Reverse osmosis in the desalination of brackish water and sea water

Reverse osmosis in the desalination of brackish water and sea water

Desalination, 36 (1981) 153-178 © Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands REVERSE OSMOSIS IN THE DESALINATIO...

1MB Sizes 1 Downloads 41 Views



Desalination, 36 (1981) 153-178 © Elsevier Scientific Publishing Company, Amsterdam -Printed in The Netherlands

REVERSE OSMOSIS IN THE DESALINATION OF BRACKISH WATER AND SEA WATER

H . LUDWIG Hager and Elsosser GmbH, 7000 Stuttgart-80 (West Germany) (Received February 2, 1980 ; in revised form August 15, 1980)

SUMMARY To make RO an economical process, plant and operating costs have been reduced by developing compact modules, led by polyamide hollow fibre modules which recover energy from the reject when desalinating highly saline brackish water and seawater. Such plants only start to pay their way at product rates of around 400 m 5 /d + . Energy costs for desalination systems with lower capacities can also be reduced considerably by operating the RO plants at conversion rates of more than 3085 . To what extent the required higher conversion rates can be obtained for a particular application depends on concentration of scale-forming content matter (calcium sulphate, strontium sulphate, barium sulphate and calcium carbonate), salt content of the raw water, concentration of fouling substances such as iron, manganese, inorganic and organic matter, risk of biological growth due to algae, shells and bacteria . The scope and type of conditioning and pre-treatment are based on these factors .

SYMBOLS Ca CF CI I~, n

PCF

-

TDS-NaCl equivalent at standard conditions (mg/1) TDS-NaCI equivalent of feed (mg/1) colloid index initial membrane permeability required number of stages . at a given recovery rate y of a module unit - productivity correction factor

0011-9164/81/0000.0000/$02 .50 @ 1981 Elsevier Scientific Publishing Company

H. LUDWIG

154 PF

- feed pressure (bar)

PFRO - feed pressure - '

module pressure drop at standard conditions

(bar) PFR - Pp - (MFR/2) (bar) APFR - module pressure drop (bar)

P,,

- product back pressure at operational conditions (bar) - product back pressure at standard conditions (bar) Q~ - operational product flux (m3 /h) Q;,, - initial product flux (m3 /h) to - initial time (s) to obtain liquid volume V t, - time (s) at end of test to obtain liquid volume V T - total test time (min .) Y - conversion rate of a module unit lior - total conversion rate 7rFRO - osmotic pressure of feed-reject stream at standard conditions (bar) 7rFR - average feed-reject osmotic pressure (bar) . - osmotic pressure of product at standard conditions (bar) ., 7r 7rn - osmotic pressure of product at operational conditions (bar) p7r - osmotic pressure differential (bar)

Pp

INTRODUCTION

Development over the past few years has shown that the technology of reverse osmosis for recovering fresh water from seawater and brackish water can no longer merely be considered a potential and, under certain favourable circ umstances, an economical process of desalination . Operating experience with various plants of differing sizes has proven that the membrane technique can be a real alternative to evaporation process . The reduction in plant and operating costs required to make RO economical has been achieved particularly by the development of compact modules . Fig. 1 shows a polyamide compact module used in seawater desalination containing a hollow fiber bundle and Fig . 2 shows a cross section through a polyamide hollow fiber skin located in the bundle . Table I shows the various types of polyamide hollow fiber modules . The high-pressure modules HEPH, which are suitable for desalination of brackish and sea water, operate at working pressures from 56 to 70 bar (depending on operating temperature) . RO plants used for desalinating seawater are normally run nowadays at conversion rates of between 15% and 30%. Operating pressures of 60 to 70 bars are required for achieving a sufficiently large flow of product at this particular rate of conversion . However, depending on the type of membrane and the design of the desalination units, a considerable share of the energy supplied is used up to attain a variably high flow through the modules .



DESALINATION OF BRACKISK AND SEA WATER

Locking ring

156

Intermediate .Open fibre tissue ends~ Fibre F 1 '-,,, .

Reject outlet

Epoxy product

head

Porous disc Locking ring

0-ring gasket' 4 Feed

(r

/

Prods ict

/ O-ring gasket

Shell

End plate

. .1

Fibres

End plate

Porous feed distribution table

Fig. 1-

Polyamide compact module.

Diameter A : Type HEPN for fresh water = 85p Type HEPH for brackish and seawater

= 1O0µ

A I li =micron =0,00004 inch =41100000 inch 42 s = 0 .0016 inch 85p = 0,0033 inch I OOM

Fig. 2 .

Section

through

a polyamide

hollow

= 0,004 inch

fibre.

