Desalination

There is an estimated 300m miles³ of water present on the earth. Of this, 96% is found in oceans, another 2% of water is tied up in glaciers and ice caps, and 1% sits in the atmosphere. This leaves only 1% of the water present on earth available for human and animal consumption, and even a large percentage of this is inaccessible. Demand is growing as a direct result of an increasing population and increased economic development, and has tripled over the last 50 years alone. To cater for this increasing demand desalination may be the only answer in the years to come.

What is desalination?

Desalination describes a range of processes which are used to reduce the amount of dissolved solids in water. It is most often used to describe the process of converting salt water (e.g. sea water) to fresh water that can then be used for drinking (potable water) and irrigation. Used for sometime on ships and submarines, this process now has new focus to provide fresh water for human use in areas where it is currently limited.

Large scale desalinisation plants tend to use large amounts of energy to produce the water as well as costly infrastructure, therefore when compared to drawing fresh water from rivers and groundwater, desalinated water is very expensive. However, you tend to find desalination plants associated with electricity generating plants, from which electricity and waste heat are readily available making them more cost effective. This combined use of resources is explored more in CHP cogeneration.

The quantity of dissolved solids in a liquid is known as total dissolved solids (TDS) and is measured in mg/l. Typically sea water has a TDS value of over 30,000mg/l, while drinking water sits within the range of 0-1000mg/l.

What are some of the desalination techniques?

There are many techniques involved in desalination, each with their own advantages and disadvantages, but broadly speaking most techniques sit within two camps.

1. Thermal distillation – evaporating the pure water from salt water using heat. The processes that fall in this category are as follows and explored below in more detail: a) multistage flash distillation, b) multiple effect distillation, c) vapour compression and d) solar humidification-dehumidification

2. The use of semi permeable membranes – processes explored that fall in this category are reverse osmosis and electro-dialysis reversal.

Desalination using Multistage Flash Distillation

Approximately 85% of desalination worldwide is completed by multi-stage flash distillation (MSF) – this is a type of thermal desalination. This process involves distilling sea water by flashing a portion of the water into steam in multiple evaporating chambers (known as stages) of what are essentially counter current heat exchanges. The process is as follows.

• The sea water enters the system, and via heat exchanges it is heated as the water travels through the top of each of the main evaporating chambers (stages).
• Eventually, once the water has travelled through all of the stages, it enters a brine heater which heats it up further to the optimum temperature for the process to take place (heating it first in the evaporating chambers limits the energy required by the brine heater).
• This water then enters the first stage, where lower ambient pressure causes the seawater to instantaneously boil, releasing heat and water vapour until reaching equilibrium with the conditions in the chamber.
• This water then enters the next stages one after another, where successively lower ambient pressure in each of these causes the seawater to again instantaneously boil as soon as it enters without reheating each time.
• The vapour from each of the stages is condensed against the heat exchanger tubes (with the heat being used in step 1), creating the freshwater which is pumped away and then the remaining brine enters the next stage and this process gets repeated.
• The sea water increases in salt concentration from stage to stage as distilled water leaves the solution. This can simply be pumped back into the original source.

Desalination using reverse osmosis

Another popular method for desalination is reverse osmosis, which involves the use of a semi-permeable membrane. Osmosis is a phenomenon used by plants to absorb water and move it within the plant itself.

Osmosis involves the movement of a solvent across a semi-permeable membrane into a solution of higher solute concentration. It results in equilibrium being met, where the two solutions on each side of the membrane have equal solutes. The difference in the concentrations is known as osmotic pressure, and the higher this is, the quicker the solvent will move. This process can be reversed if the pressure applied to solution with the greater solute concentration is higher than the osmotic pressure.

In reverse osmosis, pressure is applied to the feedwater, forcing the water molecules through a semi-permeable membrane. The water that has passed through the membrane leaves the unit as product water, and most of the dissolved impurities remain behind and are discharged in a waste stream.

There are however major problems associated with this, firstly the process is very slow, and the membranes are very delicate so can tear easily. In addition the water needs to be filtered first so that large particles don’t damage the membrane, and additives may need to be added to prevent build up of salts on the membrane.

These two methods account for most of desalination that takes place across the world. There are other methods which are described very briefly below.

Desalination using Multiple Effect Distillation

In multiple-effect distillation, evaporators are situated in series, so the energy in the steam from one series is used to evaporate water in the next one. The saline water is usually applied to evaporator tubes in the form of a thin film so that it will evaporate easily.

Desalination using Vapour Compression

The technique of vapour compression uses a mechanical energy source, such as an engine or electric motor, to power a compression turbine. The feedwater is evaporated and the turbine compresses this raising the temperature of the exhaust vapour. The vapour is then passed over a heat exchanging condenser, where it returns to the liquid state as product water. The heat removed during condensation is returned to the raw water to assist in the production of more vapour.

Desalination using Electrodialysis Reversal (EDR)

In EDR, an electrical current is used to separate out salt and impurities in the intake water. Most of the impurities in water are present in an ionized (electrically charged) state and will conduct electric current. When direct current is applied, the impurities migrate towards the positive and negative electrodes; these ions are pulled through a semi permeable membrane resulting in two streams, a desalinated stream (which is tapped off as potable water) and a salt water stream. These membranes can become blocked by ions and other impurities; however by reversing the current the solutes that attach themselves to the membrane dissolve back into the water, so this combats efficiency reductions. However, this process is only possible for brackish water; it does not work effectively on purifying seawater. Click here for an EDR example.

Desalination using Solar Humidification-Dehumidification Method

This process mimics the natural water cycle, however takes place over a much shorter period. The simplest example is using a solar still, where the sun enters a glass covered box heating water held in the bottom of the box (which is black to absorb more heat). This then causes the water to evaporate, and this then condenses on the glass cover where it gets collected. More sophisticated designs separate the solar heat gain section from the evaporation-condensation chamber. An optimised design comprises separated evaporation and condensation sections.

Thermal distillation vs semi permeable membranes for desalination

• The performance of distillation technologies is relatively unaffected by feedwater salinity
• Distillation technologies have a higher total energy consumption than membranes
• For low salinity feedwater, membrane technologies have a higher rate of recovery
• Reverse Osmosis requires pre-treatment before the desalination process, in which all solid and suspended particles are removed, its pH value is adjusted and appropriate chemical inhibitors are added to ensure scale deposits
• Distillation suffers from higher rates of corrosion and scale formation due to the higher temperatures involved.

Desalination industry development

The scarcity of fresh water is already critical in many arid regions of the world and this will increase in importance in the future. It is also highly likely that the availability of fresh water (along with fossil fuels) will be a determining factor in world stability in the future.

According to the International Desalination Association in 2009 there were over 14,000 desalination plants, producing 60million m³/day of potable water. Approximately 70% of desalinated water is currently produced and used in the Middle East, the largest plant currently in operation is the Jebel Ali Desalination Plant producing 60million m³/year, and this is a MSF plant.

Closer to home, during 2010, Thames Water opened a £250m desalination plant in Beckton, East London. The plant is the first of its kind to be built in the UK, and will be able to supply 150m litres of potable water each day. The Beckton plant is run using 100% renewable energy and uses Reverse Osmosis to produce the drinking water. It is due only to be used in times of drought and to maintain supplies in the event of an incident at another water treatment facility, but will be able to service 400,000 homes in London and the surrounding areas.

We have to wait and see whether desalination will prove to be profitable for investment in the UK, given that recent water shortages have subsided.