Desalinisation Plant | waterengineering2030

Desalination Plants

Methods of Desalination

Distillation
- Multi-stage flash distillation (MSF)
- Multiple-effect distillation (MED|ME)
- Vapor-compression (VC)
Ion exchange
Membrane processes
- Electrodialysis reversal (EDR)
- Reverse osmosis (RO)
- Nanofiltration (NF)
- Membrane distillation (MD)
- Forward osmosis (FO)
Freezing desalination
Geothermal desalination
Solar desalination
- Solar humidification-Dehumidification (HDH)
- Multiple-effect humidification (MEH)
Methane hydrate crystallization
High grade water recycling
Seawater greenhouse

I have focused on the two most frequently used types of desalination plants; RO and MSFD, which are explained and discussed in the video clips.

Multi- Stage Flash Distillation

Reverse Osmosis Desalination

BENEFITS OF DESALINISATION

- Desalination plants can provide drinking water in areas where no natural supply of potable water exists.

- One of the few water sources that is entirely independent of precipitation levels

- Desalinised water generally meets or exceeds standards for water quality.

- Desalination plants also lessen the pressure on freshwater supplies in protected areas as the water is sourced from the sea.

DISADVANTAGES OF DESALINATION

- Environmental impacts are a serious disadvantage to desalination plants. Disposal of the salt removed from the water is a major issue. This discharge, known as brine, can change the salinity and lower the amount of oxygen in the water at the disposal site, stressing or killing animals not used to the higher levels of salt. Brine is denser than seawater due to higher solute concentration. The ocean bottom is most at risk because the brine sinks and remains there long enough to damage the ecosystem. Careful reintroduction can minimise, but not remove, this problem

- The desalination process uses or produces numerous chemicals including chlorine, carbon dioxide, hydrochloric acid and anti-scalents that can be harmful in high concentrations.

- There is a very high cost to build desalination plants. Depending on their location, building a plant can cost from £195 million to £1.9 billion [as of 2008.]

- Once operational, plants also require lots of energy. Energy costs account for one-third to one-half of the total cost of producing desalinated water. Because energy is such a large portion of the total cost, the cost is also greatly affected by changes in the price of energy. The California Coast Commission estimated that a one cent increase in the cost of a kilowatt-hour of energy raises the cost of one acre-foot of desalinated water by £32.

LEFT: Plan of a typical reverse osmosis desalination plant.

RO vs. MFSD

MSFD RO

Percentage of world's 60 15

desalinated water :

Efficiency: MEDIUM LOW

ADVANTAGES & DISADVANTAGES OF MFSD:

- Can operate at 23-27kWh/m3 of distilled water.

- Little heat energy escapes because the colder salt water entering the process counterflows with the saline waste water. Most of the heat is collected by the colder saline water going towards the heater so that the energy is recycled.

- Requires power input

- Reverse osmosis, MSF distillation main competitor, requires more pretreatment of the seawater and more maintenance, as well as energy in the form of work.

ADVANTAGES AND DISADVANTAGES of RO:

- A reverse osmosis unit delivering five gallons of treated water per day may discharge between 20 and 90 gallons of wastewater per day.

- Due to its fine membrane construction, reverse osmosis not only removes harmful contaminants present in the water, but it also may strip many of the good, healthy minerals from the water. A number of peer-reviewed studies have looked at the long-term health effects of drinking demineralized water.

In conclusion, we think that MFSD is overall a cheaper and more efficient method of desalination than RO, although they both have advantages and disadvantages

Other potential developements

Other ideas that are currently being tested, developed and expanded:

DESALINATION POWERED BY WASTE HEAT:

Diesel generators (used to provide electricity in remote areas) typically produce about 40-50% of energy as low grade heat which leaves the engine via the exhaust. By connecting a membrane distillation system to the diesel engine exhaust it may be possible to use this low grade heat which is currently wasted. The membrane distillation system even actively cools the diesel generator; improving its efficiency and increasing electrical output. This could be an energy- neutral desalination solution. As the key problem discussed above with desalination is the high energy consumption, this idea could be vital to increasing the use of desalination plants, especially in poorer countries.

LOW TEMPERATURE THERMAL DISTILLATION

LTTD takes advantage of water boiling at low pressures and possibly even ambient temperatures. Vacuum pumps are used to create a low pressure, low temperature environment where water boils at an 8-10℃ temperature gradient between two volumes of water. Cooling ocean water (supplied from depths of up to 600m) is pumped through coils to condense the water vapour. The condensate is then purified. LTTD may also be able to make use of the large quantities of warm waste water available at power plants. This reduces the the energy input needed to create a temperature gradient. Recent experiments have been conducted to test this approach; a spray-flash evapouration system was tested by Saga University in Japan, and India’s National Institute of Ocean Technology opened their first plant in 2005 at Kavaratti in the Lakshadweepislands. The plant has a capacity of 100,000 litres a day and uses deep water at a temperature of 7-15℃.

THERMOIONIC PROCESS

In October 2009, Saltworks technologies announced a process that uses solar or other thermal heat to drive an ionic current that removes all sodium and chlorine ions from the water using ion-exchange units. This process uses IonFlux membranes.

PASSARELL PROCESS

The Passarell process uses reduced atmospheric pressure to drive evaporative desalination. The pure water vapor generated by distillation is then compressed and condensed. The compression process improves distillation efficiency by creating the reduced pressure in the evaporation chamber. The compressor centrifuges the pure water vapor after it is drawn through a demister (removing residual impurities); causing it to compress against tubes in the collection chamber. The compression of the vapor makes its temperature increase. The heat generated is transferred to the input water falling in the tubes, causing the water in the tubes to vaporize. Water vapor condenses on the outside of the tubes as the product pure water. By combining several physical processes, Passarell enables most of the system's energy to be recycled through its subprocesses, namely evaporation, demisting, vapor compression, condensation, and water movement within the system. Therefore it is an incredibly efficient process.

GEOTHERMALLY DRIVEN DESALINATION

In most locations, using this method is more environmentally and economically friendly than using scarce ground or surface water.

FREEZE- THAW DESALINATION

Using freezing to remove fresh water from frozen sea water.

NANOTUBE MEMBRANES

There is current research into the effectiveness of nanotube membranes for water desalination and filtration. Hermetic, sulphonated nano-composite membranes have shown to be capable of cleaning most all forms of contaminated water to the 'parts per billion' level. Nanomaterials are not susceptible to high salt concentration levels, and require substantially less energy than RO. Biomimetic membranes are also being considered.

MEMBRANELESS DESALINATION

Membraneless desalination at ambient temperature and pressure using electrokinetic shocks waves has been demonstrated. Anions and cations in salt water are exchanged for carbonate anions and calcium cations respectively using electrokinetic shockwaves in this process. Calcium and carbonate ions then react to form calcium carbonate, which then precipitates out to leave fresh water. Theoretical energy efficiency of this method is similar to electrodialysis and reverse osmosis.