DESALINATION PLANT – AN OVERVIEW

•          Desalination is a separation process used to reduce the dissolved salt content of saline water to a usable level.

•          Involves three liquid feeds

                                - brackish water or seawater

                                - low-salinity product water

                                - brine or reject water

NEED FOR DESALINATION

COMMERCIAL DESALINATION TECHNOLOGIES

•          Thermal

–        Multistage flash distillation

–        Multi effect distillation

–        Vapor compression     

•          Membrane

–       Reverse osmosis

–                      Electrodialysis

 

 

 

 

 

OSMOSIS

The phenomenon of transport of water across semi-permeable membrane from low salt concentration to high salt concentration is called Osmosis

PRINCIPLE OF REVERSE OSMOSIS

•          Pressure is applied to saline water to force the pure water molecules through a semi-permeable membrane. The majority of dissolved salts, organic, bacteria and suspended solids are unable to physically pass through the membrane and are discharged form the system in the rejected brine. The pure water is then ready for use without further treatment.

Why Reverse Osmosis ?

Reverse Osmosis : Technical advantages

Versatility:

•          For the treatment of all types of water: brackish water –sea water

•           Adapted to all sizes of installations and any sources of energy: steam, electricity

•          Setting up in all geographic zones possible

•          Aside from the need to dispose of the brine, RO has a negligible environmental impact.

Flexibility:

•          Plant sizing according to the present needs (possibilities to increase production capacity)

•           Operating cost is proportional to flow variations

Reverse Osmosis: a competitive solution

Constraints

•          The membranes are sensitive to abuse.

•          The feedwater usually needs to be pretreated to remove particulates (in order to prolong membrane life).

•          Operation of a RO plant requires a high quality standard for materials and equipment.

•          Brine must be carefully disposed of to avoid deleterious environmental impacts.

•          There is a risk of bacterial contamination of the membranes; while bacteria are retained in the brine stream, bacterial growth on the membrane itself can introduce tastes and odours into the product water.

RO membrane

Types of Membrane Foulants in SWRO

         TYPICAL REJECTION CHARACTERISTICS OF R.O. MEMBRANES
Elements and the Percent R.O. Membranes will remove

Sodium
Sulfate
Calcium
Potassium
Nitrate
Iron
Zinc
Mercury
Selenium
Phosphate
Lead
Arsenic
Magnesium
Nickel
Fluoride
Manganese
Cadmium
Barium
Cyanide
Chloride

85 - 94%
96 - 98%
94 - 98%
85 - 95%
60 –75%
94 – 98%
95 – 98%
95 – 98%
94 – 96%
96 – 98%
95 – 98%
92 – 96%
94 – 98%
96 – 98%
85 - 92%
94 – 98%
95 – 98%
95 – 98%
84 – 92%
85 – 92%

% may vary based on membrane type water pressure, temperature & TDS

 DESALINATION SYSTEM

•          Pre treatment system (PT system)

•          Sea Water Reverse Osmosis System (SWRO)

•          Post treatment and treated water storage system

•          Reverse Osmosis and Mixed Bed Ion Exchangers for DM water system

•          Chemical Storage and dosing system

                        

                        SWRO : schematic

                INDUSTRIAL MIXED BED DEIONISER

•          Single column units

•          Strongly acidic cation and strongly basic anion exchange resin beds

  Working principle

   There are four distinct stages in the operation of an industrial mixed-bed deioniser:

•          Service/exhaustion

•          Backwashing

•          Regeneration

    Rinse/remix