The invention relates to a method for optimising material recovery in a chemical pulping process. The method comprises treating of cellulosic raw material with cooking chemicals in a cooking step, where lignin is at least partly dissolved into cooking liquid phase and separated from fibres. The lignin is separated from the cooking liquid phase. The separated lignin is used for formation of a treatment composition comprising at the most 1 weight-% of aluminium. A bleaching liquid phase is treated with the treatment composition in order to remove organic humic substances from the bleaching liquid. Organic sludge is formed and it is used for energy production in a chemical recovery step.