage centrifugal pumps with energy recovery turbine Design : Capacity : 80 m3/h ; pressure head : 67 .5 bar. (5) Reverse osmosis comprising modules from Toray, Japan, with polyether composite membranes and polysulfone support . Advantages : Single stage operation ; high recovery rate of 40% compared to generally 25-30% of other membranes ; low salinity of permeate ; high temperature resistance (up to 45 °C) ; wide pH range stability and high pressure durability . Disadvantage : Non-resistance against oxygen . Design : 180 8-in . elements, type SP-120 per line and 30 pressure vessels .

(6) Free programmable logic and process controller permitting completely automatic start-up, operating control, ordinary and emergency shut-down, alarm recording and periodical data logging . Beside these highlights and the normal 20 Mm cartridge filtration the following chemical treatment is applied :

Pre-treatment up-stream RO units (1) Sterilizing shock-treatment : In case of bacteriological contamination or shell growing problems inside the wells, it is possible to inject periodically a non-oxidizing disinfectant as shock-treatment agent into the wells . (2) Intermittent sodium bisulfite dosing : An intermittent dosing (twice daily) of an increased amount of 500 ppm sodium bisulfite permits reduction of bacteriological growth problems on the ultrafiltration and RO membranes. With the same dosing equipment also during shut-down the same amount of sodium bisulfite is added . (3) Sequestering agent dosing: Depending on the recovery rate of the reverse osmosis units there is a permanent danger that the seawater becomes oversaturated with certain salts (e .g . calcium carbonate and calcium sulfate) and that the resulting scaling blocks the reverse osmosis membranes. For the avoidance of these scale formation problems approx . 5 ppm of polyacrylate (Flocon 100) are added after ultrafiltration .



SEAWATER DESALINATION BY RO - TANAJIB

193

(4) Permanent sodium bisulfite dosing : For the elimination of the residual oxygen after the vacuum deaerator a dosing pump injects approx . 20-30 ppm of sodium bisulfite into the main pipe downstream the vacuum deaerator. For the control of bacteriological growth problems and for the conservation of stainless steel pumps and pipes with the same dosing equipment also during shut-down an increased amount of 500 ppm sodium bisulfite is added . (5) Sulfuric acid dosing : For the avoidance of any calcium carbonate precipitation during the concentrating process on the reverse osmosis membranes a small amount of approx. 10 ppm sulfuric acid can be injected into the main pipe after the pressure increasing pumps. Post-treatment down-stream RO units (1) Correction of the pH value : The permeate leaving the reverse osmosis units is relatively aggressive and would cause corrosions if no correction, that means increasing the pH value would be realized . As pH correction chemical a small amount of approx . 15 ppm of caustic soda is injected into the drinking water transmission pipe from the plant to the storage tanks. (2) Sterilization : Independent of the fact that no bacteria pass the reverse osmosis unit it is necessary to add a sterilizing agent to the drinking water in order to have a certain protection against a post-contamination with bacteria during the storage and transportation of the drinking water. As sterilizing agent we apply calcium hypochlorite. MATERIAL SELECTION

We know very well that seawater is very aggressive and corrosive . This behavior is even intensified if acid is added to the seawater and if the seawater is pressurized as it is done in a reverse osmosis unit . For this reason a careful selection of all materials being in contact with seawater was made . In our special case we decided on plastic materials (e .g. C-PVC, polyethylene and fiber reinforced polyester) in the low pressure section of the plant and on stainless steel for the high pressure section of the plant (except the RO pressure vessels) . As stainless steel we chose material with the indication 1 .4439 = AISI 317 LN for all those high pressure pipes and vessels in which water flows permanently during operation and which may be rinsed with permeate before shut-down . All other stainless steel parts have the indication 1 .4539 = Uddeholm 940 L. Only the well pumps are made of zinc-free marine bronze . A few special parts are performed as carbon steel with inner rubber lining. In connection with the question of material selection again I want to emphasize the importance of the absence of oxygen in the water . With the vacuum deaerator and the residual elimination of oxygen by dosing of sodium bisulfite we guarantee an oxygen-free water during the operation

194

W .HEYDEN

of the plant. By adding an excess amount of 500 ppm sodium bisulfite before shut-down we do the same during stand-by and standstill periods . This measure has permitted us certain concessions when selecting the materials .

PERFORMANCE DATA, OPERATION AND MAINTENANCE Fig . 4 gives some idea about the main parameters measured during 1983 (temperature and TDS in the raw water and conductivity in the product water) . Water analyses made during mid-June and mid-September 1983 are given in Tables I and II . Until today (October 1984), more than one and a half years after the start-up of the first two RO lines, product quantity and quality has always met the requirements of Aramco's specification . Intermediate deteriorations of product quality due to a short-time oxygen passage and/or an anaerobic fouling (growth of sulfate reducing bacteria), could always be removed by adequate cleaning procedures or application of special membrane treatment agents. q 100

90

80

70

60

CDNDUCTIVITYIx ORS ml

50

TD51+103 ppmI 40

30 TEMPERATURE ['0

20

10

i

C C

T

E

4

N

Fig . 4 . Changes of main parameters during the year 1983 .



