Preliminary techno-economic assessment of recovering water and caustic soda from alkaline bleach plant effluent

Increasingly stringent environmental regulations and increasing input costs negatively affects the sustainability and competitiveness of the pulp and paper industry. As a result, it has become necessary that broader strategies aimed at helping the industry to improve the environment performance whilst also reducing the operation costs are initiated. Recovering water and chemical from the bleach plant effluent streams could be one of such initiatives. In this paper, a preliminary techno-economic assessment of recovering water and caustic soda from alkaline bleach plant effluent using ultrafiltration and reverse osmosis membrane systems is presented. The scope of this work included; assessing the practicality and the economic performance of this membrane process. In terms of the practicality, the data acquired revealed that the proposed concept can only be applied to the alkaline effluents from the alkaline extraction stage, where caustic soda, oxygen, and peroxide are used as the pulp bleaching chemicals. Economic performance of this membrane system, however, is dependent on the effluent flow rates and the amount of water recovered. To be economically viable, the flow rates must higher than 480 m3/day and water recovery rate need to be higher than 70%. At this optimum effluent flow and water recovery rates, the process cost is approximately R 19/m3. Depending on the flow rates, the investment cost is in the range of R 5-25 million and the payback time is between 3 and 4 years. However, motivations of caustic recovery should be based on environmental rather than economic benefits. Mills can use the detailed information/data acquired in this study to determine whether it would be beneficial for them to implement this type of integrated waste management strategy in their processes.