The main aim of this paper is to improve the current sawmilling process by utilization of the products from thermal treatment of wood residues. Firstly, batch thermal treatments of rubberwood sawdust were conducted under various conditions to study the effects of the conditions on the yields and the characteristics of the products, pyroligneous acid and activated carbons. Sufficient amounts of crude pyroligneous acid and activated carbon were obtained. In all conditions, the produced pyroligneous acid contained the same amount of components which are essential as preservative, e.g., phenolic compounds. The activated carbon especially that produced under steam atmosphere exhibited a relatively high surface area that is suitable as an adsorbent. Secondly, the model aqueous solution of a single harmful phenolic compound contained in the pyroligneous acid was subjected to batch equilibrium adsorptions with the activated carbon prepared by the above treatment. The activated carbon effectively adsorbed the phenolic compounds in the pyroligneous acid. The overall capacity of the rubberwood activated carbon to fully adsorb any component in pyroligneous acid was considerably higher than the amount of the component produced from the same rubberwood sawdust, showing the feasibility of treating the wastewater containing pyroligneous acid with the activated carbon. Finally, based on these experimental results, we synthesized the outline of an improved sawmilling process: the wood residues are thermally treated to obtain pyroligneous acid, activated carbon, and off-gas; the pyroligneous acid is used as wood preservative instead of the conventional toxic one, the wastewater containing the pyroligneous acid is treated by the activated carbon, and the off-gas with addition of residues is used as a heat and energy source for these additional operations. The proposed method is promising and reliable to give the desired performance to improve the manufacturing process and overcome environmental problems.