Investigation of Silt Density Index and Modified Fouling Index Responses to Fouling Concentration and Particle Size

Abstract The primary constraint that hinders the efficiency and structural integrity of membrane systems is membrane fouling. This study assesses the sensitivity of two prevalent fouling indices, the Silt Density Index (SDI) and the Modified Fouling Index (MFI-0.45), in relation to different types of pollutants, their concentrations, and particle sizes. The findings indicate that MFI-0.45 exhibits a markedly greater sensitivity to variations in pollutant concentration and particle size than SDI. MFI-0.45 values for alumina nanoparticles and silica particles demonstrated up to a tenfold increase, whereas SDI values displayed negligible variations, signifying their restricted sensitivity. Likewise, organic contaminants such as sodium alginate and humic acid exhibited a sixfold increase in MFI-0.45 values, but SDI outcomes remained erratic or undetectable. In mixed foulant scenarios, the incorporation of humic acid (1–5 mg/L) into 10 mg/L silica solutions reduced SDI values (e.g., from 5.69 to 4.13), likely due to the adsorption of organic molecules onto particle surfaces, which diminished direct deposition on the membrane; however, the measurement of SDI values was unfeasible when sodium alginate was mixed with particles, as the membrane filters became rapidly and entirely obstructed. For MFI-0.45 mixed foulant scenarios, addition of humic acid (1–5 mg/L) to alumina nanoparticles (5 mg/L) resulted in a substantial increase in MFI-0.45 values (e.g., from 2.14 to 3.73 s/L 2 ).The addition of humic acid (1–5 mg/L) incrementally resulted in a significant linear increase in MFI-0.45 values for silica particulates (10 mg/L), which was indicative of the formation of more resistant, denser fouling layers. When combined with particles, sodium alginate induced a rapid decrease in flux, which was followed by complete clogging, resulting in elevated MFI-0.45 values. This behavior was consistent with that observed in solo experiments. MFI-0.45 effectively captured the cumulative fouling potential, whereas SDI remained largely unaffected by the presence of organic pollutants. The response of SDI to fluctuations in foulant concentration and particle size in the feed solutions was often inadequate, while MFI-0.45 values demonstrated a high level of consistency as the particle size and concentration increased.