Water utilities and developers

Water treatment

Water desalination

Halcrow offers a full consultancy service in the design and installation of control and equipment for water treatment processes. Modern plants include microprocessor /computer control and monitoring of all the process equipment.

With the increasing demand for clean, potable water allied to the increase in cost of developing, locating and maintaining traditional water sources.

The desalination of poor grade water becomes more economic.

Halcrow has knowledge of the technique of desalination by the use of membranes.

With the application of a pressurised source of water onto a membrane it is possible to reverse the osmotic effect to desalinate waters of most dissolved salts. This method of Reverse Osmosis has been available since the 1950's.

The Sea Shall Quench (pt I) A Matter of Membranes (pt II) Man’s first attempt at desalination – the conversion of saline water to fresh – is attributed to Aristotle who postulated that salt might be removed from seawater by a process of successive filtration.With water resources under pressure, it seems that Aristotle’s postulation has not only found its place in history but is very alive and relevant to the demands of the 21st Century, as Neil Campbell* highlights in the first of this two part article on desalination. By Neil Campbell MPhil CSci CChem FRSC FCIWEM, Halcrow Water Services, UK

Desalination

Desalination is a technique whereby water containing a high concentration of salt is rendered fit for either human consumption or irrigation.

Water containing salt is broadly classified into two categories : Saline and Brackish.

Saline waters such as Sea water, contain up to 42000 mg.l-1 of salts, whereas brackish waters contain upwards of 2000 mg.l-1<.

To be fit for human consumption, water should contain less than 1500 mg.l-1 of salts (usually expressed as Total Dissolved Solids).

The process of Desalination has been known for a long time and can be accomplished by methods such as :

	Distillation: by Multi-Stage Flash (MSF), or Multiple Effect Distillation (MED) which both rely on boiling the water and collecting the condensate, or Vapour Compression (VC). This technique is very costly, and is usually carried out in conjunction with power generation in order to utilise the waste heat / steam generated at these facilities and hence bring down the cost of production to economic levels.
	Ion exchange: In this method, the water to be treated is passed through beds of Ion Exchange Resins where Cations (such as Sodium, Magnesium, Calcium, Strontium etc) are replaced by exchange with Hydrogen ions attached to the resin, and Anions (such as Chloride, Sulphate, Bromide etc) are replaced by exchange with Hydroxide ions attached to a different type of resin. The resins require frequent regeneration with Sulphuric Acid and Caustic Soda.
	Membrane separation: by Electro-Dialysis where ions are caused to migrate through a membrane under the influence of an electric charge, or Reverse Osmosis where the solvent (pure water) is forced through a semi-permeable membrane by applying a high pressure.

Of all the desalination methods available, Reverse Osmosis is the one which has seen the most rapid growth in the last 30 years.

A study undertaken by Black & Veatch in 1992 showed that the industry was growing by approximately 70% per annum.

At that time, almost 4 million cubic metres of water were being produced every day by 9000 desalination plants worldwide - with 30% of this being produced by RO.

The main reason for this rapid growth has been the technological developments which have occurred in recent years with membrane manufacture.

Membrane flux has progressively increased while retaining high salt rejection ratios at progressively lower operating pressures.

This development has been spurred on by a need to conserve progressively dwindling water resources in high population centres such as California and Florida, drought prone areas such as the Middle East, and in areas where the tourism industry has placed an excessive burden on limited water resources of islands such as Grand Cayman, Cyprus, Malta and Lanzarote.