Mitigation effect and mechanism of different pretreatment methods on reverse osmosis membrane fouling in coal chemical wastewater treatment

Membrane fouling presents a major challenge in the operation of reverse osmosis (RO) systems, particularly when treating complex industrial wastewaters such as coal chemical wastewater (CCW). This study systematically evaluates the effectiveness of three pretreatment methods, softening, adsorption, and ozonation, on mitigating membrane fouling in RO systems. The results demonstrated that all three pretreatment methods effectively delayed membrane fouling, increasing normalized membrane flux by 17.2 % to 21.9 %. Softening reduced membrane fouling by removing 79.1 % of Ca2+ and 27.8 % of Mg2+, preventing the formation of large amounts of inorganic-organic aggregates. Adsorption and ozonation mitigated membrane fouling by decreasing the concentration, molecular weight, and hydrophobicity of organic matter in CCW. Specifically, ozonation removed 84.7 % of fluorescent organics and 18.2 % of hydrophobic fractions, while adsorption removed 70.8 % of fluorescent organics and 11.0 % of hydrophobic fractions. Interface free energy analysis further revealed the molecular-level mechanisms of membrane fouling, showing that these pretreatment methods effectively reduced the adhesion free energy between the membrane and foulants by altering CCW characteristics, thus significantly delaying the progression of membrane fouling. This study underscores the importance of selecting appropriate pretreatment strategies to enhance the efficiency and longevity of RO systems in industrial wastewater treatment.