Like digesters, there are many types of brownstock washers in mills -- vacuum drum filters, pressure filters, atmospheric diffusers, pressure diffusers, twin-roll presses, etc. Whatever the equipment, the purpose is to separate the spent liquor (which is sent to the chemical recovery system) from the fibrous pulp going forward along the fiberline  The efficiency of the brownstock washing operation has a very big influence on mill economics, chemical recovery, and environmental compliance.

Vacuum drum filter          Atmospheric diffuser          Pressure diffuser