The invention relates to a method and apparatus for obtaining strong white liquor and lime mud with low residual alkali content from causticized liquor. The first filter (FWL) receiving causticized liquor from causticizing vessels (CT1-CT2-CT3) is designed as pressurized two stage disc filter with 2 separate vat parts. In the first vat part 10a is non-diluted white liquor obtained as a filtrate from the causticized liquor. The first lime mud cake accumulated on the filter discs in the first vat part is scraped off and diluted before being fed to the second vat part 10b. In the second vat part is weak liquor obtained as a filtrate from the reslurried first lime mud cake. The second lime mud cake accumulated on the filter discs in the second vat part is washed before being scraped off and diluted before being fed to subsequent lime mud handling. By the invention is lower TRS emissions from lime kiln obtained compared with 2-stage lime mud washing using atmospheric lime mud thickening filters.