TABLE I OPERATING DATA FOR POLYAMIDE HOLLOW FIBRE N 10 Normal output (m1/d) at 25`C, 1 day 3.74 mg/i NaCl Conrerson rate (%) Salt passage (%) Allowable operating prmurm P. as (bar) Diameter (rum) Length (mm) pH and range Counter pressure permeate mss . (bar)

140 400 2,0

N21

N42

7 .9

15 .8

B9 N 140

N 155

52 .8 58 .7 1500 25-90 -10

N 250

64 .3

94,6

0 .95

6.2 18 .9 30,000 10-50 ^-1,5

310 1350

56 63 70 140 500

35 30 25 140 1190 5-9

28 133 670

133 1200

240 1250 4-11

240 1250

3,5

Prerdter fmeneaa : 51t colloid index 'C 3, temperature range 0-35°C Freechlorinemas.,mg/l : ph<8-0 .1 ;pH>9-0 .25 Hollow fiber membrane : polyamide. shell: filament-wound ffiberglan epoxy, endplates and segmented ring%: fiberglass epoxy operating potion horizontal or yerticxl .

240 1250

11 03

11 16

B10 H 50

N 170

3 .5

H 55 20 .8

at maz. temp . °C 264 264 1500 1500



156

H . LUDWIG

In 1969 research was conducted by the US Department of the Interior, Office of Saline Water, with the aim of partially recovering dissipated energy in the reject stream of a desalination plant [1] . This analysis on the usage of mechanical energy recovering systems has shown that when applying these systems, recovery rates of between 5% and 25% are possible, at a conversion rate of 40% (Fig. 3) . The use of hydraulic turbines, which recover energy either via direct mechanical coupling to the high pressure pumps or via the production of electricity, is however only of interest according to this investigation for capacities of approx . 106 gpd (380 m3/d) and above ; the most favourable range of application of such systems on the other hand begins at around a product capacity of approx . 106 gpd (3800 m3/d) . By increasing the conversion rate, the energy consumption based on the product capacity can be very effectively reduced even on plants with a low capacity (Fig . 4)A seawater desalination plant working at an operational pressure of 70 bar, consumes approximately 13 kWh/m3 of product water at a conversion rate

100 95

la_

Conversion rate = 40% = constant Operational pressure = 100 bar = constant

i I

90 I-` a 85 E e0 U n 75 V U C 70 O d 65 60 55

U

1 . Energy consumption without recovery 2 With recovery by means of a turbine coupled mechanically with a high pressure pump . 3 . Energy consumption with recovery by means of a turbine and generator

I I t 1000 15002000 300040005000 10000 20000 30000 500 Product capacity [m' /d] Fig . 3 . Energy recovery during seawater desalination . Possible recovery rate in dependence of the product capacity . 50



1 57

DESALINATION OF BRACKISH AND SEA WATER

of 20% . If the rate of conversion is increased to 40%, the energy requirement drops to approximately 6 .5 kWh/m 3, i.e . by 50% . Moreover the necessary feed flow of RO decreases by the same rate, with corresponding consequences on the capacity of the pre-treatment and the associated chemical and operational costs. Whether a certain type of seawater, with such a high-upgrading of the salt content, can be desalinated using a membrane process, depends on various pre-conditions . In order to prevent fouling and maintain the product output and the salt rejection of a RO plant in long term operation, depending on the composition of the raw water, a more or less extensive pre-cleaning and pre-treatment of the water is required to remove suspended and undissolved inorganic and organic matter, iron, manganese, etc .

Operational temperature =20° C Seawater salinity = 30 g/kg Degree of efficiency pump= 75%

18.00

2.50 2 .40

17.00

2 .30

1600-

2.20

15p0`

-2 .10

14 .00 ?~ L

E

1 00

Y

~`

m U3

Y

1 .90 Y

1200 Ia

. 200

\

Operating 1 .80 ~ P~ \ 28 bar operating pre-re 56 bar Operating pressure V% 1 .70 s m

70 bar .0011

.00-zoo-6600-S00_ .00-8 1Q0()-9

` -1

1 .60

``

1L

1 .50

~~

1 .40 1 .30 1 .20 1 .10

1

20

30

40

50

60

1

1

70

80

1 .00 90

Conversion rate [%] Fig. 4. Energy consumption in terms of product Np when desalting fresh water and seawater.



