Hanna Seppäläinen: Towards Closed Water Loops in Modern Bioproduct Mills: Recycling Acidic D0 Filtrate from ECF Bleaching

This project targets closure of the bleaching water cycle in modern bioproduct mills by recycling the acidic D₀ filtrate as process water and recovering chemicals, thereby reducing wastewater treatment and improving resource efficiency. Feasibility is enabled by today’s lower filtrate volume (~5 m³/ADt), which brings evaporation capacity within integrable range and leverages the ~70 °C feed to cut steam demand. 

Laboratory work applies filtration and neutralization as pretreatments before vacuum evaporation – yielding a condensate composed mainly of water and methanol, while chlorine remains in the concentrate. The concentrate is then incinerated and the resulting brine purified to recover NaCl for potential return to the chlorate plant, closing both water and chemical loops. Performance is evaluated for bleaching operated with HCl pH control or negligible acid addition – conditions that suppress sulfates and emphasize chlorides, and a life‑cycle assessment will quantify environmental and economic impacts of the developed closure concept.