WATER DESALINATION

Prof. David Hasson

Technion

TOPICS:

* desalination fundamentals
* current desalination technologies
* desalination in Israel

DESALINATION FUNDAMENTALS

The Technological Problem:
To separate potable water from salty water by a technology meeting the criteria of reliability & economic viability.

Main Cost Centers:

CAPITAL INVESTMENT  separation equipment                35 - 50 (%)   supporting equipment                15 - 20 (%)                                                                                 50 - 70 (%)  AUXILLIARY COSTS   intake, discharge, energy     supply, instruments               15 - 20 (%)   structures, engineering & erection  15 - 30 (%)                                                                                     100 (%)   WATER COSTS COMPONENTS   capital costs                       30 - 50 (%)   energy costs                        30 - 50 (%)   other costs (labor, chemicals, etc) 20 - 30 (%)                                                                                   100 (%) 

INVESTMENTS COSTS for medium to large to medium plants are of the order of US $ 500 to 1000 per cu m per day

TYPICAL COSTS OF DESALTED WATER     sea water desalting            1 - 2  US $ / cu m     brackish water desalting     0.3 - 1  US $ / cu m 

CURRENT DESALINATION TECHNOLOGIES

1. Thermal Distillation Processes:
separation achieved by forming water vapours and condensing them.

1.1 Multistage flash distillation (msf):
pure water vapour formed by repeated release of the pressure of pressurized salty water in successive pressure steps
– Energy supply: low pressure steam

1.2 Multiple effect distillation (med):
pure water vapour formed by repeated boiling of the saity water in successive temperature steps – energy supply: low pressure steam

1.3 Vapor compression distillation (vc):
energy economy achieved by recycling the energy stored in the vapour by means of a motor driven compressor

                     UNIT CAPACITY COMPARISON                        (1 MGD = 3,800 Cu m per Day)   Process                    Sold to Date      Projected Largest   Multi-Stage Flash              9.0                20.0  Multiple Effect                4.5                15.0  Vapor Compression              0.5                 0.75    COMPARISON OF ENERGY EFFICIENCY OF DISTILLATION PROCESSES   Typical Figures for Single Purpose Plants   Process           Recovery    Process &    Total Primary     Equivalent                      (%)      Pump Use      Consumption     Fuel Oil Use                                             (Btu/kgal)       (gal oil/                                                              kgal water)  Multi-Stage,         21      83.3 Btu/Ib     903,100            6.3  Flash                       1O kWh/kgal   Multiple Effect      31      66.7 Btu/Ib     599,300            4.2  Vapor Compression    50      60  kWh/kgal    512,000            3.6   ENERGY SAVINGS IN DUAL PURPOSE PLANTS (combined power and water production)   Process                 Operating                  Heat Use                        Temperature (F)         (gal oil/kgal water)                                                                                              Single     Dual                                                Purpose   Purpose  Multi-Stage Flash          235                   6.3       1.6  Multiple Effect            160                   4.2       1.2

MEMBRANE PROCESSES

1. Reverse Osmosis (RO)

* a sophisticated "filtration" process
* based on the discovery (Loeb and Sourarijan) that certain thin plastic sheets allow pressurized water to percolate through them while hindering salt passage.
* energy consumption -high pressure pumping
* most economical technology for brackish water desalination

2. Electrodialysis (EDR)

* removal of salt through passage of electric current through "anion & cation" permeable membranes
* limited to low salinity waters
* electrical energy required

                           Representative Costs                      Water       Capital Cost          Water Cost  Process            Source    (s/gpd) : (s/m3d)  (S/1000gal) : (S/m3)   Reverse Osmosis  Seawater       4.97 : 1313            4.14 : 1.09                  Brackish       1.93 :  509            1.30 : 0.34  Electrodialysis  Brackish       2.42 :  637            1.35 : 0.36                            Unit Capacity Comparison                                                    UNIT SIZE (MGD)  Process                              Sold          Projected                                     To Date         Largest   Reverse Osmosis-Seawater             1               5  Reverse Osmosis-Brackish Water       2.5            10  Electrodialysis                      1               5 

DESALINATION IN ISRAEL

MILESTONES

Early ’60s R&D on Zarchin freezing process

1964 – 1 mgd dual purpose msf plant in Eilat

1968- 200 cu m ro plant built in Yotvata

Late ’60s – I.D.E develops low temprature vc units based on cheap aluminium tubes

Early ’70s – I.D.E develops low temperature aluminium tubes med units

1971 – 2500 cu med plant erected in Mashabbe Sadeh (UN development project)

1974 – 1 mgd med plant built in Eilat by I.D.E

Early ’80s – 5 mgd med demonstration plant erected in Ashdod (joint US-Israel development project)

Late ’70s – large scale development efforts initiated by Mekorot to introduce increasingly efficient RO units for supplying water to Eilat and Negev settlement

’80s – development of various solar energy systems

Late ’80s – large scale piloting by Mekorot of seawater RO

SOLAR ENERGY ENDEAVOURS

– Various solar systems examined:

Luz high temperature parabolic mirror concentrators
Paz-Pimat intermediate temperature parabolic mirror concentrator
Ormat salinity gradient ponds
Solar ponds with Arel company transparent absorber insulators honeycomb panels

Estimated water costs from 100,000 m3/d solar desalination stations

 Interest    Luz SEGS     Solar pond    IT trough    Luz SEGS    rate      and LT-MED   and LT-MED    and TVC      and SWRO    (%)        (c/m3)       (c/m3)        (c/m3)       (c/m3)      6         73-79         67-73         112-123      77-81     8         80-84         74-81         123-134      82-86    10         87-91         80-88         132-144      88-91   

These solar pond and LT-MED water costs are competitive with those from a dual purpose LT-MED plant operating with fossil fuel energy at fuel prices from $150-210/ton. It should be stressed, however, that these are target figures – the best that can be achieved. They are based on the performance of an advanced, high efficiency LT MED plant and a solar pond operating at a very high average annual insolation) rate, 2400 kWh/m2/y. While this latter figurc may be typical at low Red Sea and Persian-Arabian Gulf latitudes, at Israel’s Red Sea coast it will be 20% lower. Along its Mediterranean coast insolation will be 25% lower.

SUMMARY OF SIGNIFICANT ACHIEVEMENTS:

– Availabilty of advanced med and vc technologies exported successfully world wide by J.D.E. Technologies Ltd

– Advanced design and operation experience on RO accumulated at Mekorot and elsewhere

– Frontier R&D work on integration of desalination with solar energy

– Noteworthy industry-university research collaborations