158

H . LUDWI(

PRE-CLEANING The risk estimation of colloidal fouling is an important factor for the assessment of the required pre-cleaning measures . Colloidal fouling is causec by entrapment of inorganic and organic colloids in the raw water on the membrane surface, being destabilized during the RO process and result in decrease of salt rejection and permeate flux . The hazard of colloidal foulinl may be ascertained by calculating the colloid index (CI) of the water to be treated as follows : - (tA / tt)X100 (1 Cl = T where T = Total test time in minutes (usually 15) t 4 = initial time (s) to obtain liquid volume V, and t t = time (s), at em of test to obtain same liquid volume V. This measuring method applies 0 .45 micron membrane filters, filtering a defined raw water quantity at a certain pressure for a certain test period [2] . Increasing the product conversion means increasing the fouling effect . The zeta potential which is useful to determining the colloid stability in raw waters with a TDS of 1500 mg/I, may not be applied anymore for the evaluation of this factor for brackish water and sea water . The CI varies, as a subject to the sea water and well water sources . For example, at take off of surface water it may shift depending on the distance of the water sources from the shore, from 10 to not measurable higher values . Water drawn from shore wells, or in great distances from the shore, were found to be of the value of 1 . Table H illustrates the reduction of CI which can be obtained by the various pre-cleaning processes, furthermore the application range of these different processes is shown where optimal results have been reached . Filtration, using sand or multilayer filters, is an effective method to reduce the CI to smaller and equal 3 of surface waters (lakes, rivers, or reservoirs) and sea water, by having an in-line coagulation upstream of the filter unit, even without the application of flocculants . This CI is recommendable for the troublefree operation of a RO plant with adequate short flushing intervals . As with surface waters in general, pre-cleaning may be simplified essentially, if the raw water will not be drawn off directly, but from a shore drilled well . In that case, to obtain the above mentioned CI, the sole filtration may be sufficient or the in-line coagulationHydroanthrazite and sand are some of the filter media, used in coagulation filtration with multi-layer filters, applying iron salts and alum salts as flocculants . By application of direct filtration of more heavy polluted surface waters or well waters, this process accomplishes an insufficient treatment effect . Here, the in-line coagulation, using special filter material, with a larger and more active surface and particular polyelectrolytes, produces a

DESALINATION OF BRACKISH AND SEA WA'r1slt

159

TABLE U REDUCTION OF THE COLLOID INDEX USING VARIOUS PRECLEANING METHODS (151 Process

Gravel filtration, Multilayer filtration Contact sludge reactor and filtration Coagulation filtration (Multilayer filter and flocculants) Inline-coagulation (Servofilt S, Servofloc A) Precoat filtration

Range of Application at Ci

Reduction of Cl (%)

< 100-200

x 50 mas

< 50-100

> 95 max

< 50

> 90 nrax

< 50 < 6

> 95 max > 95 max

better effect, reacting even in the colloidal area . With heavy polluted surface waters, flocculation and sedimentation, for instance, the employment of a contact sludge reactor, will be unavoidable, independently of the chosen RO system [3] .

PRE-TREATMENT Pre-treatment approaches will prevent scaling . The most important scale forming agents to be encountered in the water treatment application, are calcium hydrogen carbonate, calcium sulfate, and silica, but other compounds, such as calcium fluoride, barium and strontium sulfate in sea waters also can be limiting factors for the rate of product recovery . Scale-forming substances (calcium sulphate, strontium sulphate, barium sulphate and calcium carbonate) can be stabilised, if necessary either by adding suitable stabilising agents (hexametaphosphate, polycarboxylic acids, organo phosphates), can be removed using some form of treatment or converted to easily soluble compounds (acid dosing for removing calcium carbonate) . The solubility of the scale-forming agents depends on the ionic strength of the seawater [4, 5, 6] . Calcium sulphate can be kept in solution by dosing special phosphate, independent of the existing ionic strength, up to a solubility product of



H . LUDWIG

160 10

10

mg/I

Special phosphate

1e4

0 .01

10

01 Ionic

strength

I

Fig. 5 . Dependence of CaSO4 solubility product Ksp on the ionic strength . 3

around 10" (Fig . 5) . It also has the same solubility, but without stabilisation, at an ionic strength of 0 .7 . However seawater with a TDS of 35,000 mg/kg, an average salinity, already has an ionic strength of 0 .72 . In an RO plant, operated at a recovery rate of 45%, the reject has an ionic strength of 1 .23, so that under these conditions and with seawater with an even higher salt content, calcium sulphate is still soluble and stabilisation is not necessary . Only with water from inland seas containing a lower salt content, can it be indispensable to carry out conditioning using hardness stabilisers to attain a conversion rate of more than 30% (Table III) .

TABLE III TDS-NaCI EQUIVALENTS, IONIC STRENGTHS, OSMOTIC PRESSURES AND CONVERSION RATE POSSIBLE DUE TO SCALING BEHAVIOUR OF VARIOUS TYPE OF SEAWATER

