Critical Flux Behavior and Mass Transport in Fouling of Forward Osmosis Membrane Filtration

Abstract

Forward osmosis (FO) membrane process has emerged as a promising technology for application in desalination and water treatment. The integrated FO-RO process could carry out both desalination and water treatment simultaneously if seawater and wastewater (WW) are employed in the FO membrane process. However, fouling can be an anticipated and inevitable issue in the FO process. Membrane fouling is closely related to the concept of the critical flux. This concept has been well applied in the pressure driven membrane process. Nevertheless, hitherto, little attention has been directed to the role of the critical flux of osmotically driven processes (FO) in controlling membrane. Therefore, a systematic study on the critical flux of FO is necessary for fouling control in wastewater reuse and desalination. This work not only focus on the comprehensive evaluation of critical flux behavior-based membrane fouling control for wastewater reuse and desalination but also validate mass transport in fouling of FO membrane. The structure of the thesis is divided into 5 specific tasks.

In Task 1 (Chapter III), the existence of critical flux concept in the FO process has been successfully demonstrated through a reliable stepping method (DS concentration stepping). The critical flux behavior in the FO processes was evidently affected by the foulant type and the membrane type. PA-TFC membrane outperformed the CTA membrane in terms of critical flux, which suggests that the former might be favored for practical applications. Organic fouling (organic macromolecules types e.g., polysaccharide, humic acid, protein and their initial concentration) need to be paid attention as they cause serious fouling. This fact is considered in Task 2 (Chapter IV). A comprehensive assessment of PA-TFC FO critical flux behavior for a variety of organic fouling types and concentrations was carried out. Based on these critical values, guidelines for fouling control, of relevance to wastewaters, are proposed. Importantly, the operational flux and complex mixtures had strong influence on the morphology of organic fouling layer. These conditions led to the formation of a cohesive and compact cake layer. This task confirmed that plant operation below the critical flux can still generate a small degree of fouling but this fouling is reversible and reversibility is vital for the minimization of chemical cleaning. A threshold value of 25 LMH was preliminarily proposed for fouling control, at which beneficial energy consumption can be obtained.

Since the flux decline in FO membrane is due to the couple effect of concentration polarizations and fouling, the Task 3 (Chapter V) has been directed towards a quantitative evaluation of the synergistic outcome of concentration polarizations (CPs) and Cake-Enhanced concentration polarization (C-ECP) on overall FO performance. In this task, a good foundation for an understanding of transport phenomena and fouling in the FO process has been achieved. As the results, operation at an initial 34 LMH favored the formation of a thicker and more compact cake layer which resulted in significant increase in both cake structural parameter (four-fold) and cake layer enhanced concentration polarization (ten-fold). After 40 h operation without physical cleaning the additional effect of cake layer enhanced concentration polarization and fouling resistance consumed 25% of the total driving force; the significant internal concentration polarization still had the greatest impact. In contrast operation at the lower flux of 25 LMH generated less fouling with a lower cake structural parameter (119 µm). The resultant flux decline was only 3%.

Understanding the insightful impact of divalent cations mass transport on the complex organic fouling of forward osmosis (FO) membrane is vital when seawater/brine is utilized in a FO hybrid process. In this Task 4 (Chapter VI), the work deeply focused on quantifying the influence of divalent cations on not only critical flux but also the organic fouling mechanism. The presence of divalent cations in aqueous solutions caused a decrease in critical flux values. The fouling mechanism is mainly governed by intermolecular and ion-mediated interactions. The findings suggested the interaction of divalent cations with organic matters close to the membrane surface is more noticeable than that in the feed solution. These interactions combined with an operation at the flux ≥ 30 LMH led to the formation of a compact and cohesive cake layer (4.8 -9.1 µm) where the great divalent cations were deposited on membrane surface after 30 h operation (Ca2+: 162.13 - 256.72 g/m2, Mg2+: 182.60 - 374.38 g/m2). In contrast, 21 LMH below critical value exhibited minor fouling with a very thin cake layer (0.5 µm) and minimized deposited divalent cations (Ca2+: 79.47 g/m2, Mg2+: 46.65 g/m2). Overall, the extended operation of 30 h suggested 16-21 LMH were threshold values for FO fouling control.

For a real practical application, the optimal operating condition under critical flux-based fouling control need to explored. Therefore, the task 5 (Chapter VII) was carried out. In this chapter, we investigated the effect of hydrodynamic conditions on the change of critical/threshold flux. The study comprehensively evaluated different cross-flow velocity (CFV) in the feed side and draw side, being 6.66, 9.99, 12.32, 19.98 cm/s. It was found that the hydrodynamic conditions i.e., different CFV of both sides had a certain impact on the change of the critical/threshold behavior. Critical flux values were ranged 13-18 LMH, depending on choosing hydrodynamic condition. However, to reduce membrane cost, the study indicated that threshold flux should be recommended for a guideline in which it exhibited higher critical values (24-25 LMH). Overall, for a thin film composite membrane and wastewater with a foulant concentration of 160 mg/L an operation of CFV in feed/ CFV in the draw ratio of 1.5-2 is a recommended value to attain the fouling control but retaining a high critical/threshold values.