Organic fouling in membrane separation processes can be influenced by several factors [27,28,29]. These factors are included in the below sections.

Sodium alginate (SA), bovine serum albumin (BSA), Aldrich humic acid (AHA), natural organic matter from the Suwannee River (SRNOM), and octanoic acid (OA) are commonly used to represent different types of organic matter in studies related to water treatment processes. SA is often used to represent polysaccharides, BSA represents proteins, AHA or SRNOM represents terrestrial humic substances, and OA represents fatty acids. Several studies have reported that SA exhibits the greatest potential for membrane fouling among these organic compounds. This can be attributed to the specific interactions between SA and calcium ions (Ca²⁺), which can result in the formation of unique egg-box-shaped gel structures. These gel structures can adhere strongly to membrane surfaces, leading to fouling [30]. The presence of high organic foulants, mixtures of organic foulants, concentration of monovalent and divalent ions in the feed solution, and draw solutes in the feed water resulted in severe organic fouling behavior [31]. The properties of polysaccharides can also affect the degree of organic fouling potential in water treatment processes. The severity of flux decline, which is a measure of the reduction in water flow rate due to fouling, appears to be related to the molecular weight (MW) and solution viscosity of the organic compounds. Larger molecular sizes of organic matter are likely to reduce the shear force associated with the feed cross-flow velocity, leading to a more pronounced flux decline. For example, a study by Xie et al. [28] found that flux decline was more severe on the order of xanthan (1000–50,000 kDa) > sodium alginate (SA) (200 kDa) > pullulan (75 kDa). Xanthan gum, with its high molecular weight and solution viscosity, showed the greatest fouling potential among these polysaccharides.

The fouling aspect of a hollow fiber ultrafiltration membrane (UF) was investigated using combinations of dissolved organic matter (DOM), including humic acid (HA), bovine serum albumin (BSA), and sodium alginate (SA) representing humic substances, proteins and polysaccharides, respectively. The findings showed a considerable correlation between fouling resistance and the concentration of small molecules in DOMs, as well as the solution’s zeta potential, based on statistical analysis. The study found that the impact of small molecules on membrane fouling was more significant compared to the zeta potential of the solution, indicating that the concentration of small molecules in DOMs played a more critical role in determining the fouling behavior of the UF membrane [31]. These findings suggest that controlling the concentration of small molecules in the solution is an important factor in mitigating fouling in UF membrane separation processes.

Operating conditions such as temperature, pressure, and flow rate can also affect organic fouling. Higher temperatures can increase the rate of fouling, while higher pressures can help mitigate fouling. The flow rate can affect the shear stress on the membrane surface, which can impact the extent of fouling.

BSA fouling was significantly pronounced at the pH 4.7 isoelectric point of BSA, where there is a minimum repulsion force between BSA molecules. As the pH moved away from the isoelectric point, the fouling of the membrane became less severe. At pH 3.0, increasing the ionic strength resulted in severe fouling, probably due to compression of the electric double layer (EDL) [32].

The rate of NOM fouling increases with higher ionic strength, pH, and applied pressure due to various mechanisms such as electrostatic repulsion, hydrophobic forces, hydrophobicity, valley blocking, and compaction [33].

Ultrafiltration experiments were conducted on whey proteins at different pH values of 3, 9, and 10. The resulting permeate fluxes were measured as 68 to 85, 91 to 87, and 89 to 125 Lm⁻² h⁻¹, respectively. However, when the pH was close to the isoelectric points of the major proteins (at pH 4 and 5), the resulting permeate fluxes were lower, ranging from 40 to 25 and from 51 to 25 Lm⁻² h⁻¹, respectively. These results suggest that the pH of the protein solution plays a significant role in determining the permeate flux during ultrafiltration. When the pH is close to the isoelectric point of the major proteins, the proteins are less soluble and are more likely to aggregate, leading to reduced permeate flux [32]. Therefore, controlling the pH of the protein solution is an important factor in optimizing the performance of ultrafiltration processes for protein separation.

The properties of the membrane, including surface charge, pore size, and hydrophobicity, can also affect organic fouling. Membranes with a higher surface charge or smaller pore size can be more prone to fouling, while more hydrophobic membranes may be more resistant to fouling. The fouling of polyvinylidene fluoride (PVDF) membranes can be significantly affected by their hydrophobicity and pore size. As the hydrophobicity of the PVDF membrane increases, it becomes more prone to organic matter fouling, which tends to adhere more strongly to hydrophobic surfaces [34]. Similarly, as the pore size of the PVDF membrane decreases, it becomes more susceptible to fouling by organic matter, which can become trapped in the smaller pores and accumulate over time. This can lead to a reduction in membrane permeability and an increase in transmembrane pressure required to maintain a constant flow rate [34]. The fouling of membranes in the presence of NOM can be affected by several factors, such as the membrane surface structure and chemical properties. The rate of NOM fouling increases with surface roughness, membrane charge, and hydrophobicity [33].