TDS mg/kg

TDS-NnCI equivalent mg/kg

Raw water Mal product Ionic CaSO, strength .10 ° mot e /kg

Osrn. press . bars

Puss . Y %

Osm . P . . at pass. Y ban

Reject Ionic Osm . P . . strength at Y- 45% at puss. Y bars

13 .600 35 .100 41,800

11 .100 32.100 38,400

34 29 47

8 9 262 31 .5

4065 58

15 .0 76.6 78.7

0 .50 1 96 1 .69

• Ar CaSO, stabilization

Y = Conversion rate

0 31 1 .72 0 87

16 .1 49 .5 588

Ionic strength at V - 45% 0 .52 1 .23 1 .47



DESALINATION OF BRACKISH AND SEA WATER

161

In order to avoid precipitations by calcium carbonate, empiric acidification may be done . With brackish water treatment the saturation index in concentrate of the RO can be calculated according to Iangelier--Strohecker . Fig. 6 illustrates the flow scheme of a quite extensive pre-treatment unit part of a sea water desalination plant . In the first pre-treatment stage, polyelectrolyte will be added upstream multi layer filters, while at high bacteria count, also chlorine dosing will be effected upstream this filter . The second pretreatment stage consists of an in-line coagulation with the following dosing equipment for sodium bisulfite shock dosing for the sterilization of the RO plant . As an alternative, UV sterilization may be applied . If chlorine dosing is required for sterilization, continuous dosing of bisulfite for dechlorination will be effected at this point . Another dechlorination technique used, may be an activated carbon filter, which is outlined with a dotted line . For treatment of well water, generally, a mechanical pre-treatment stage will suffice (multi-layer filter or in-line coagulation), together with a cartridge filter, installed upstream of the RO unit . For small RO systems (output 1 to 15 m 3/d), installed in large numbers on ships, to provide potable water, it is also dispensed with extensive pre-treatment measures, and merely cartridge filters are used . However, this equipment will only be operated on open sea, at a distance of 3 to 10 miles from the coast, where the sea water contamination is sufficiently low and a conversion of 10% may be attained . These units are operated without the acidification or stabilization approach, using only chemical reagents for module cleaning [7) . Fouling involves the trapping of some type of material within the RO device itself, or on the surface of the membrane, even if pre-cleaning and pre-treatment have properly been carried out . Adequate pre-treatment of the RO feed is characterised by the fact, that by rinsing scaling and fouling can be eliminated reversibly and that the intervals between the individual rinsing processes are long enough . Such flushing processes are indispensable to maintain stable long term performance. Therefore, during start-up of the plant or during the first months of operation, it is recommendable to determine the type of the flushing solution and the frequency of the flushing processes, in consideration of the foulants. Biological growth in the pre-treatment and desalination stages which is a disturbance factor particularly when desalinating water from tropical seas, can be kept in bounds by chlorine dosing and shock treatment of the membranes using sodium . bisulphite, also used for removing chlorine from the water before it enters the desalination stage . Usually a shock treatment in the feed supply, using sodium bisulfite will suffice, treating the feed in 12 hour intervals for 30 minutes with 500 mg/1 sodium bisulfite . The increasing application of noncellulose acetate membranes, such as polyamide hollow fibre and composite membrane modules, which are more

5

b

w 0 p

a

,$

0

O

na

M

ro M

p

Surface Source



P\

Well Source

Chlorine

Acid

Polyelectrolyfe

Inline Coagulation

O (3)

I

Acid

~~

7

Sodium %--/ 8lsulflte

Ultra fine Cartridge Filter I SN

(D ./orb . Cl, . QNa/ISO Dosing or Acfivaled Carbon

Surface, many germs

Shock

( /or

Surface, tew germs

0 . NaHSO,

NoHSO, Dosing or Activated Carbon

Q •C ly . () Q) or

(D or (D . MoHSO, Shock

Well, many germs

Pretreament Modus

Well, few germs

Raw Water Soucre

cJ

Dechlarinutkx, Arbvrtled PmLun Filter or M l1cf1j Shuck Treatment

Prerleaning Measures at 'different Raw Water Conditions

Mulb Layer riper

(I)

1-

Osmosis

f Content rote Per meat

Hover cc

N

m



1, Feed lank 2. Rinsing tank 3 . High pressure pump 4 . Fine filter feed 6. Rinsing 6, Heat exchanger + cooler



7. 8. 9. 10 . 11 . 12 .

I

i 1

7

7

Individual module Module group Separation stage Reject drain control valve Rinsing pump Preparation tank for conditioning chemicals

L-- .L- _L_J

I

I

I

I

PS

7

7

A

14

Product

P Return rinsing device open during . rinslng

to

Reject



164

H. LUDWIG

sensitive to chlorine than cellulose acetate, necessitates the search for other possibilities of sterilisation, such as UV radiation with extremely high radiation intensity [9] . The salt content as well as the maximum allowable operating pressure of a module system also set limits on the attainable conversion rate . At a salt content of 35,000 mgfkg and a conversion rate of 45%, an osmotic pressure of 49 bars is obtained in the reject . At a salt content of approximately 42,000 mg/l (which is likely to occur as far as the Persian Gulf is concerned), the osmotic pressure in the reject already has a value of about 60 bar . The optimal arrangement of RO compact modules in the plant is another precondition for a stable long term performance . The higher the product conversion expected, the more stages are required . Normally, a RO plant for fresh water and brackish water treatment, using compact modules, will consist of at the most 3 stages (Fig . 7) . For sea water desalination, due to a much lower product conversion, the modules will not be staged but run in parallel operation, thus allowing a quite simple plant construction (Fig . 8) . The minimum allowable reject flux and ratio of product to reject, attained with a certain module system, determines the type of arrangement of the desalination units in the plant (Table IV) . TABLE IV PRODUCT REJECT RATIO AND MAXIMUM ALLOWABLE CONVERSION RATE FOR VARIOUS RO MODULE SYSTEMS IN USE FOR SEAWATER DESALINATION Module system

