基于LIF-PIV/CFD的纳滤浓差极化行为解析、控制机制及在处理EfOM中的应用

The phenomena of concentration polarization (CP) is one important factor to take responsible for the flux decline and membrane fouling in nanofiltration (NF) for water engineering, due to osmotic pressure raise, increase of resistance to permeation (e.g. gel formation and scaling) and fouling susceptibility. However, owing mainly to the difficulty of CP visualization and in-situ characterization, numerous previous researchs had to focus on mathematical modeling and indirect measurement, making the inconsistent results on CP behavior, and constraining its understanding and effective control . This project is proposed to investigate the mechanism of CP formation and its transformation behavior in NF by means of CFD simulation and advanced visualization-imaging technique of non-invasive laser-induced fluorescence (LIF) and micro-scale particle image velocimetry (Micro-PIV). With CFD simulation and PIV observation, the CP hydraulic boundary layer can be well visualized. Accompanying with the mass transfer analysis, the interactive correlation among feeding characteristics, hydraulic dynamic profile, membrane surface properties and CP behavior will be obtained, which is essential to understanding of CP mechanisms and NF process. The LIF technique will be used to directly determine the rate of nano-size particle deposition onto membrane surface around polarization layer, and hence to analyze the relationship between CP profile and fouling layer formation. Special attention will be paid to the influence of membrane surface properties, including surface morphological characteristics and roughness, on the CP layer and its minimization function onto nano-particle deposition through CFD modeling and LIF-PIV investigation. Accordingly, the optimum NF membrane surface properties will be determined and fabricated, which will be used to treat effluent organic matter to comprehensively investigate its functions in terms of CP control and fouling alleviation. Therefore, the results herein will be very useful to understanding the NF process and CP behavior, which is also important to the NF application.

浓差极化是影响纳滤水处理应用中分离效能的主要因素之一，由于较难可视化和准确表征，目前的研究主要是数值模拟和间接测定，限制了对其行为机理与控制的深入理解。本申请利用激光诱导荧光-显微粒子图像测速技术与计算流体力学（LIF-PIV/CFD）研究纳滤分离不同特征基质过程的浓差极化行为与控制机制。通过LIF-PIV可视化表征与CFD模拟，结合传质过程解析，揭示浓差极化与基质特性、膜表面特征及水力动力学的内在关联；分析纳米颗粒在极化边界层附近的沉降行为，解析浓差极化与膜污染层发展的关系；研究膜表面特征变化（粗糙度与微观形貌改造）改变水力流场进而破坏极化稳定化与压缩极化层效应，结合LIF-PIV解析颗粒沉降行为变化，形成基于膜表面优化的极化控制与膜污染缓解方法，并在处理污水二级生化出水有机物过程中进行应用研究。本申请可望深入理解纳滤与浓差极化行为，为纳滤在污水深度处理中的高效低耗应用提供理论依据。

浓差极化（Concentration polarization, CP）是影响纳滤分离效能的主要因素之一，本研究在分析纳滤(Nanofiltration, NF)膜通量衰减行为和影响因素的前提下，通过激光诱导荧光（Laser Induced Fluorescence，LIF）, 粒子图像测速（Particle Image Velocimetry，PIV）可视化技术和应用计算流体力学（Computational Fluid Dynamics，CFD）模拟，揭示CP与膜表面水力条件的内在关联，解析CP与膜污染层发展的关系。错流式NF分离EfOM实验表明：过滤初期CP层的形成造成快速的通量衰减，且膜通量衰减程度及速率与EfOM中腐殖质和蛋白质类物质密切相关。为进一步确定CP层的特征行为与影响因素，构建了LIF-PIV实时表征方法，成功实现了溶质分离阶段NF表面CP层的可视化表征。在NF分离HA过程中，CP层稳定后的浓度为初始浓度的2~5倍，其厚度分布在15~30 μm，提高错流速度可以有效缓解CP的形成。分析了不同进水基质下CP的传质系数、稳定化时间与阻力等特征参数，结果表明亲水性多糖类有机物海藻酸钠（sodium alginate，SA）形成的CP传质系数（7.69~28.81×104 cm/s）显著低于相同操作条件下腐殖酸（humic acid，HA）类的（2.06~4.72×103 cm/s），海藻酸钠溶液会造成CP较大的溶质传递比阻。同时，纳滤分离SA过程中，CP稳定速率也较快，约20 min内稳定，主要原因是多糖类物质溶质分子间较大的作用力造成的。快速形成的高比阻CP可加速生成表面密实的凝胶层，促进膜污染的迅速发生。在准确确定CP层特点的基础上，利用CFD数值求解膜腔内部流体流动的微分方程，在稳态控制方程中引入动态控制方程，利用CP的可视化结果对模型进行实验参数校准，建立准确模拟NF错流过滤过程CP行为的CFD模型。实时模拟不同位置处的污染物质浓度，动态模拟CP的变化过程。最后，利用CFD模型研究膜表面微观形貌特征对CP的干扰与破坏作用。结果表明，相比于三角形和光滑形貌，圆弧形膜形貌能够提高CP内的传质系数，有效破坏CP层。