Simulation of pulp mill wastewater recycling after tertiary treatment

The aim of this work is to study the possibilities of effluent recycling in a bleached Kraft pulp mill, for a better water management. To avoid problems associated with effluent recycling (corrosion, odors, loss in pulp and paper quality), wastewaters have to be treated before recycling. This study is particularly focused on organic matter removal. Several treatments are applied on a biological secondary effluent: adsorption on activated carbon, coagulation with ferric chloride or alum sulfate, precipitation with lime, ozonation and catalytic ozonation. These techniques are compared in terms of COD (Chemical Oxygen Demand) removal. Catalytic ozonation is finally chosen as the most effective solution to achieve 50% of COD removal in the effluent. The characteristics of the effluent treated according to this technique are then used to simulate the impact of its reuse in the process for pulp production. The study is focused on the changes in these parameters in the various stages of bleaching and final washing when water is replaced by the wastewater treated or directly issued from the wastewater treatment plant. The simulation demonstrates the need of a tertiary treatment to eliminate COD in order to avoid possible reactant overconsumption and decrease in pulp brightness. Chloride and sulfate ions which could cause corrosion should also be removed.