Combined alginate-humic acid fouling mechanism and mitigation during microfiltration: Effect of alginate viscosity

Highlights

• Presence of humic acid accelerated alginate fouling under continuous filtration.

• Alginate-humic acid fouling mechanism was dependent with flux and alginate nature.

• Intermittent filtration of alginate-humic acid promoted reversible fouling.

• Clean water (50 °C) flushing was effective in alginate-humic acid fouling control.

Abstract

In membrane-based water and wastewater treatment processes, understanding the interactive effect of various types of organic components on membrane fouling is crucial in order to identify effective fouling mitigation strategies. In this study, the effect of humic acid on alginate filtration was examined under both dead-end and crossflow constant flux microfiltration conditions using low viscosity and medium viscosity alginate as model foulants. The presence of humic acid (100 mg/L) appeared to accelerate alginate fouling under both dead-end and crossflow filtration conditions, following cake layer filtration mechanism (dead-end) and a combined intermittent pore blocking and cake layer filtration mechanism (crossflow) respectively. Under the crossflow filtration condition, the fouling mechanism of the alginate-humic acid mixture was dependent with the permeate flux and alginate nature, e.g., less pore blocking with more cake layer fouling were present at a higher permeate flux during filtration medium viscosity alginate-humic acid mixture. When intermittent relaxation was employed during crossflow filtration, the presence of humic acid could alleviate medium viscosity alginate fouling. This was attributed by a significant shift of irreversible fouling to reversible cake layer fouling, which was readily removed by shear force during relaxation. A comparison of membrane performance during intermittent filtration with periodical flushing using clean water, NaClO (5%), and persulfate (100 mg/L) at 50 °C indicates that high temperature clean water flushing was effective in maintaining membrane performance during filtrating alginate-humic acid complex, which majorly contributed to a reduction of reversible cake layer fouling.

Introduction

Globally, many countries and regions are facing increasing challenges associated with water scarcity and water pollution. To protect sustainable environments and human health, the UN has addressed the sustainable development goal, aiming to ensure access to water and sanitation for all by 2030 [1]. In recent years, membrane-based separation processes have been widely applied in treating nature water resources (e.g., river water, lake water, seawater) for producing drinking water [[2], [3], [4]] and various types of wastewater (e.g., municipal and industrial wastewater) for their safe discharge to environments [5,6].

However, membrane fouling has been recognized as a major obstacle during membrane filtration of water and wastewater, leading to reduced water productivity and increased operation cost [7]. This phenomenon is attributed by particles, microbial cells, colloids, and soluble organics/inorganics derived from water and wastewater treatment processes, which potentially block the membrane pores or/and deposit on the membrane surface [8,9]. Especially, plenty of research work has emphasized that the soluble organics, including polysaccharides-, proteins-, humic acid-like substances play major roles in causing membrane fouling in various membrane-based water and wastewater treatment processes [[10], [11], [12], [13]]. It should be noted that the individual organic contributions to membrane fouling were dissimilar in the reported studies, possibly associated with different feed water characteristics, operation conditions, membrane properties, and analytical methods.

Due to the complicated nature of the foulants during membrane-based water and wastewater treatment processes, the investigation on the interactive effect of these organic components (polysaccharides, proteins, humic acid) on membrane fouling has been well performed. Several findings have emphasized that the complex of two or three of these organic substances could contribute more greatly to membrane fouling compared to that of individual component [[14], [15], [16], [17]]. This was thought to be associated with an increased attraction energy between the foulant-foulant complex and the membrane surface, thus accelerating membrane fouling [13,18,19]. On the other hand, several studies have revealed that the structure and molecular compositions of the organic matter could have significant impacts on membrane fouling, such as alginate vs. xanthan gum [20]; different alginate blocks [19]; humic acid vs. fulvic acid [18]. However, so far, the effect of individual organic property on the membrane fouling of the combined organics (such as polysaccharides and humic acid) has not been well investigated, especially under the crossflow filtration mode (generally applied in practical membrane processes).

It is well documented that alginate is a type of polysaccharides, which is the most widely used model foulant for investigating polysaccharides fouling behaviour during membrane filtration [13,19,20]. Generally, alginate consists of blocks of (1,4)-linked β-D-mannuronate (M) and α-L-guluronate (G) residues in an arrangement of consecutive G residues, consecutive M residues, and alternating M and G [21]. A higher content of G residues has been reported to increase alginate viscosity, which could further influence the mechanical behaviours of alginate solution [21,22]. On the other hand, researchers have illustrated that high molecular weight of polysaccharides (such as xanthan gum and pectin) potentially led to severer membrane fouling due to their limited mass transfer along membrane surface [23,24].

In this study, two types of alginate (low viscosity and medium viscosity) and humic acid were selected as model foulants. The membrane fouling behaviours of combined alginate-humic acid solution were monitored under constant flux conditions in both dead-end and crossflow microfiltration (MF) membrane filtration systems. The membrane fouling mechanisms were illustrated by fitting the experimental data with the reported membrane fouling models. The effects of operation conditions (intermittent interval, cleaning protocols) on the fouling potential of combined alginate-humic acid were examined. It is expected that an in-depth investigation on combined organic fouling in terms of different organic properties allow us well understanding membrane fouling mechanisms of soluble organics and further identifying suitable cleaning protocols in membrane-based water and wastewater treatment processes.