Certain natural and synthetic materials have the ability to remove mineral ions from water in exchange for others. For example, in passing water through a simple cation exchange softener all of calcium and magnesium ions are removed and replaced with sodium ions. Since simple cation exchange does not reduce the total solids of the water supply, it is sometimes used in conjunction with precipitation type softening. One of the most common and efficient combination treatments is the hot lime-zeolite process. This involves pretreatment of the water with lime to reduce hardness, alkalinity and in some cases silica, and subsequent treatment with a cation exchange softener. This system of treatment accomplishes several functions: softening, alkalinity and silica reduction, some oxygen reduction, and removal of suspended matter and turbidity.
Chemical treatment of water inside the boiler is usually essential and complements external treatment by taking care of any impurities entering the boiler with the feed water (hardness, oxygen, silica, etc.). In many cases external treatment of the water supply is not necessary and the water can be treated only by internal methods.

Internal treatment

Internal treatment can constitute the unique treatment when boilers operate at low or moderate pressure, when large amounts of condensed steam are used for feed water, or when good quality raw water is available. The purpose of an internal treatment is to

1) react with any feed-water hardness and prevent it from precipitating on the boiler metal as scale;

2) condition any suspended matter such as hardness sludge or iron oxide in the boiler and make it non-adherent to the boiler metal;

3) provide anti-foam protection to allow a reasonable concentration of dissolved and suspended solids in the boiler water without foam carry-over;

4) eliminate oxygen from the water and provide enough alkalinity to prevent boiler corrosion.

In addition, as supplementary measures an internal treatment should prevent corrosion & scaling of the feed-water system and protect against corrosion in the steam condensate systems.