Blowdown is a method of solids control. In boilers and scrubbers, solids accumulation (both dissolved and sometimes, in the case of the scrubber, suspended solids) can interfere with the operation of the unit. In the boiler, high suspended solids and/or high dissolved solids can cause deposits on the inside of the boiler tubes. The deposits have a much lower heat conductivity than the boiler tube metal, and reduces the efficiency of the boiler. Solids accumulate because of evaporative processes, and in order to control the solids buildup, one has to waste high solids water and replace it with lower solids water. That is blowdown.

The same thing is true in a scrubber, only the solids buildup tend to be particulate if it is operating on flue gasses, and salts if it is operating on a chemical stream off gas. The salt buildup beyond a certain point will reduce the efficiency of the scrubber by approaching the solubility point of the scrubbed material in the scrubber liquid. When the solubility limit is reached, the absorption stops and the scrubber efficiency goes to almost zero. Alternatively, the scrubber may, when the solubility limit is reached, begin to deposit solids on the walls of the pipes, etc. Again, in order to reduce the solids accumulation, the scrubber water rich in solids or salts is wasted or blown down and replaced with low solids water.