In an organic fouling simulation study, dextran (DEX), bovine serum albumin (BSA), and Aldrich humic acid (HA) were used as model foulants representing polysaccharides, proteins, and humic substances, respectively. The study found that hydrophobic interaction was the primary mechanism that influenced adsorptive fouling, rather than electrostatic interaction. The results suggested that the hydrophobicity of both the polyvinylidene fluoride (PVDF) membrane microfiltration and the foulant played a significant role in the adsorptive fouling, with the higher hydrophobicity increasing the extent of fouling [35]. The study gives emphasis to the need to consider both membrane and foulant hydrophobicity in developing effective fouling mitigation strategies for membrane separation processes.

Pretreatment methods such as coagulation or adsorption can affect the concentration and composition of the foulants, which can impact the extent of fouling. Cost-effective pretreatment of wastewater can bring several benefits, such as disinfection, removal of large suspended particles through settling, and reduction of total suspended solids (TSS) in the wastewater. Furthermore, effective pretreatment can also result in a lower fouling propensity of the feed wastewater, which can improve the efficiency and lifespan of the membrane.

Pretreatment techniques may be used to reduce the incidence of membrane fouling in wastewater treatment systems. These methods involve eliminating or altering the compounds responsible for fouling prior to their contact with the membrane surface. Coagulation has been shown to be highly effective in mitigating membrane fouling and is therefore extensively used in multiple industrial sectors for wastewater treatment [36,37].

The choice of membrane material, whether organic or inorganic, can have a significant impact on fouling in membrane-based processes [38]. Organic membranes are typically made from polymers such as cellulose acetate, polyamide, polyethersulfone, or polyvinylidene fluoride. The properties of organic membranes can vary significantly depending on the specific polymer used [34]. Here’s how organic membrane materials affect fouling: The surface properties of organic membranes, such as hydrophilicity or hydrophobicity, play a crucial role in fouling. Hydrophilic membranes tend to be less prone to fouling as they repel organic foulants and promote easier cleaning. However, they may be more susceptible to fouling by inorganic foulants such as colloidal particles or minerals. Hydrophobic membranes, on the other hand, can repel organic foulants but may be more prone to fouling by hydrophilic substances. The pore size and distribution of organic membranes influence fouling by determining the size of particles or solutes that can pass through. Membranes with smaller pore sizes are generally more resistant to fouling by larger particles but may be more prone to fouling by smaller molecules that can penetrate the pores. The surface charge of organic membranes affects fouling by influencing the interaction between the membrane and charged foulants. Electrostatic repulsion between similarly charged foulants and the membrane surface can reduce fouling. Membrane materials can be modified to have a positive or negative charge to enhance fouling resistance.

Inorganic membranes are typically composed of materials such as ceramics, metals, or metal oxides (e.g., alumina, titania, zirconia). Inorganic membranes offer distinct characteristics that can influence fouling behavior [37]: Chemical Stability: Inorganic membranes generally exhibit high chemical stability, making them resistant to degradation when exposed to harsh chemical environments. This stability can reduce fouling caused by chemical reactions or exposure to aggressive substances. Inorganic membranes tend to have superior mechanical strength compared to organic membranes. This strength can help withstand physical stresses, such as pressure or cleaning procedures, reducing the likelihood of membrane damage and fouling. The surface roughness of inorganic membranes can impact fouling. Smoother surfaces typically experience less fouling as there are fewer sites for foulants to adhere to. However, excessively smooth surfaces may promote the formation of a thin, dense fouling layer due to reduced hydrodynamic shear forces. Inorganic membranes often exhibit excellent thermal stability, allowing their use in high-temperature processes. This stability can help minimize fouling caused by heat-induced reactions or thermal degradation of foulants. Overall, the choice of membrane material, whether organic or inorganic, should be carefully considered to mitigate fouling. Factors such as surface characteristics, pore size, surface charge, chemical stability, mechanical strength, surface roughness, and thermal stability all play significant roles in determining the fouling behavior of a membrane.

In general, understanding the factors that contribute to organic fouling is critical to developing effective control and mitigation strategies. By optimizing the operating conditions, membrane properties, and pretreatment processes, it is possible to reduce the extent of fouling and improve the performance of membrane separation processes. A summary of factors influencing organic fouling can be indicated in Table 1.

Factors Influencing Organic Fouling	Description	Reference
Organic composition and concentration in the feed solution	The properties of organic compounds, such as their size, molecular weight, hydrophobicity/hydrophilicity, charge, and tendency to form aggregates, can influence their fouling behavior. Certain compounds may have a higher affinity for membrane surfaces or be more prone to fouling the membrane pores.	[28,29,32,37]
Operating conditions	Operating conditions, including transmembrane pressure, crossflow velocity, temperature, and pH, can influence organic fouling. Higher pressures and velocities can help minimize fouling by reducing the deposition of foulants on the membrane surface. Temperature and pH can affect the solubility and aggregation behavior of organic compounds.	[34,35]
Membrane properties	The material and surface characteristics of the membrane, such as surface charge, hydrophilicity/hydrophobicity, roughness, and pore size, can affect the interaction between the membrane and organic foulants. Surface properties that reduce fouling include hydrophilic surfaces and negatively charged membranes.	[33,35,36]
Pretreatment wastewater	The effectiveness of pre-treatment processes, such as coagulation, flocculation, or activated carbon adsorption, in removing or reducing organic foulants before they reach the membrane can impact fouling.	[38,39]