超滤饮用水处理中化学清洗造成膜损伤的机理及其调控研究

Chemical cleaning is a vital step in maintaining the permeability of the membrane, but membrane degradation caused by chemical cleaning might compromise rejection performance and lifetime of the membrane, which would raise the risk of virus contamination in product water and eventually lead to a substantial increase of cost due to membrane replacement. Based on the idea of membrane degradation assessment - degradation mechanism investigation - control of membrane degradation, this project tries to explore the mechanisms and control of degradation of PVDF ultrafiltration membrane. Firstly, the sensitivity of various properties of membrane, including membrane performance, surface properties and microstructure as well as chemical composition, towards cleaning agents such as oxidant, acid and base will be investigated, and proper techniques to assess membrane degradation will be identified. Moreover, in order to illustrate the pathway and mechanism of membrane degradation, the degradation behavior of PVDF polymer and additive will be studied in-depth by QCM-D, and change in composition and molecular structure of membrane material will be characterized systematically. Besides, the influence of chemical cleaning parameters on membrane degradation will be investigated, and accordingly chemical cleaning strategies to minimize membrane degradation without compromising cleaning efficiency will be proposed. This study will provide theoretical and technical supports for the safe and effective chemical cleaning of ultrafiltration membrane in potable water treatment.

化学清洗是超滤系统运行中的必要环节，但化学清洗造成的膜损伤会影响膜的截留性能和使用寿命，使超滤出水的生物安全性降低，并最终导致膜的提前更换而使水厂成本大幅增加。本项目针对饮用水处理中广泛使用的PVDF超滤膜，以膜损伤评价-膜损伤机理解析-膜损伤调控为主线，通过模拟化学清洗实验研究膜的使用性能、界面性质和微观结构及其化学组成等不同性质对氧化剂、酸、碱等清洗剂的敏感性，建立科学合理的膜损伤评价方法体系；利用耗散型石英晶体微天平探明化学清洗剂作用下膜主体材料和添加剂的降解特性，通过多种结构表征手段揭示膜损伤前后膜材料组成和分子结构的变化，阐明化学清洗导致膜损伤的途径和机理；明确化学清洗条件对膜损伤的影响规律，提出能够在保证化学清洗效率的前提下控制膜损伤的化学清洗策略。本研究的预期成果将为饮用水处理中超滤膜的安全有效化学清洗提供理论依据和技术支撑。

化学清洗是超滤系统运行中的必要环节，但化学清洗剂与高分子膜材料及添加剂反应会改变膜的化学组成与理化性质，进而影响膜的截留性能和使用寿命。本研究通过模拟化学清洗实验研究了次氯酸钠清洗条件对膜损伤的影响规律与机理。不同pH的次氯酸钠均使聚醚砜膜对有机物的截留能力下降，纯水通量明显增大，中性和弱碱性条件下损伤最为严重；聚偏氟乙烯膜对次氯酸钠耐受性较好，且不同pH下差异很小。膜表面化学组成分析表明，中性和弱碱性条件下聚醚砜分子链断裂最为显著，在膜上形成了强酸性的磺酸基官能团，pH 3-9范围内大部分亲水性添加剂被氧化，且氧化产物在弱碱性条件下大量流失；在考察的剂量范围内，次氯酸钠未造成聚偏氟乙烯分子结构的变化，但亲水性添加剂在酸性至弱碱性范围内被氧化。针对次氯酸钠清洗造成膜损伤以及清洗过程中与膜污染物和膜材料反应生成卤代有机物问题，评价了过氧化氢作为清洗剂的可行性，指出溶液pH是影响过氧化氢清洗效率的重要因素，利用有机物分子量分布和荷电性分析了过氧化氢对有机污染的清洗机理；考察了碱性条件下过氧化氢对膜理化性质和性能的影响，发现在相同剂量下过氧化氢对聚醚砜和聚偏氟乙烯膜的损伤显著低于次氯酸钠。