Table Of ContentN u c l e ar
D e s a l i n a t i on
Joint Symposium held at the Institution of Civil Engineers, 1-7 Great George Street,
London SW1, on Monday, 30 October 1967, by the British Nuclear Energy Society
and the British Nuclear Forum
THE BRITISH NUCLEAR ENERGY SOCIETY • 1-7 GREAT GEORGE STREET • LONDON SWl
ISBN: 978-0-7277-4478-4
© The British Nuclear Energy Society, 1968
The British Nuclear Energy Society is not responsible
for statements or opinions in this publication
CONTENTS
Papers:
1. The need for desalination
by Dr R. G. Allen, BSc, PhD, FInstP
2. A survey of present and future desalination processes
by K. D. B. Johnson, MA, BSc, AMIChemE
3. The physics of desalination by multi-stage flash distillation
by Professor R. S. Silver, CBE, MA, DSc, MIMechE, FInstP
4. Single and dual purpose desalination plants
by Dr D. W. Clelland, BSc, PhD, ARTC, ERIC, AMIChemE
Economics: effect of water/power ratios and the role of dual process-
dual purpose plants
bv Dr S. Chambers, BSc, PhD, and Dr F. C. Wood, MSc, PhD, AMIChemE
Discussion :
Morning session
Afternoon session
Paper i
THE NEED FOR
DESALINATION
R. G. Allen, BSc, PhD, FInstP
Director, The Water Research Association,
Medmenham, Mar low, Bucks
Desalination is in the early stages of its development as a water resource. It is already making its contribution as
a straight addition to the water supply in cases where there is an absolute water deficiency, and it is now coming
into use in areas where further conventional water supplies would be expensive, as the most economical supplement
to existing systems. The world demand for desalination plant is thus expanding rapidly. The most successful
process for sea-water conversion remains multi-stage flash distillation, judged on both technical and economic
grounds, and this advantage is likely to be retained for some time. In the United Kingdom desalination as a base-
load resource is considered unlikely before 1990, but its earlier application for peak load operation is expected.
Water resources in the United Kingdom and in arid countries have prolonged dry seasons each year, and are subject to
The state of the water resources and water supplies in the extreme droughts. These regions are mainly:
United Kingdom has become a subject for public dis in the middle and west of the North American continent;
cussion within the last few years, and it is now generally in the temperate areas of South America;
understood that the problem here is one of organization
around the Mediterranean;
and investment, rather than of inventing some new tech
nique to overcome a state of natural deficiency. The pres in the Middle East;
ent demand for public supply is for an average of some in the whole northern half of Africa;
2500 million gallons a day, with about the same quantity in an enormous area of Asia and Russia;
directly abstracted by industry, and that still leaves ten
in a large part of South Africa; and
times that quantity which runs off the surface of England,
in most of Australia.
Wales and Scotland without being taken into use.1
2. However, the rainfall is not very conveniently distri In these areas the deficiency of water is such as to form the
buted in time and space. Most of it falls in Wales, north major barrier to progress, limiting the productivity of
west England and Scotland, which is not where most of the existing communities, and preventing the development of
people are. Most falls in the winter months, whilst the new ones.
highest demands are in the summer, and the amounts and 5. Desalination does not at present offer the key to this
distribution vary from year to year in an unpredictable way. situation, because for the greater part of these areas there
3. These effects are magnified by evaporation losses, is no easy access to the sea or to brackish waters, and be
which vary much less throughout the country than rainfall cause many communities are too poor to afford desalina
does and are typically between 16 and 20 inches a year. In tion. Many communities indeed cannot even afford simple
areas whose annual precipitation may lie between 50 and civil engineering works to make good use of the poor
100 inches, the loss of 16 inches by evaporation still leaves a natural water resources they have.