Membrane material

Product/ reject

Y max. per unit (%)

Du punt hollow fibre bundle Perinasep B-10 Spiral wound module Tubular module

Polyamide

1 :1 .1 to 1 :2 1 :4 to 1 :6 1:25 to 1 :40

35-50

CA or composite membranes Cellulose acetate

15-20 2.5-5

If a tubular module has a product reject ratio of 1 :25 up to 1 :40, this decreases with increased compactness of the module from 1 :4 up to 1 :6 with the spiral-wound module to 1 :1 .1 to 1 :2 with the polyamide hollow fibre module. The maximum allowable conversion rate of an individual desalination unit can be calculated from the product reject ratio, which is around 2.5 to 50%o with the stated module systems [10,11 1 . In order to increase the total conversion rate of the desalination unit, reject staging is used . The required number of stages at the stated maximum conversion rate of an indi-

0

Conditioning

High Pressure Compact Module 2 Permeat Rinsing and Intermediate Tank 3 Rinsing Tank I

'4

O

.4

8 Cooler 9 Concentrate Outlet Valve and Control Valve 10 Product Standstill Rinsing

Run back to Rinsing Device open at Rinsing Action

Osmosis

4 Dosing Device for Conditioning Agent 5 High Pressure Pump 6 Rinsing Pump 7 Fine Filter 5-10/1

Reverse

Ui

ftj

a

a



x. LUDWIG

166

vidual desalination unit and the total conversion rate of the plant can be roughly estimated using the following equation : Assuming

Yt = Y2 = Yn then Y = 100 [1-(Yat /100) 1 1"] and n = log[1-(Y ot /100)]/log[1-(Y/100)] . If a total conversion rate of 25% using a tubular module with a maximum recovery rate of 5% per unit is to be attained, 5 reject stagings are necessary for this (Fig. 9) . Using the spiral wound module, a conversion rate of 35% with 2 to 3 reject stages can be obtained, a recovery rate of 45% with 3 to 4 stages . In a multi-stage reject staging (Fig . 7), the individual modules are connected within module groups in reject staging and these groups of modules are then 90

a ao 7 70

1-5 =number of stages

5 30

20 I AO 5 1

1

20

25

0

35

Ystage (%) Fig. 9 . Requires, number of stages N for reject staging at given recovery rate of module unit state and of total plant ( Ytot)•



DESALINATION OF BRACKISH AND SEA WATER

167

in turn arranged in the same type of staging again in series. It is thus possible to obtain relatively high conversion rates even with modules with a high product reject ratio . As the polyamide hollow fibre module has an allowable maximum conversion rate of 3595 to 5090 per single unit, high rates of conversion can be attained with this module in parallel arrangement without using staging(Fig .8) . Of all the commercially available module systems, the spiral-wound module is the most similar to the polyamide hollow fibre module, taken from the point of view of possible conversion rate and also compactness (Fig. 10), [11] . With the illustrated type of module an individual unit is charged with raw water through lateral openings in the shell . Product and reject are led off at the top . In the spiralwound module, the desalination units are inserted in a pressure vessel and thus interconnected, so that within the resulting module, which may contain up to 6 units, multi-stage reject staging can be mounted. A desalination unit corresponds in length and diameter approximately to a polyamide hollow fibre module . A module containing 4 to 6 units of this

Gasket

Reject

a

Product Module Materials

Product collector RO membrane Flow diagram Product collecting line

Feed

Product Fig_ 10 . Spiral-wound module I



168

H . LUDWIG

Fig- 11- Spiral-wound-module H .

system is equivalent in capacity and rate of recovery virtually to that of a hollow fibre desalination permeator (Fig . 11), [101 . One reason why a certain minimum reject flux rate in a module should be maintained, may be the increase in concentration polarisation . A further factor limiting the maximum conversion rate can be an increase in the danger of fouling if the reject limit value drops below the set standard . R is therefore often possible with raw water which has been very well pretreated or contains very few contaminants to let the minimum allowable reject flow drop off to a certain degree, without detrimental effect on the operation of the desalination plant . This applies, in particular, to module systems with membranes for low specific product capacity, as for example the polyamide hollow fibre membrane, whose influence of concentration polarisation on membrane behaviour is very low . In order to obtain high rates of plant recovery even with low allowable recovery of individual units, reject return is often applied, i .e . a partial stream of the reject is branched off and led back into the inlet of the stage (Fig . 12). The minimum allowable reject flow is maintained by this internal re-cycling. This mode of operation however has a great effect on the product flux and the treatment effect of a desalination plant, particularly with raw water with a high salt content . The drop in capacity of the modules due to alternations in the salt contents of feed and reject (reject return and increase of conversion rate) can be calculated using the known equation for product flus



