Institute of Environmental Engineering, RWTH Aachen University, Mies-van-der-Rohe-Strasse 1, D52074 Aachen, Germany.
Biological and Environmental Sciences and Engineering Division, Water Desalination and Reuse Center King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia.
Water Res. 2022 Feb 15;210:118031. doi: 10.1016/j.watres.2021.118031. Epub 2022 Jan 2.
The application of membrane technology for water treatment and reuse is hampered by the development of a microbial biofilm. Biofilm growth in micro-and ultrafiltration (MF/UF) membrane modules, on both the membrane surface and feed spacer, can form a secondary membrane and exert resistance to permeation and crossflow, increasing energy demand and decreasing permeate quantity and quality. In recent years, exhaustive efforts were made to understand the chemical, structural and hydraulic characteristics of membrane biofilms. In this review, we critically assess which specific structural features of membrane biofilms exert resistance to forced water passage in MF/UF membranes systems applied to water and wastewater treatment, and how biofilm physical structure can be engineered by process operation to impose less hydraulic resistance ("below-the-pain threshold"). Counter-intuitively, biofilms with greater thickness do not always cause a higher hydraulic resistance than thinner biofilms. Dense biofilms, however, had consistently higher hydraulic resistances compared to less dense biofilms. The mechanism by which density exerts hydraulic resistance is reported in the literature to be dependant on the biofilms' internal packing structure and EPS chemical composition (e.g., porosity, polymer concentration). Current reports of internal porosity in membrane biofilms are not supported by adequate experimental evidence or by a reliable methodology, limiting a unified understanding of biofilm internal structure. Identifying the dependency of hydraulic resistance on biofilm density invites efforts to control the hydraulic resistance of membrane biofilms by engineering internal biofilm structure. Regulation of biofilm internal structure is possible by alteration of key determinants such as feed water nutrient composition/concentration, hydraulic shear stress and resistance and can engineer biofilm structural development to decrease density and therein hydraulic resistance. Future efforts should seek to determine the extent to which the concept of "biofilm engineering" can be extended to other biofilm parameters such as mechanical stability and the implication for biofilm control/removal in engineered water systems (e.g., pipelines and/or, cooling towers) susceptible to biofouling.
膜技术在水处理和再利用中的应用受到微生物生物膜形成的阻碍。微滤和超滤（MF/UF）膜组件中的生物膜生长，无论是在膜表面还是在进料间隔物上，都可以形成二次膜，并对渗透和错流施加阻力，增加能源需求并降低渗透物的数量和质量。近年来，人们已经付出了巨大的努力来了解膜生物膜的化学、结构和水力特性。在这篇综述中，我们批判性地评估了膜生物膜的哪些特定结构特征对 MF/UF 膜系统中用于水处理和废水处理的强制水通过施加阻力，以及如何通过工艺操作对生物膜物理结构进行工程设计，以施加较小的水力阻力（“低于疼痛阈值”）。具有讽刺意味的是，较厚的生物膜并不总是比较薄的生物膜造成更高的水力阻力。然而，与密度较低的生物膜相比，致密的生物膜始终具有更高的水力阻力。文献中报道，密度施加水力阻力的机制取决于生物膜的内部堆积结构和 EPS 化学组成（例如，孔隙率、聚合物浓度）。目前关于膜生物膜内部孔隙率的报告没有得到充分的实验证据或可靠方法的支持，限制了对生物膜内部结构的统一理解。确定水力阻力对生物膜密度的依赖性，可以通过工程生物膜内部结构来控制膜生物膜的水力阻力。通过改变关键决定因素，如进料水营养成分/浓度、水力剪切应力和阻力，可以调节生物膜内部结构，从而降低密度，进而降低水力阻力。未来的研究应该努力确定“生物膜工程”的概念在多大程度上可以扩展到其他生物膜参数，例如机械稳定性，以及对易受生物污染的工程水系统（例如管道和/或冷却塔）中的生物膜控制/去除的影响。
