It is generally agreed that where possible on steam boilers, the principal feedwater treatment should be external to the boiler.

External water treatment processes can be listed as:

1. Reverse osmosis - A process where pure water is forced through a semi-permeable membrane leaving a concentrated solution of impurities, which is rejected to waste.

2. Lime; lime/soda softening - With lime softening, hydrated lime (calcium hydroxide) reacts with calcium and magnesium bicarbonates to form a removable sludge. This reduces the alkaline (temporary) hardness. Lime/soda (soda ash) softening reduces non-alkaline (permanent) hardness by chemical reaction.

3. Ion exchange - Is by far the most widely used method of water treatment for shell boilers producing saturated steam. This module will concentrate on the following processes by which water is treated: Base exchange, Dealkalisation and Demineralisation.