high proportion of the water for possible use. In the south 6. In areas of the world where water supplies from con
east, however, rainfall in some years is so low that evapora ventional sources are becoming depleted desalination can
tion may deprive underground strata from getting any win be expected to make a contribution in one of two ways,
ter replenishment. In Essex, for instance, the underground both depending upon easy access to the sea or to large
component of the flow in some rivers may fail during natural resources of brackish waters to provide the source
drought periods, leaving only the flow of treated effluents material and for the disposal of the effluent from the pro
from sewage works and industrial discharges. cess. Firstly, where the existing resources are virtually all
4. Average annual rainfalls considerably smaller than used up already further increases in demand could be met
those in south-east England are found over more than half by straight additions in the form of desalinated water, pro
the land surface of the earth, and many of the arid regions vided the cost is accepted by the community. Secondly, a
1
more sophisticated approach is needed when there is water 10. The lines of approach which can be used in the United
available, perhaps varying in quantity and with time, where Kingdom are principally the following:
supplies from a desalination plant can be incorporated as (i) treatment of sewage and industrial effluents to a higher
a component of the total water resource system. standard, so that rivers that are at present too contaminated
7. Water needs to be thought of not only as a specific for water supply by conventional treatment processes be
commodity having value as a domestic or industrial piped come usable, and so that in other rivers the use and re-use
supply, but of course in relation to its many other important of the water by towns and industries in succession can be
uses. It is particularly important to distinguish water supply increased without the water becoming objectionable;
in that sense from the need for water supply for irrigation
(ii) bulk transportation from areas of water surplus. The
purposes. Water for irrigation is used once only, the major
cheapest way to do this is in the rivers themselves; not only
part of it disappearing back into the atmosphere. Most of
may the capital expenditure be less, but the water is then
our potable and industrial water supplies eventually return
available for re-use as it travels;
to rivers and underground aquifers there to be used again.
Irrigation water does not have to be of the highest quality, (iii) by storage, in surface reservoirs and underground.
but it is required in very large quantities and at very low Future storage reservoirs are likely to be designed for
prices if it is to be of social benefit, especially in arid un amenity use more deliberately than in the past, and to be
developed regions. In speaking of the potential value of employed for supply to rivers rather than for direct supply
desalination to poor agricultural communities this distinc to water treatment plant. Natural storage in underground
tion is sometimes overlooked, but supplying water for the aquifers will no doubt be extended by artificially recharging
two purposes presents problems of quite a different order. them from rivers during times of abundant flow;
Even in a rich agricultural country such as the UK a price
(iv) by artificial stimulation of rainfall, although the scale
for irrigation water of 2s. per thousand gallons, as applied,
of distances that applies in the UK would make a high
would usually be considered too high.
degree of precision necessary;
8. Thus in comparison to the world situation the des
cription 'water shortage' cannot be used of the United (v) desalting of sea and brackish waters.
Kingdom. We only have to plan on a national scale and
increase our investment in water resources to a figure which
Desalination by distillation
will still be quite modest, and any part of the country can
Desalination plant is being installed for use both in con
have as much water as it wants.
junction with existing conventional schemes, and in arid
9. The framework for national planning in the United
areas that have virtually no other supply. The technology
Kingdom was established by the Water Resources Act of
is advancing rapidly, principally as a result of an annual
1963, which set up River Authorities responsible for all
research and development investment of rather more than
significant surface and underground water resources in
£10 m by the Office of Saline Water in the USA, and in
England and Wales, and established the Water Resources
this country as a result of a £11 m programme administered
Board to co-ordinate and advise. Faced with the probability
by UKAEA. Reviews of desalination are available in the
of having to provide for double the present demand by the
literature, principally references 2 and 3.
turn of the century, forward planning under the new system
12. The point has been reached where the processes
is gaining in momentum. Data are being assembled that
likely to be of use have probably all been identified in
will enable a much more precise approach to be made to
principle, and their likely development during the next
the design of inter-related river systems. Large and un
decade or so can be estimated. Steady advances in the tech
orthodox schemes, such as barrages, are being studied
nology of the most promising processes are resulting, and
seriously, and a conscious effort is being made to take advan
of course distillation is firmly established as the present
tage of the opportunities for thinking big which the Act
best method for sea-water conversion. It seems likely to
presents.
