Towards closed water loops in modern bioproduct mills: evaporation of organic-acid-rich D ₀ filtrate from ECF bleaching

Abstract In modern bioproduct mills, process improvements have reduced acidic bleaching filtrate volumes to 5 m 3 /ADt, making its treatment increasingly feasible. This study investigates closing the bleaching water loop by evaporating the D 0 filtrate from an ECF D 0 -E op -D 1 bleaching sequence operated without added acids for pH-control. Under these conditions, the D 0 filtrate contains enhanced levels of organic acids. Pretreatment by filtration removed fiber-derived solids and Ca/Si-rich non-process elements, while neutralization improved process safety and cleanliness, and condensate quality. Evaporation produced 4.8 m 3 /ADt of clear condensate suitable for internal reuse, with no detectable chloride, chlorate, or sulfate and methanol as the principal volatile; neutralization suppressed formic and acetic acids to non-volatile salts. The concentrate, after incineration, yielded ash dominated by 61–67 % NaCl, accompanied by carbonate-containing compounds reflecting the high organic-acid feed. Recovery of 13 kg/ADt NaCl for chlorate-plant feed is plausible but will require brine purification to meet vacuum-salt specifications. Overall, D 0 filtrate evaporation enables condensate reuse, recovery of ∼1.5 kg/ADt methanol, and concentrate valorization, advancing water-loop closure in modern bioproduct mills.