Introduction to desalination

Desalination is the process of removing salt from saline water including seawater, brackish water (less saline that seawater) and hypersaline water (more saline than sea water).

The majority of the world’s desalination plants today (by both number of plants and volume of water treated) employ either Reverse Osmosis (RO), Multi Stage Flash (MSF) or Multiple Effect Distillation (MED).

In general most systems can be considered to be membrane, thermal or electrically based.  Membrane systems hold back the salt while allowing water to pass through a semi-permeable membrane.  Thermal systems boil off and re-condense the water, leaving the salt behind. Electrical systems pass current through the saline water attracting or repelling the charged salt ions.  Ion exchange is somewhat different in that it exchanges unwanted ions for less undesirable ions.  Hybrid systems uses a combination of techniques – most commonly combining thermal & membrane processes.

Methods of desalination

1. Membrane
   1. Reverse Osmosis (RO)
      1. Cellulose acetate membranes
      2. Polyamide membranes
      3. Thin-Film Composite (TFC) membranes
      4. Nanotube desalination
         1. Carbon nanotubes (CNT)
         2. Boron nitride nanotubes (BNNT)
      5. Graphene membranes
      6. Aquaporin and biomimetic membranes
      7. Closed circuit desalination (CCD)
   2. Nanofiltration (NF)
   3. Forward Osmosis (FO)
2. Thermal
   1. Distillation
      1. Multi Stage Flash (MSF)
      2. Multiple Effect Distillation (MED)
         1. MED with Thermal Vapour Compression (MED-TVC)
         2. MED with Mechanical Vapour Compression (MED-MVC)
      3. Solar
         1. Solar stills
         2. Seawater greenhouse
         3. Concentrated Solar Still (CSS)
         4. Plasmonics
      4. Humidification-dehumidification (HDH)
      5. Multiple effect-humidification
      6. Membrane Distillation (MD)
   2. Freeze Thaw
3. Electrical
   1. Electrodialysis Reversal (EDR)
   2. Capacitive Deionisation (CDI)
   3. Electro Deionisation (EDI)
   4. Nanojunction Desalination
4. Ion Exchange
5. Hybrid desalination

Pre & post-treatment

Pre-treatment

In addition to the critical step of desalting, pre-treatment is often required to protect the desalination step, most methods of coagulation, clarification & filtration can be used.

Thermal systems like MSF and MED tend to need much less pre-treatment than other systems, often only screening and chemical treatment (adding chlorine, antiscalant and antifoam) is required.

Chlorination is typically used to avoid biological fouling of the desalination step.  Where chlorine intolerant membranes are used, dechlorination may be essential in order to avoid oxidation of the membrane.  Dechlorination is not normally required with thermal systems.

Post-treatment

Distillation results in a product water with very low mineral content.  This water can be aggressive (causing corrosion to metal pipes) and would not be considered potable.  Remineralisation is therefore conducted to stabilise the water.

Even though the pore size may be much smaller than the membranes typically used for disinfection, Reverse Osmosis (RO) is not generally considered to be a disinfection step, as significant quantities of water can pass the membrane.  It is therefore normal to have a separate step of primary disinfection following reverse osmosis.  Even where the desalination step results in completely sterile water, it would also be normal to add a residual disinfectant in order to avoid recontamination in distribution.