169

DESALINATION OF BRACKISH AND SEA WATER

Saline solution 66°T. aa'x '~~~7. - Dosing Anti ~if required Zs 78 262ti. Pressure gauge 1 Sac -pressure valve 0 Flowmeter Reject collecting tank

pressure G High pump Circulation pump Fig. 12 . Multi-stage for high salt concentrations . (16)

through RO membranes : PCF= K (PF -APFR/2-Pp - Aa) A7r = vFR - vp (1) Qop - QM X PCF Reject return increases the salt content in the feed to the module according to the ratio raw water inlet and reject return . Although the salt concentration in the reject remains constant, the mixture of feed and reject salt content and thus the mean osmotic pressure feed-reject resulting from this combination are increased and the driving force for the product flux decreases . The differential pressure which is formed in the individual stages has a similar effect, i .e . with an increasing number of reject stages and constant operational pressure, the product flux is reduced . Salt rejection and salt pacSgge is affected by the operational _pressure and alternations in the salt content in the feed and reject . Deviation from the basic values, which apply to standard conditions can be calculated using a correction factor, by which the basic salt paste of the individual ions is multiplied [12] .

(;p

SPCF r

vFR

PPR9 -pp .

(2) CF

PFR - Pp - vFR

vp

vFR e



170

H . LUDWIG

This equation is determined by the quotients of a) standard salt content to the effective salt content, b) standard pressure to the effective operational pressure, c) effective osmotic pressure to the osmotic pressure under normal conditions . By re-cycling the reject, both the feed salt concentration and the effective mean osmotic pressure in the feed reject is increased, whereby the salt passage is automatically increased as well . Re-cycling the reject in order to achieve high rates of conversion in seawater desalination is a technique which in the region of high conversion rates and salt concentrations, should only be applied, providing an exact knowledge of membrane behaviour is available and only then, if the allowable salt content in the product may be relatively high or can be reduced by a postdesalination stage . The polyamide hollo_.w fibre module has under normal conditions (56 bars operational pressure, 25° C, 30,000 mg/l NaCl, rate of conversion 30%) a salt passage of 1 .5% 110] . Composite spiralwound modules and cellulose acetate flat membranes have similar passages of salt . As the equation for calculating the salt passage correction factor illustrates, the effect of the variation in salt passage of desalination membranes, due to the alternations in the conversion rate and the associated alternation in the mean osmotic pressure feed-reject, diminishes with increasing operational pressure of the module system . If a salt passage of 1 .5% is calculated for seawater with a TDS-NaCl equivalent of 30,000 mg/I at a conversion rate of 30% and an operational pressure of 56 bars, the salt passage decreases by increasing the operational pressure to 70 bars to approximately 1% . Thus a salt content in the product, even with reject recycling for seawater of normal salinity is obtained, virtually corresponding to the WHO guidelines for drinking water . A high operational pressure has an even more favourable effect on seawater with a TDS-NaC1 equivalent of 40,000 mg/l . In this instance an operational pressure of 70 bars is absolutely pre-conditional for attaining conversion rates exceeding 35%. A salt passage, corresponding to normal conditions of 1 .5%, is obtained in this case at a conversion rate of 35% . Increasing the conversion rate to 40% causes the salt passage to increase to approximately 2% . Because of the high feed salt content however, a post-desalination stage is required in order to meet WHO guidelines for drinking water . Energy consumption of a desalination plant is, under normal conditions (56 bars, 30,000 mg/I NaCl, 30% conversion rate) around 7 kWh/m 3 product water, assuming a degree of efficiency of 75% for the high pressure pumps (Fig. 4) . If the conversion rate with the same salt content is increased to 40% at an operational pressure of 70 bars, the energy consumption in terms of product water of the desalination stage decreases to approximately 6 .5 kWh/ M3, i .e . a reduction of approximately 7% . If it would be possible to increage



171

DESALINATION OF BRACKISH AND SEA WATER

the conversion rate further to 50% by re-cycling the reject, the energy consumption of the high pressure feed pump could be reduced to approximately 5 .5 kWh/m3. However a part of the conserved energy had to be used by re-cycling the reject, either by operating a separate circulation pump or by increasing the capacity of the feed pump . At the same time, an increase in the salt passage of approximately 30% would have to be tolerated, because reject return increases the salt content in the feed to the module according to the ratio raw water inlet and reject return . In order to obtain the same product salt content in a desalination stage, run at a high rate of conversion and at a high operational pressure, as when operating at a low conversion rate, a further desalination stage can be installed in product staging downstream of the main desalination unit, run at a conversion rate of up to 90% . The energy consumption of this low pressure stage is then around 1.2 kWh/m3 . Under favourable conditions, the energy consumption of this product-staged desalination unit can on the whole be less than that of a system with a low conversion rate in parallel arrangement or in reject staging . The design and planning of the operating conditions for plants used to desalinate seawater with an extremely variable composition and temperatures subject to seasonal fluctuations, demand an exact knowledge of membrane behaviour and a considerable amount of calculation . Fig. 13 shows to what degree the salt content, the turbidity and how the temperature in tropical seas can alter in the course of the seasons . The temperature serves not only as a parameter influencing the product capacity during momentary operation of the plant (according to the following equations) but also in combination with the operational pressure as a criterion for long-term behaviour of the membrane (Fig . 14) .