SEAWATER DESALINATION BY RO - TANAJIB TABLE I WATER ANALYSES IN MID-JUNE 1983 Feed Temp . ( °C) pH

C1S0 2NO; HCO; cal . Mg" K' Nat Si0 2 CO, TDS

Brine

25 .00 7 .60

25 .00 7 .07

Product 25 .00 6 .01

PPM

PPM

PPm

22,588 .00 3,305 .00 6 .30 140 .30 441 .00 1,516 .00 425 .00 12,649 .05 10 .00 5 .48 41,080 .66

37,589 .02 5,563 .57 10 .34 204 .12 734 .62 2,525 .38 705 .44 21,060 .95 16 .58 17 .61 68,410 .02

182 .00 13 .22 0 .92 10 .79 1 .14 1 .36 16 .58 119 .62 0 .49 17 .61 346 .02

-0 .33

-0 .56

-4 .09

SDSI

TABLE II WATER ANALYSES IN MID-SEPTEMBER 1983 Feed Temp . ( °C) pH

Brine

31 .00 7 .50

31 .00 7 .00

Product 31 .00 5 .87

PPM

PPM

PPM

C1_ SO ; NO; HCO; Cal' Mg" K' Na' SiO 2 CO, TDS

24,060 .00 3,520 .00 10 .70 135 .00 469 .00 1,615 .00 452 .00 13,470 .08 10 .00 6 .67 43,741 .78

40,038 .60 5,921 .95 17 .56 194 .98 781 .27 2,690 .29 750 .26 22,427 .24 16.58 18 .93 72,838.72

262 .83 12 .94 1 .17 8 .27 1 .71 5 .88 13 .16 172 .29 0 .36 18 .93 478 .61

SDSI

-0 .43

-0.62

-4 .44

195

196

W .HEYDEN

Fig. 5 . (a) Gallery of seawater beach wells . (b) Cartridge filters. (c) Two ultrafiltration lines.



SEAWATER DESALINATION BY RO - TANAJIB

197

Fig . 5 . (d) Vacuum deaerator . (e) Line 1 of the reverse osmosis unit (in front on the lefthand side you can see the high-pressure multi-stage centrifugal pump with energy recovery turbine) . (f) Two Diesel generator containers (capacity of each generator : 818 kVA).



198

W .HEYDEN

TABLE III POWER CONSUMPTION FIGURES kWh/m' 1 . Shallow beach well pumps and OF pumps 2. Vacuum pump of deaerator and booster pumps 3 . High pressure pumps 4 . Pressure increasing pumps (pumping of drinking water to storage tanks) 5 . Brine discharge pumps 6 . Housing facilities (air-conditioning, light etc.)

%

1 .32

14 .4

0 .95 5 .93

10 .4 64 .9

0 .20 0 .46

2 .2 5 .0

0 .28

3 .0

Total power consumption

9 .14

100 .0

Specific fuel (Diesel) consumption (1/m')

2.4

TABLE IV CONSUMPTION PER 100,000 m' PRODUCT AND RESULTING SPECIFIC COSTS (Exchange rate : 1 .00 SR = 0.80 DM) DM/kg Chemicals Sodium metabisulfite Antiscalant Flocon 100 NaOH, 49% Citric acid HTH (calcium hypochlorite) H,SO„ 95% Ammonia, 25%

kg

(1 .36) : (8 .00) : (1 .08) : (5 .60) : (6 .00) : (0.98) : (2.00) :

DM/m' 7,700' 650 3,500 530 360 1,100 350

0 .105 0 .052 0 .038 0 .030 0 .022 0 .011 0 .007

DM/1

I

DM/m 3

(0.11) : (12.00) :

240,000 450

0.264 0.054

DM/p

p

DM/m'

Other consumables

Fuel (Diesel) Lubricants Spare parts Filter cartridges Other spare parts

(14 .00) :

Total specific costs for consumables (DM/m') :

600

0.084 0.200 0.867

*Excess amount because of frequent shut-downs beyond Preussag's control .

SEAWATER DESALINATION BY RO-TANAJIB

199

TABLE V OPERATION AND MAINTENANCE COSTS WITHOUT DEPRECIATION (Exchange rate : 1 .00 SR = 0 .80 DM) DM/m'

%

Chemicals Sodium metabisulfite Antiscalant Flocon 100 NaOH, 49% Citric acid HTH H,SO„ 95% Ammonia, 25%

0 .105 0 .052 0 .038 0 .030 0 .022 0 .011 0 .007

7 .05 3 .49 2 .55 2 .01 1 .48 0 .74 0 .47

Other consumables Fuel (Diesel) Lubricants

0.264 0.054

17 .72 3 .62

Filter cartridges Other spare parts

0.084 0.200

5 .64 13 .42

Labour Engineers (2/day) Helpers (3/day)

0 .456 0 .167

30 .60 11 .21

Total operation and maintenance costs

1 .490

100 .00

Spare parts

The Tanajib RO seawater desalination plant has produced approx . 800,000 m 3 until today . Operation and maintenance services provided by Preussag include : operation, repair, preventive maintenance, calibration, testing and monthly reports on plant performance (quality and quantity), all required consumables (chemicals, filters, oils, greases, etc .), fuel and power, maintenance of adequate access for water hauling and general housekeeping and maintenance . As already mentioned these comprehensive additional services are very strictly penalized, so it was in our interest to design a very safe and economically operating plant . The following consumption figures (see Tables III and IV) and operation and maintenance costs (see Table V) emphasize this . The excellent experiences until today with the first temporary Tanajib seawater desalination plant allow me to conclude that the RO technology can hopefully avoid major setbacks and may face the competition with the distillation processes with confidence .