Acid precipitation coupled membrane-dispersion advanced oxidation process (MAOP) to treat crystallization mother liquor of pulp wastewater

Treatment to crystallization mother liquor containing high concentration of organic and inorganic substances is a challenge in zero liquid discharge of industrial wastewater. Acid precipitation coupled membrane-dispersion advanced oxidation process (MAOP) was proposed for organics degradation before salt crystallization by evaporation. With acid–MAOP treatment COD_Cr in mother liquor of pulping wastewater was eliminated by 55.2% from ultrahigh initial concentration up to 12,500 mg·L⁻¹. The decolorization rate was 96.5%. Recovered salt was mainly NaCl (83.3 wt%) having whiteness 50 brighter than industrial baysalt of whiteness 45. The oxidation conditions were optimized as CO₃ = 0.11 g·L⁻¹ and CH₂O₂ = 2.0 g·L⁻¹ with dispersing rate 0.53 ml·min⁻¹ for 100 min reaction toward acidified liquor of pH = 2. Acidification has notably improved evaporation efficiency during crystallization. Addition of H₂O₂ made through membrane dispersion has eliminated hydroxyl radical “quench effect” and enhanced the degradation capacity, in particular, the breakage of carbon–chloride bonds (of both aliphatic and aromatic). As a result, the proposed coupling method has improved organic pollutant reduction so as the purity of salt from the wastewater mixture which can facilitate water and salt recycling in industry.