Boiler feedwater is water used to supply ("feed") a boiler to generate steam or hot water. At thermal power stations the feedwater is usually stored, pre-heated and conditioned in a feedwater tank and supplied to the boiler by a boiler feedwater pump.
The feedwater must be specially treated to avoid problems in the boiler and downstream systems. Untreated boiler feed water can cause corrosion and fouling.
Corrosive compounds, especially O2 and CO2 must be removed, usually by use of a deaerator. Residual amounts can be removed chemically, by use of oxygen scavengers. Additionally, feed water is typically alkalized to a pH of 9.0 or higher, to reduce oxidation and to support the formation of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion. This is usually done by dosing alkaline agents into the feedwater, such as sodium hydroxide (caustic soda) or ammonia.
Deposits reduce the heat transfer in the boiler, reduce the flow rate and eventually block boiler tubes. Any non-volatile salts and minerals that will remain when the feedwater is evaporated must be removed, because they will become concentrated in the liquid phase and require excessive "blow-down" (draining) to prevent the formation of solid precipitates. Even worse are minerals that form scale. Therefore, the make-up water added to replace any losses of feedwater must be demineralized/deionized water, unless a purge valve is used to remove dissolved minerals.
Priming and foaming
Steam locomotives usually do not have condensers so the feedwater is not recycled and water consumption is high. The use of deionized water would be prohibitively expensive so other types of water treatment are used. Chemicals employed typically include sodium carbonate, sodium bisulfite, tannin, phosphate and an anti-foaming agent.
Treatment systems have included:
- Alfloc, developed by British Railways and Imperial Chemical Industries 
- Traitement Integral Armand (TIA), developed by Louis Armand
- Porta Treatment, developed by Livio Dante Porta