in which pure water crystals are formed and are separated off while released dry matter remains in the enriched filtrate which can be conducted to the recovery system of the factory the recovery system can vary for example a portion of the filtrate can be used for dissolving the soda smelt from the recovery boiler the remainder of the filtrate can be concentrated and mixed with the black liquor if required the whole of the filtrate can be subjected to a limited pre evaporation or freeze crystallization to a liquid quantity which is adapted to the maximum quantity which can be fed to the soda dissolvers alternatively the whole of the filtrate can be pre evaporated to the lowest possible quantity of liquid which is then mixed with the black liquor which goes to the usual evaporation system of the factory in order to limit energy consumption in the pre evaporation stages or in the freeze crystallization system the quantity of filtrate leaving the washing apparatus of the q stage can be decreased by reusing a portion of this filtrate as dilution liquid prior to the q stage in this case to achieve a balance in liquid management the supply of fresh water to the washing apparatus of the peroxide stage must be decreased to a corresponding degree and replaced with recirculated filtrate from the ozone stage the degree of closure recirculation which is possible will depend on how many metal ions are introduced in the q stage together with the oxygen delignified pulp as a result of the closure an enrichment of both metal ions and released organic substances occurs in the filtrate of the q stage owing to.