remain so for the next decade or more.
lb of steam input lb of water output
jj SIMPLE BOILING
j~f"j SUBMERGED TUBE
10
|jj MULTISTAGE FLASH
i MULTIPLE EFFECT 10
LONG TUBE VERTICAL
Fig 1 Energy consumption in distillation processes
2
STEAM FROM BOILER
1000 psia J STEAM TURBINE CONDENSER
1000° F
ALTERNATOR
BRINE HEATER MULTISTAGE-FLASH PLANT
Fig 2 Coupled water/power plant
Distillation processes this case is some 18 times bigger than the largest so far con
To date distillation has been used for the production of structed, which is of 2*2 mgd capacity. It is good to see that
boiler water feed and other specialized purposes, and for cost estimates by UKAEA assume capital charges that are
domestic water supply in areas where unusual climatic and realistic financing rates.
economic conditions prevail. Since 1957 the most significant
advance in distillation processes has been the change from The market for multi-stage flash distillation plant
submerged-tube multiple-effect plant to multi-stage flash
The demand for distilled sea-water for potable use as dis
evaporation, the process pioneered by Professor Silver in
tinct from industrial use has so far been in arid coastal areas
the UK. The reduction in energy consumption that has
and in small islands. In the arid coastal areas freshwater
resulted from this change is shown in Fig. 1. The figures
sources have generally been remote geographically or poli
given are typical of those used in practice, but would depend
tically ; in the small islands the rainfall has often been ade
upon the design of the plant.4 A further form of multiple-
quate but lack of suitable sites for impoundment and the
effect unit has been developed in the USA, and could prove
nature of the terrain resulting in rapid runoff has led to
competitive.
water shortages. Guernsey is an example.
14. More than half of the installed distillation plant is 17. Up to the present, plants have been located where
coupled with electricity generation plant. Fig. 2 shows a rather unusual economic conditions prevail. For example,
possible arrangement for a coupled installation where the Kuwait, Bahrain, Aruba, Curacao, and Qatar are wealthy
low pressure steam that might otherwise have driven the oil communities where water at prices above 5s. a thousand
turbine is taken to the distillation plant. Arrangements of gallons, undistributed, is accepted, and the cost would be
much greater complexity are necessary when the distillation still higher were not the fuel so cheap. The Virgin Islands,
plant is to be operated other than at base load. Coupled the Bahamas, Bermuda, and the Canary Islands, have depen
plant is found in all sizes, from the 0-3 mgd plant at Khark ded upon a desalinated water supply to promote their
Island, Iran, to the 127 mgd and 1500 MW plant for the flourishing tourist industries, and only now is plant being
Metropolitan Water District of California, USA, the con installed or contemplated for normal water supply in less
struction of the first stage of which has been approved. Of extreme circumstances, as for example in Mexico, Israel,
course it is the very large distillation plant that promises to and California.
produce water at really low cost, and one may expect the
18. Few land-based units were installed before 1957 and
development of coupled plant of large size to be one of the
the graph shown in Fig. 3 illustrates the growth of installed
features of the next decade. In all but the largest population
capacity from 1957 to 1969. The extrapolation to 1969 is
centres, however, such outputs may be too great to be assi
based upon plants firmly planned for construction within
milated into existing schemes. The Metropolitan Water
the next two years.
District of California averages a total output of 2000 mgd,
19. Fig. 3 suggests a startling growth rate, and it would
but the whole output of the London Metropolitan Water
appear that the installed capacity is trebling every five years.
Board, for instance, is only about 450 mgd, with an annual
On this basis by 1974 the total capacity will be above 200
increase of some 3%.
mgd and by 1979 over 600 mgd; it may be doubted, however,
15. The Californian plant referred to above will be the whether there is yet enough market experience to indicate
first large nuclear plant, although there is a small one at how the curve should be extrapolated to cover the type of
McMurdo Sound in the Antarctic. In California both the use to which desalination will be put when it is fully estab
power and water plant will operate at base load, and the lished. This expansion includes the Metropolitan Water
price quoted is 26 cents per thousand gallons. This sur District of California's programme, already referred to,
prisingly low cost results from taking the most favourable the first 42 mgd unit of which is planned for the early 1970s,
conditions of interest rate, amortization period and so with a further 85 mgd capacity before 1979. In addition,
forth, so that the charge to capital is very much lower than Israel is now considering the construction of a 100 mgd
ruling industrial rates. In addition the unit being costed in plant for operation before 1974.