'Snv0

Vt TDS-NaCI l5 equivalent

rn tOal N m m o I

U'0 m RO"n WY i0 -i E F70 1] 2s0a0 -lo

,

iy`f ~ r NOV ~, r~ r , , Temperature i

l

Turbidity

11 . . .1 a , .I Dec. Sept. Jan Dec . Sept . Jan . 1977 1978 1977 1978 Fig. 1.3- Seasonal fluctuations of TDS-NaC1 equivalent, turbidity and . temperature of seawater .



H. LUDWIG

172

Operational pressure = 56 bar = constant • =25° C O =35° C

0 .75 030 0 .65 1

0-60

1

3 Operating time (years) 2 pressure polyamide hollow Fig . 14 . Pressure/temperature long term behaviour of high . fibre membranes for seawater desalination 0

1

Temperature dependence of product flow of the DU PONT polyamide hollow fibre module is given by Lpt 25

= 1 .03

1- zs

Lo where Lp, = product flow at t° C, P L, 25 = pruduct flow at 25° C and t = operational temperatureOften it is not possible to keep the march of temperature constant at feasible costs, by cooling the raw water • before it enters the desalination plant, due to the high air temperature and water vapour saturation in tropical areas . The plant must be designed in such a manner that despite seasonal high temperatures, it still operates at full capacity after several years of operation and that the feed and reject flow of the modules are not altered by fluctuations in temperature so that operating states can result which alter capacity and salt passage of the plant in an unforeseen way . The demands for full capacity in long-term operation and constancy of feed and reject flow conditions to the modules are contrarily at times in opposition to one another. A high water temperature requires corresponding reserves of capacity, i .e .



DESALINATION OF BRACKISH AND SEA WATER

173

additional membrane surface for maintaining product capacity, even after many years of operation . However a large number of modules causes a drop in the minimum reject flow below the set standard if the raw water temperature decreases and therefore product flow is reduced . In order that not too high capacity reserves for long-term behaviour of a plant are installed and thus have a detrimental effect on the momentary operational behaviour at low temperatures, it is necessary to be very familiar with the seasonal fluctuations in the temperature of the seawater and thus be in a position to determine the approximate long-term temperature load on the membranes with seawater subject to extreme fluctuations in temperature and in particular to maximum temperatures, which virtually reach the maximum allowable operational temperature of the RO modules . Optimisation of a desalination plant with such varying raw water conditions entails a great deal of work and can in an economical way only be done using data programming . The calculation of course, becomes all the more extensive, depending on how many reject and product stages are interstaged, affecting the product capacity factor and the salt passage factor by the change in differential pressures in the stages, by re-cycling reject for the post-desalination stage etc . By varying the operational pressure, fluctuations in the raw water temperature in a correspondingly optimised plant can be compensated to such a degree that fluctuations of the product capacity scarcely occur . However, an alternation in the salt passage is always associated with this, which, especially with seawater having a high salt content, can cause a certain salt content limit in the product water to be exceeded at high raw water temperatures. These fluctuations can be balanced out again by installing a postdesalination .

ECONOMIC CONSIDERATIONS

As Fig. 15 indicates, energy consumption of RO systems for seawater desalination even for high salinity seawater is lower than for evaporation processes . Even in areas where energy costs (fuel or electricity) are low, a detailed analysis may show RO desalination processes to be advantageous, or at least competitive with evaporation systems [13] . Energy costs are only one component of the total operating costs . The operating costs of a Saudi Arabian RO seawater desalination system with an output of 285 m 3/d, are subdivided in Table V . The total operating costs, including energy, operation material and service as well as maintenance, however, without amortization, in the projected area amount to DM 1 .42/m 3 of product . The portion of operating costs for cartridge filter replacement is quite high in this case due to the simple rough filtration by the pre-cleaning stage . This cost factor could



H . LUDWIG

1 74 kWh m3

M91 t 1000

2000

4000

10000

20000

40000

Fig. 15 . Practical value of energy consumption dependent on salt concentration for electrodialysis (A), MSF evaporation (B), and Reverse osmosis (C) .

TABLE V OPERATING COSTS OF A REVERSE OSMOSIS SEAWATER DESALINATION PLANT Plant capacity 285 m3/d = 12 m3/h, seawater salt content 43,000 mg/I, water temperature 15-35° C, operating pressure 67 bar, conversion 25%, permeate salinity < 500 mg/I Costs

Consumption of chemicals

Basic costs

Costs (DM/m 3 )

Energy Filter cartridges Conditioning agent Rinsing agent Operation Maintenance Total operating costs

10kWh/m3 every 7 days 5 g/m3 Every 2 weeks 15 min ./d 80 h/week

DM 0 .08/kWh DM 575/exch .

