Abstract

The coagulation-ultrafiltration short-flow process effectively reduces sedimentation infrastructure costs, yet residual anionic polyacrylamide (APAM) with dissolved organic matter poses challenges to UF membrane fouling control and process stability. During this study, the mechanism by which coagulant APAM residues influence the fouling behavior of UF membranes during the coagulation–ultrafiltration process was investigated through fouling kinetics, morphological analysis, and preliminary molecular dynamics simulations. For bovine serum albumin (BSA, Mw: 133.3 kDa), APAM concentrations lower than 5 mg/L alleviated fouling through charge neutralization between APAM's anionic groups and BSA's cationic residues. Fouling mitigation reversed beyond 5 mg/L APAM, with minimal fouling observed at this threshold. Preliminary molecular dynamics revealed APAM may induce BSA structural reorientation, exposing hydrophilic groups outward to slow fouling kinetics while increasing irreversible pore clogging. For humic acid (HA, Mw: 4 kDa), morphological transitions caused a fouling inflection at 10 mg/L APAM, achieving minimal fouling with 53.41 % irreversibility. HA aggregates evolved through particulate, flocculent, and core-shell structures as APAM increased. Simulation data demonstrated APAM concentrations altered intermolecular forces, triggering abrupt HA floc size changes and structural transformations. The results suggest that controlling the residual APAM concentration in the short-flow process is essential for better managing membrane fouling, offering technical insights for optimizing the stable operation of the coagulation-ultrafiltration process.

Introduction

As global water scarcity intensifies and contamination of drinking water sources becomes more frequent, the demand for clean water continues to rise [1]. Consequently, the development of advanced surface water treatment technologies to secure a safe and reliable drinking water supply has become increasingly urgent [2,3]. The coagulation-sedimentation-ultrafiltration (C–S–UF) process is widely employed in domestic and international surface water treatment projects. This process efficiently removes suspended solids and bacteria, ensuring the water supply's safety [4]. However, the inclusion of sedimentation tanks in this process not only increases construction costs but also extends treatment time.

To address these limitations, researchers have increasingly focused on coagulation-ultrafiltration (C–UF) as a short-process alternative to mitigate the high construction costs associated with sedimentation tanks [5]. Although coagulation effectively removes suspended solids, its ability to remove dissolved organic matter (DOMs) from water is limited. Ultrafiltration technology has garnered significant attention due to its advantages, including stable effluent quality, high turbidity removal efficiency, and effective pathogen removal. Ultrafiltration can partially remove large molecules of DOMs; however, as a primary source of membrane fouling, DOMs compromise the stability of ultrafiltration [6]. When integrated into a short-flow configuration, the C–UF process eliminates the need for sedimentation, simplifying operation and enabling substantial reduction of turbidity and suspended solids concentrations via coagulation coupled with membrane filtration [7]. Unlike conventional C–S–UF systems, short-flow setups expose membranes to residual coagulants and non-settleable DOM within a short period after dosing, introducing distinct membrane fouling risks. Although the term “short-flow” remains informal, such configurations are increasingly adopted in compact or decentralized systems, particularly in space-limited regions like Singapore and Japan. For example, Singapore's Tuas Water Plant has successfully employed short-flow C–UF since 2018 to treat 300,000 m3 of water per day [8]. This growing adoption underscores the need to better understand coagulant–DOM–membrane interactions under these unique conditions. Coagulants have a limited effect on the removal of DOM, and residual coagulants induce a shift in the properties of DOM, which alters the tendency of membrane fouling, and thus affects the stable operation of the process in a short flow.

Among various coagulants, polyacrylamide (PAM) demonstrates superior performance in removing DOMs [9]. As the most widely used synthetic organic polymer coagulant, PAM plays a key role in enhancing effluent water quality. Studies have shown that PAM not only enhances the aggregation of fine particles but also effectively reduces the concentration of DOMs. However, residual PAM in water can contribute to membrane fouling in the subsequent short-flow ultrafiltration process, thereby impacting the efficiency of the ultrafiltration system [10].The combined use of non-ionic polyacrylamide (NPAM) and polymeric aluminum chloride (PAC) in a short coagulation-ultrafiltration process for drinking water treatment has been shown to significantly reduce ultrafiltration and microfiltration membrane fouling [[11], [12], [13]]. In contrast, other studies have shown that cationic polyacrylamide (CPAM) can exacerbate membrane fouling due to the strong electrostatic attraction between the positively charged polymer and the negatively charged membrane surface, leading to the formation of a cake layer of residual PAM on the membrane [14]. Conversely, it has been shown that the cake layer formed by the direct deposition of coagulant can help mitigate membrane fouling [15,16]. Different types of PAMs exert varying effects on membrane fouling, with CPAM exacerbating fouling. Due to the versatility and toxicity of CPAM, anionic polyacrylamide (APAM) is commonly used for flocculation in water purification [17]. However, systematic studies on the mechanisms by which residual APAM affects membrane fouling caused by different types of DOMs remain lacking.

Moreover, the reversible and irreversible transformations of membrane fouling significantly impact membrane performance. Studies have shown that the type of DOMs influences the nature of membrane fouling, with certain DOM types more readily inducing irreversible fouling, thereby accelerating membrane degradation [18,19]. Meanwhile, it has been suggested that interactions between coagulants, such as PAM, and specific DOMs can alter membrane fouling behavior, making fouling more difficult to reverse and significantly increasing the likelihood of irreversible fouling [20]. However, few studies have investigated the reversible/irreversible transformations of membrane fouling induced by APAM concentrations with typical DOMs.

This study aims to address this gap by investigating the impact of residual APAM in a short-flow coagulation-ultrafiltration process on the subsequent membrane filtration behavior. The effect of APAM concentration changes on the potential and particle size was investigated using typical DOMs, including BSA (Mw: 133.3 kDa) and HA (Mw: 4 kDa), which induced membrane fouling shifts and changes in microscopic morphology. Firstly, the effects of variations in the concentration of residual APAM on membrane filtration performance and on the characteristics of the membrane fouling layer were investigated. Additionally, the microscopic morphology and characteristics of the fouling layer were analyzed using atomic force microscopy (AFM). To further investigate the reversible/irreversible transformation of membrane fouling, preliminary MD simulations were conducted to explore the underlying mechanisms. This study elucidates the influence of APAM concentration on the reversible/irreversible transformation of membrane fouling, providing a theoretical basis for understanding the mechanism of residual coagulant-induced membrane fouling. These findings can help optimize the stable operation of the coagulation-ultrafiltration short-flow process.

The effect of APMA concentration on DOM particle size and potential

The variations in zeta potential and particle size of BSA and HA with increasing concentrations of APAM are presented in Fig. 2. For the typical macromolecule BSA, the zeta potential of the solution decreased sharply from −11.0 to −27.2 mV, while the particle size increased significantly from 248.7 to 292.8 nm as the APAM concentration increased from 0 to 5 mg/L. The absolute value of the zeta potential increased with the APAM concentration, which can be attributed to the higher negative charge

Conclusion

This study systematically investigated how residual concentrations of APAM modulate membrane fouling behavior in a short C–UF process using two model foulants: BSA and HA. The key conclusions are as follows:

Filtration, QCM-D adsorption–desorption, and AFM morphology results demonstrated that residual coagulants can induce changes in DOM-induced membrane fouling behavior. Membrane fouling showed concentration-dependent reversibility and structural transformation.