3
of suitable feedwater, and present indications are that the
/ use of electrodialysis will meet the needs of modest supplies
/
/ for small communities in remote areas. Electrodialysis is
/
among the methods which have been supported by the
/ UKAEA desalination programme, and with this assistance
William Boby and Co. Ltd have made very significant
improvements with great economy.
Reverse osmosis
This is the most promising of the newer desalination tech
niques and is most suitable for the conversion of brackish
waters. The process depends upon the passage of water under
hydraulic pressure through semi-permeable membranes,
the most successful of which are of modified cellulose
acetate. The process is at an early stage of development and
is unlikely to make a great impact in brackish water con
version for at least five years, and small plant sizes are to
be expected. Membranes for sea-water conversion are being
developed but it is likely to be many years before an econo
mic process is available.
Use of desalination in the United Kingdom
I have already described the water resource planning prob
lem in the UK as being one of selecting the cheapest method
of accommodating the differences in distance and time be
1 tween rainfall and water demand.
., I I - I .—I— I I I . I I
1957 60 62 65 66 67 68 69 1970 25. With regard to distance, in principle one might use
Year a desalination plant at some dry coastal point in the south
Fig 3 Growth of installed desalination plant capacity. The extra east as a simple alternative to bringing water from the
polation to 1969 is based on plants firmly planned for construction
north-west by a more conventional means; the report of the
Water Resources Board, ' Water Supplies in South-East
England'5 quotes local water deficiencies of
20. The United Kingdom Water Resources Board is 100 mgd in 1971
also on record for the opinion that operational experience 400 mgd in 1981 and
with a large-scale desalination plant in this country is now 1100 mgd in 2001
required, and is exploring the possibilities with the appro in certain areas. However, the conclusion is also drawn that
priate government departments. the region as a whole has the resources to meet all needs
up to 1981 by conventional methods, and that requirements
up to the year 2001 can be met by such schemes as artificial
Desalination by other processes
recharge of aquifers, and transfer of water from the rivers
Freezing Severn and Wye to the Thames.
In these processes saline water is frozen to give salt-free ice 26. The possibility can also be visualized of desalination
crystals which, when melted, give a substantially pure water. plant being used as a special case in an area of abundant
Development has emphasized the usefulness of direct con water, in order to preserve the amenity values of water in
tact heat exchange, thus avoiding the use of the rather expen the locality.
sive heat transfer materials. Progress has been hampered 27. It must be recognized, however, that both these
by mechanical problems of handling the ice and, having types of use are unlikely within the next few decades, in the
regard to the nature of the refrigeration processes used, it light of present planning at national level for the use of
seems unlikely that these processes will be used in the large conventional resources.
plant range but will find application in units up to 5 mgd in
28. A more likely possible application of desalination
size.
would be for direct supply to industry for high quality water
requirements such as boiler feed. It will be appreciated that
Electrodialysis
a great deal of naturally derived water used in industry
More than 150 electrodialysis plants have been constructed needs further refinement for such purposes.
for the conversion of mildly brackish waters, that is waters 29. It is significant that the characteristics of con
with less than 10 000 ppm of total dissolved solids, but most ventional impounding schemes and of desalination plant
of these plants are quite small with a capacity less than differ in two important ways:
0-1 mgd. The process is based on the transfer of ions through (i) desalination plant can have a lower ratio of fixed
selective membranes, under the influence of an applied elec costs to operating costs than reservoirs and their ancillary
trical potential, and the energy costs are directly related to pumps, pipework, etc. Desalination costs therefore rise less
the level of dissolved solids The size of plants which will be than those of conventional water supply as the load factor
constructed in the future is dependent upon the availability falls;
4
Demand
FOR MEETING PEAK LOADS
Desalination may be cheaper than
Existing supply limit
additional conventional sources
Jan. Dec.
