Coupling effects of organic fouling and inorganic fouling: Influence of organic content and ion concentrations

• HA either exacerbates or inhibits fouling depending on the Ca 2+ concentration. • At low Ca 2+ concentrations (∼20 mg/L), HA increased fouling by over 100%. • At moderate Ca 2+ concentrations (∼50 mg/L), HA reduced fouling by chelating calcium ions. • Increasing Ca²⁺ enhances its interaction with HA, leading to more severe fouling. • Fouling shifted from CaCO 3 in non-HA conditions to organic and particulate with HA. Fouling poses significant challenges in various water distribution systems. Current research primarily focuses on investigating single types of fouling, with limited studies on the combined effects of different types of foulants. This study investigated the interaction between inorganic fouling and organic fouling. Six calcium ion concentrations (20, 50, 100, 150, 200, 250 mg/L, inorganic fouling) and two humic acid (HA, organic fouling) concentrations (0 and 0.4 g/L) were tested in a custom-built system. Results showed a strong coupling effect between HA and calcium ions. In the non-HA treatment group, more than 96% of the blocking material was calcium carbonate; while in the HA treatment group, almost no calcium carbonate was found, mainly organic matter, kaolinite and silica. At low Ca 2+ concentrations (20 mg/L), HA increased composite fouling by 106.1% compared to the non-HA group, primarily due to the inability of low calcium concentrations to form inorganic fouling, leading to an increase in organic fouling with HA application. However, at moderate concentrations (50 mg/L), HA reduced composite fouling by 18.1%, as it chelated calcium ions, effectively inhibiting calcium carbonate formation despite increasing organic fouling and particulate contents. As the concentration of calcium ions increases, they interact with the functional groups on humic acid molecules, resulting in the formation of complexes between calcium ions and humic acid, and exacerbating the deposition of organic matter in the contaminated layer. These findings offer insights for improving fouling management in water distribution systems by adjusting calcium ion and humic acid levels.