Modeling and estimation of organic fouling thermal conductivity and its impact on membrane permeate flux in DCMD for wastewater treatment

초록

This study aims to investigate the impact of organic fouling on membrane performance in Direct Contact Membrane Distillation (DCMD) for wastewater treatment, with a focus on the thermal conductivity of the fouling layer and its influence on permeate flux decline. Sodium alginate (SA) and bovine serum albumin (BSA), two common organic foulants, were selected to simulate fouling behavior under various pHs and temperature conditions. The experimental results showed a flux decline during the first hour, followed by a steady state for all conditions. At 35°C of feed temperature, lower permeate flux was registered around 1 L/m2 ·hr (LMH) with no significant effect of pH. Nevertheless, at 50°C, higher flux was observed around 4–6 LMH. Moreover, at 50°C, the effect of pH was more obvious; in fact, increasing pH reduced flux with SA solution while it increased flux with BSA. Numerical analysis based on pore-blocking models identified cake layer formation as the dominant fouling mechanism. This result was confirmed through SEM images. Experiments also showed that fouling decreased membrane hydrophobicity, with contact angles dropping to 40° for SA and 36° for BSA. Moreover, the observation of increased permeate conductivity, especially at pH 8, confirmed the possibility of a wetting phenomenon due to fouling. A numerical model was proposed to estimate the temperatures at the membrane interfaces as well as the thermal conductivity of the fouling layer, based on (i) permeate flux measured experimentally before and after fouling development and (ii) the feed and permeate bulk temperatures. The estimated thermal conductivities for the SA fouling layer varied between 0.46 × 10⁻² and 2.89 × 10⁻² W/m·K. Higher thermal conductivities were estimated for the BSA fouling layer, increasing from 0.89 × 10⁻² to 5.61 × 10⁻² W/m·K. All calculated values were much lower than the thermal conductivities of the respective bulk foulants, which indicates that the obtained organic fouling layer should have more impact on membrane permeability rather than heat resistance. Moreover, the heat resistance of BSA foulants would be more significant than that of SA. These findings highlight the critical role of permeability loss in DCMD fouling and underscore the importance of optimizing operating conditions and developing fouling-resistant membranes to enhance energy efficiency and long-term performance in wastewater treatment applications. © 2025 The Institution of Chemical Engineers

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