The continuous digester was developed to improve the large discharge of volatile organic compounds in the waste liquid (black liquor) generated after digestion in the conventional batch-type digester. A substantial energy saving was achieved as a result of the development of a large-capacity treatment process equipped with a heat recovery system in this continuous digestion process. Continuous digester not only saves energy but also contributes to the expansion of production capacity and improves the working environment by reducing volatile organic compounds (VOC). The technology is suitable for upscaling the digester and for use of Conifers as raw materials (NEDO, 2008. p. 157).

There kinds of continuous digesters have been developed:

• Modified continuous cooking (MCC) digesteris one where cooking is performed with dispersed charging of the white liquor (cooking chemicals) in the middle and lower zones of the digester to realize a countercurrent flow with the raw material flow.
• Isothermal continuous cooking digester is one where cooking is performed uniformly by raising the temperature in the washing zone at the bottom of the MCC type digester to maintain isothermal conditions in the digester as a whole.
• Low solid fraction continuous cooking digester has reduced dissolved lignin concentration in the second half of cooking and reduced total alkaline concentration, achieving a selective increase which improves the high sulfide digestion method in the initial stage of cooking (NEDO, 2008. p. 157). - See also Lo Solid Continuos Cooking System.