Membrane technology is a promising solution to address the global challenge of water scarcity. This issue can be tackled through two main approaches using membranes: desalination of saline and brackish water and the recycling of municipal and industrial wastewater as a valuable source of clean water [1]. Significant progress in water desalination has been made in the Middle East, where nearly half of the world's desalination capacity is located [2]. Treated wastewater reuse is common in Western Europe and the MENA region, but densely populated areas in South and Southeast Asia often lack efficient wastewater treatment [3]. The treatment of saline water primarily involves the reverse osmosis (RO) process, while sewage effluent treatment is a more complex process, involving microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis. Emerging processes such as electrodialysis (ED), forward osmosis (FO), and membrane distillation (MD), membrane bioreactor (MBR) treatment have also been considered for developing zero liquid discharge cycles [4]. Despite the advantages of membrane processes, fouling remains a significant challenge. Membrane fouling occurs through the deposition of insoluble salts, macromolecules, colloids, and particles on the membrane surface or inside membrane pores. Four main types of fouling can be distinguished based on the separation process and foulant properties: inorganic (scaling), organic, colloidal, and biofouling (Fig. 1) [5].

Inorganic fouling, or mineral scaling, occurs primarily in RO or NF processes due to concentration polarization effects when the concentration of slightly soluble salts surpasses solubility limits [6]. Organic fouling is caused by the deposition and accumulation of organic substances, such as humic substances and polysaccharides. Colloidal fouling occurs due to the deposition of dissolved particles, including inorganic and organic colloids. It occurs in two stages – at the first stage colloids deposit in the pore reducing the aperture. With time this process can leads to the complete blocking of membrane pores. On the second stage the formation of a thick cake layer on the membrane surface occurs [7]. Biofouling involves the adhesion and accumulation of microorganisms with biofilm development on the membrane surface [8]. As a consequence, the developed fouling layer affects membrane performance in two distinct ways: the formed cake introduces additional hydraulic resistance, suppressing permeate flux; meanwhile, the foulant layer creates stagnant zones, amplifying concentration polarization. This results in increased osmotic pressure difference between the feed side boundary layer and the permeate side of the active layer, as well as a reduction in permeate flux and membrane selectivity. Biofouling processes contribute to membrane biodeterioration, causing damage to the membrane structure through the release of acidic byproducts of microbial biological activity. To sustain membrane performance and address fouling, chemical cleaning processes are commonly employed. However, these cleaning procedures disrupt membrane operation and contribute to a shortened membrane lifecycle, as well as increasing plant operating costs and environmental impacts [9].

Various strategies and techniques are developed to prevent fouling during membrane water treatment processes. Feedwater pretreatment techniques, such as disinfection, pH adjustment, and coagulation-flocculation are commonly used. Optimizing process parameters such as temperature, pressure, and hydrodynamic conditions can also reduce fouling. For removing the fouling layer physical cleaning (including air flushing, back washing etc.) and chemical cleaning procedures using acids, bases, surfactants, and chelating agents are utilized [5]. At the same time, the significant impact of membrane surface properties on fouling deposition has led to extensive research on development of surface modification techniques for existing membranes and the development of novel membranes with improved antifouling characteristics. These techniques aim to enhance antifouling properties through passive strategies, preventing foulant adsorption or facilitating fouling removal, and active strategies such as deactivating cells or decomposing foulants under external influence (Fig. 2) [14]. The choice of the fouling mitigation strategy depends on the fouling type.

For quantitative description of membrane antifouling, characteristics such as flux decline ratio (FDR) and flux recovery ratio (FRR) are typically used:FRR%=Jw2Jw1∙100where Jw1 -pure water flux before fouling experiment and Jw2 – pure water flux after fouling experiment and membrane cleaning/flushing.FDR%=Jf1−Jf2Jf1∙100where Jf1 and Jf2 – corresponds to initial and final fluxes during the filtration of feed stream containing foulants. The flux loss occurs due to reversible (Rr) and irreversible (Rir) fouling:Rr=Jw2−Jf2Jw1Rir=Jw1−Jw2Jw1

The total flux loss is the sum of the reversible and irreversible fouling: Rr + Rir.