i i i i i i _j i i i i i—j
because
.2 with desalination, the'fixed costs'
TO
are proportionately lower
Operating
cost
^ Operating
cost
Fixed cost
o
C3
Fixed cost
Constant use Intermittent use
Fig 4
Reservoir level
Supplementary desalination plant makes it possible to take considerably
more from a reservoir without increasing the risk of future water shortage
Increased
Reservoir yield at acceptable Desalted
water added
risk of running dry Normal
\ \N
7ig 5
(ii) there can be no certainty as to the quantities and cases be a cheaper way of getting more water than the addi
timing of the future rainfall which will provide the raw tion of further conventional water resource capacity.
water for conventional water supply schemes, whereas de 35. The above examples demonstrate that desalination
salination plant always has more raw water than it can use. may become competitive at reduced load factors before a
break-even at base load is reached. In this respect the more
30. The economics of all water supply systems suffer
common comparisons of baseload costs may be misleading.
from the load factor consideration; not only are the rates
of consumption very different as between day and night,
weekday and weekend, winter and summer, in virtually all Conclusions
undertakings, but in some cases there are very pronounced (i) Multi-stage flash distillation is the most successful
special loads. In particular, seaside towns sometimes have desalination process, judged on both technical and economic
very high peak consumptions in the holiday season. grounds for the conversion of sea water at the present time.
31. In such cases, the fact that desalination plant can It is likely to remain so for at least ten years.
be designed so that costs rise less steeply as load factor falls
(ii) The world demand for desalination plant is expand
than they do with conventional water schemes, may make
ing rapidly. The present total installed capacity is about
it economic to employ desalination first in this country for
55 mgd and it can be predicted that this will rise to 200 mgd
such intermittent operation (Fig. 4). At least one might say
by 1974, and 600 mgd by 1979.
that in any situation where desalination costs at base load
are the same as those for conventional supply, then desalina (iii) The low cost of conventional water supply leads to
tion will be the cheaper at lower load factor. the conclusion that desalination as a base-load supply in
32. In order to appreciate the manner in which the the UK will find no significant application before 1990.
second advantage of desalination (that the feedwater supply (iv) Desalination is likely to find application as a peak-
is not subject to natural variability) can be made to show an load supply in the UK before 1990.
economic advantage it is necessary to consider the question
of probability in conventional water resource planning.
33. Since future rainfall cannot be known, the data
References
necessary for the design of impounding works can only be
estimated from past records, and the longer the period over 1. Collinge V. K. Water resource planning and desalina
tion. Paper presented at the Water Research Association
which records of rainfall are taken the further apart the
Conference on Desalination as a Supplementary Water
extremes are likely to be. It therefore follows that reser Resource, September, 1966
voirs and other water abstraction works cannot be designed 2. Burley M. J. and Mawer P. A. Desalination as a supple
to provide a given supply with absolute certainty, but only ment to conventional water supply. Part 1—A technical
to an acceptable probability. One might accept the prob and economic assessment of desalination processes. Water
ability of running short two or three times in 100 years, for Research Association Technical Paper No. TP50. The
instance. Water Research Association, Medmenham, 1966
34. For a given reservoir, if the rate of draw-off is in 3. Spiegler K. S., ed. Principles of desalination. Academic
Press, New York, 1966
creased then the probability of failure is also increased.
The use of a desalination plant to supplement the reservoir 4. Stewart J. M. Some practical aspects of desalination by
evaporation. Paper No. SWD/52, 1st International
by adding water whenever the level falls below the safe level Symposium on Water Desalination, Washington, October
would enable the draw-off to be increased without failure 1965
becoming more likely (Fig. 5). Again, the desalination plant 5. Water Resources Board. Water supplies in South East
would be working at low load factor, and might in some England. HMSO, London, 1966
6
