Reverse osmosis water purification describes a process that need strike no fear among boat owners. This is despite the presence of the dreaded ‘O’ word which also commonly signals the onset of expensive damage to submerged gelcoat.

However, there are no threats from reverse osmosis. Reverse osmosis water treatment techniques are both useful and benign, since they can be employed to overcome one of many ocean voyager’s most pressing problems – maintaining a reliable supply of drinking water. To understand reverse osmosis you must first understand osmosis itself. The process involves a solvent (in the maritime case water) and dissolved substances (known as ‘solutes’). Sea water contains  a solute in the form of salt whereas fresh water does not. If they were placed in a container divided by a semipermeable membrane, the osmotic action would impel the fresh water to permeate the membrane in an effort to dilute the sea water.

This process would continue until their solute concentrations were equal and its the pressures thus caused that produce those dreaded blisters. Unless man intervenes, that is. By applying pressure (typically upwards of 40 bar (600 psi)) to the high solute concentration (salty) side of the membrane the process is reversed – hence ‘reverse osmosis’. The salt is removed and ‘bingo!’ we have potable water.

The diagram shows a typical boat system. An electric pump creates the pressure – often assisted by diverting some of the pressurised water around to the back of the piston. A compact installation on a boat with a 12V system might yield about 6 litres of fresh water per hour with a current draw of around 4A. Some smaller units can be operated by hand in emergencies.

RO desalinators like to be used regularly. If  left unused for more than a few days, the membranes must be ‘pickled’ with a special solution that prevents the built up of micro-organisms that can clog and damage them. These problems are most acute in tropical areas. Once pickled, the membrane should be OK for up to a year.

Also, the sea water needs careful pre-filtering before it gets anywhere near the RO unit itself. In areas where there’s lots of contamination – silt, weed etc – it’s sometimes necessary to have a pair of filters in line. This tends to make reverse osmosis water treatment less appropriate for inshore areas. But, of course, with land not far away these are not usually locations where drinking water is in short supply.