Membrane fouling : mechanisms, modeling, and mitigation

Date

2019-08-14

Authors

Kirschner, Alon Yeshayahu

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Abstract

Membrane systems are used for water treatment in many industries due to their small footprint, low chemical and energy use, and ease of operation. However, membrane fouling remains a challenge, especially for highly concentrated feeds. Fouling increases hydraulic resistance, lowers water permeance and increases energy consumption. Fouled membranes require expensive cleaning or replacement, increasing operating costs. This study focuses on understanding fouling mechanisms in constant flux crossflow operation, commonly used in industry, and on development of novel fouling-resistant membrane coatings. A model combining two accepted fouling mechanisms, intermediate pore blocking and cake filtration, was developed to describe fouling in constant flux crossflow ultrafiltration (UF). The model was fit to experimental fouling results using rigid and deformable particles. Observations of the model’s accuracy at different fluxes shed light on the physical meaning of the threshold flux: the threshold flux is the flux below which cake buildup is negligible and above which cake filtration becomes the dominant fouling mechanism. Further development of the model may enable fouling prediction. To mitigate fouling in oil-water separations, two novel membrane coatings were developed. The first coating was based on polydopamine (PDA), a well-established fouling-resistant coating material. Poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), a polymer zwitterion, was co-deposited with PDA to form a composite coating on polysulfone (PS) UF membranes. Fouling experiments showed that addition of PMPC to PDA significantly improved fouling resistance. The difference in fouling performance is likely due to the strongly hydrophilic surface properties contributed by PMPC. The co-deposition method opens opportunities for expansion of the concept in which PDA acts as a robust platform for the integration of non-fouling co-adsorbates. The second coating addresses a weakness of PDA coatings – their sensitivity to aqueous chlorine. Chlorine is widely used as a disinfectant in water purification processes. Chlorine oxidation results in rapid removal of PDA coatings from membrane surfaces, rendering them vulnerable to fouling. Poly(N-methylaniline) (PNMA) is a polyaniline derivative which contains a tertiary amine, rather than a secondary amine as in PDA, making PNMA less vulnerable to chlorine oxidation. PNMA-modified membranes were more stable and had higher fouling resistance than PDA-modified membranes after chlorine exposure.

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