0.80 0-33 0.10 0 .03 0 .01 0 .15

DM 75/rinsing DM 12 .50/h DM 12 .501h

1 .42

DESALINATION OF BRACKISH AND SEA WATER

175

be reduced by almost 50% for instance by additional application of an in-line coagulation filtration . Furthermore, it is to be considered that the feed salinity is 43,000 mg/l . The salinity of the seawater to be treated is already shown another factor affecting the energy costs of a RO system . For example the treatment cost (amortization included) with electricity at DM 0.10/kWh, operation pressure 56 bars and a conversion rate of 30%, 4000 m3 /d plant are estimated as follows (14) (Table VI) . TDS of feed (mg/I) 20,000 35,000 40,000

Treatment costs (DM/m 3) 1 .5 2 .2 2 .7

These plant efficiencies may be considerably improved if the operation pressure will be increased to 70 bar . The costs for seawater treatment, considering the same preconditions, then result in (Table VII) . TDS of feed (mg/1) 20,000 35,000 40,000

Treatment costs (DM/m 3) 1 .2 1 .6 2 .0

Based on the fact that it is possible to operate the HEPH compact modules at operating pressures of up to 70 bar, the efficiency of a sea water desalination plant can be substantially increased . Figs . 16-20 show RO plants of various capacities and types of construction .

Fig . 16. Feed and reject side of a multi-line RO plant . Capacity 3 times 25 m 3 /h with horizontal modules . [17] (Photo : Hajer & Elsasser)

176

H . LUDWIG

Fig . 17. Product side of a multi-line plant . Capacity 3 times 25 m 3 /h. [171 . (Photo : Hager & Elsasser)

Fig. 18. A small RO compact unit . Capacity 5 m 3 /h with vertical modules . (Photo : Hager & Elsasser)

With some of the compact module systems available nowadays, it might be possible, when operating at a high conversion rate and for seawater with a high salt content, using additional product staging to cut energy costs of RO even further. Its usage under the mentioned operating conditions does however necessitate particularly with extreme fluctuations in temperature and in the salt content of the raw water, a detailed knowledge of the effect of the upgraded salt concentration and the decreased reject flow on salt reject and product capacity in long-term operation .



DESALINATION OF BRACKISH AND SEA WATER

19

177

20

Figs . 19 and 20 . A 20 m 3 /h RO plant with vertical modules from various perspectives . (Photo : Hager & Elsasser) Fluctuations in the product salt content and in the product capacity can be considerably reduced when treating seawater with a high salt content and operating at high conversion rates, if, by boosting the operational pressure, the difference to the mean osmotic pressure of feed and reject can be increased . Moreover further development of existing module systems towards greater pressure durability in the field of maximum operational temperature, too, is a precondition, which can only be met by further basic research in the field of membrane manufacture and module construction .

REFERENCES 1 . Office of Saline Water, Res . Devel_ Progr_ Rept. No . 457, Aug_ 1969 . 2 . Du Pont, Engineering Design Manual No. 491 . 3 . Trompeter, Summers, 45th Annual Conference National Water Supply Improvement Association, Oklahoma City, July 11-15,1976 . 4 . R . Minturn, Office of Saline Water, Res . Devel . Progr . Rept . No. ORNL TM 4330, 1973 . 5 . L .B. Yeats, Office of Saline Water, Res . Devel . Progr. Rept . No . ORNL 4941, 1973 . 6 Du Pont, Engineering Design Manual, No . 410 . 7 . H . Lerat, Desalination, 19 (1977) 201-210 . 8 . P .O . Goodwyn, OWRT Report No . PB-262 207 .



178

H . LUDWIG

9 . N .W . Rosenblatt, Proc . Fifth Intern . Symp . on Fresh Water from the Sea, 4 (1976) 397-40810. Du Pont, Engineering Design Manual, No . 210-27311 . Toray, Technical Bulletin . 12. Du Pont, Engineering Design Manual, No . 510. 13 . J . Carmona and R . de Bussy, Comision Federal de Electricidad, Mexico City, June 1976 . 14 . Ranson R., Firmendruck Du Pont 1976 . 15 . Marquardt, Ludwig, Meerestechnik, 8 (1977), Nr . 5, J. 163-168 . 16 . Marquardt, Vom Wasser, 44 . Band 1975 . 17 . Marquardt, Waldbauer, EVT-Register 32, 1977, J . 32-38 . 18. Marquardt, DV6W Schriftenreihe Wasser, Band 206, 1980 Sei 12-1-12/50 . 19 . Marquardt, Meerestechnik Nr. 3 & 4 1980, VD1-Verlag Dusseldorf. 20 . Marquardt, EVT-Register 36/1979, Sei 73-80 .