Abstract This thesis aims to increase our knowledge about the characteristics of chemical pulp process and wastewaters and how problematic substances, e.g. wood extractives, could be removed effectively and selectively by coagulation–flocculation with either internal or external water treatment. Characterization was performed by investigating kraft pulp bleaching filtrates, as well as wastewater, before (influent) and after (effluent) the activated sludge treatment by means of a range of chemical analyses and by carrying out size fractionation studies. Cationic polyelectrolytes were used to purify oxygen stage bleaching filtrate, and charge analyses (zeta potential, charge quantity) were carried out in order to understand the coagulation phenomenon. In activated sludge treatment, the enhancement of particle removal, either by filtration or using a chemical in the primary clarifier, would lead to savings in aeration costs and result in a more stable process. Microfiltration already with a large pore size (8 µm) removed 30–50% of the wood extractives from the influent. Separate treatment stages for certain wastewater fractions, e.g. debarking plant effluent, would ensure cost-efficiency. After the activated sludge process, the wood extractives were present as particles (18%) and < 3 kDa fraction (82%). β-sitosterol occurred only in particles in the effluent. The release of harmful components into the environment could be decreased by microfiltration (e.g. 0.45 µm) of the final effluent or using a chemical in the secondary clarifier. Interestingly a huge increase in BOD was realized in the 3 kDa fraction of both influent and effluent, which indicated the presence of toxic substances in the larger fractions. After passing the effluent into the water system, there might be a similar jump in the BOD because the effluent is diluted many-fold. This would contribute to the formation of areas with an oxygen deficit. In the coagulation–flocculation studies, effective and selective removal of wood extractives (92%) from the oxygen stage filtrate was obtained with a cationic polyelectrolyte of medium molecular weight and medium charge density at 72 °C and pH 5–6. The multimodal zeta potential distribution gave more information than the average zeta potential. Aggregation of colloidal particles occurred when only one zeta potential was observed. The number of different zeta potentials diminished with decreasing pH and after exceeding a certain polyelectrolyte dosage level.