The present invention relates to a method and arrangement for the treatment of liquid material based on organic waste products, in particular sludge from sewage disposal plants and the like, wherein the sludge material is added and mixed with chemicals, in particular sulphuric acid, nitric acid, and/or ammonia, during the vaporization and degasification of liquid from the material to increase the solids content thereof. The present invention is characterized, inter alia, in that the material is continuously introduced at the upper part of a vertical mixing vessel (1), in which the material is subject to mixing, after which the material is passed on into a reactor tank (8) for sulphuric acid treatment and at the same time, while sinking through the sulphuric acid treatment reactor tank (8), is exposed to an impact action from a number of rotary processing means (9) disposed in the reactor tank (8), after which the material is passed on into a reactor tank (14) for ammonia treatment and at the same time, while sinking through the ammonia treatment reactor tank (14), is exposed to an impact action from a number of rotary processing means (9) disposed in the reactor tank (14), after which the material is finally passed on into a drier (15) in which the material is dried until a desired solids content is achieved.