Developing better ceramic membranes for water and wastewater Treatment: Where microstructure integrates with chemistry and functionalities

Highlights

• Engineering of ceramic membranes (CMs) leads to better filtration performance.

• Microstructures are integrated with surface chemistry and functionalities in CMs.

• Surface-patterned and gradient CMs are realized by additive manufacturing techniques.

• Functionalized CMs promise for antifouling and high-efficiency filtration.

Abstract

Ceramic membranes are being increasingly applied in water/wastewater treatment, chemical, beverage and pharmaceutical industry, due to their excellent filtration/separation performance, chemical, mechanical, thermal and long-term stability. This work presents a comprehensive review on the structure design, chemistry manipulation and functionalization of advanced ceramic membranes for their better performance in water/wastewater treatment. It begins with looking into engineering the microstructure features of advanced ceramic membranes, especially the intermediate and top active layers, aiming at reducing the mass transport resistance and the likelihood of membrane fouling. Strategies to tune both the porosity and pore configuration in the intermediate layer, minimize their thickness and even complete elimination are then analyzed. Recent advances in surface patterning of ceramic membranes enabled by additive manufacturing techniques are also highlighted. In parallel, emerging methodologies in manipulating the chemistry aspects of the top layer, in terms of surface hydrophilicity and surface charges, are examined, in order to regulate the interactions between the membrane surface and water/foulant molecules. Going beyond the conventional membranes, these functionalized ceramic membranes with the coupling of external stimulus are further involved for high-efficiency filtration and antifouling ability, with the focus on structural optimization at various scales. Finally, perspectives and opportunities on the marriage between microstructure and chemistry are discussed for new generation ceramic membranes and their application in water and wastewater treatment.

Introduction

Membrane filtration as an energy-efficient and chemical-less/free technology has been widely adopted for water and wastewater treatment [1], [2]. Ceramic membranes with intrinsic hydrophilicity and excellent chemical, mechanical and thermal stability are among the competitive candidates [3], [4], [5]. Specifically, the hydrophilic nature enables a high flux and antifouling for separation and filtration in aqueous-based liquid systems. Besides, the chemical stability endows ceramic membrane the ability to be applied with aggressive biological/chemical treatment [6], [7]. The chemical properties of ceramic membranes can be controlled to further enhance the filtration efficiency and retard membrane fouling [8], [9]. Together with outstanding mechanical stability, ceramic membranes possess much-elongated service life (up to 20 years) in water and wastewater treatment.

Ceramic membranes commonly possess an asymmetrical structure, consisting of a macro-porous support for mechanical strength, a top filtration layer with the designed pore size for selective separation and an interlayer(s) with middle-sized pores/microstructure to bridge the two parts [10]. Accordingly, the fabrication of ceramic membranes typically involves the multiple steps of forming the porous ceramic support, intermediate layer(s) and the top layer, where the porous support and each layer requires the preparation of different ceramic slurries/pastes, shaping/casting of the green body and then partial sintering at high temperatures, successively. The overall large footprint of the fabrication process together with the typical starting materials (such as Al₂O₃, TiO₂, ZrO₂, etc.) is responsible for the high manufacturing cost of ceramic membranes. For example, the price of common alumina ceramic membranes is in the range of 300–600 USD/m², which is higher than that of a typical polymeric membrane. To this end, low-cost raw materials [11], [12] and modified preparation processes, such as by lowering the sintering temperature and co-sintering [13], [14], [15], have been explored to reduce the membrane cost. For example, Zou et al. [16] recently reviewed the fabrication of ceramic membranes from the angle of low-energy consumption, types of membrane materials, and innovation in fabrication techniques. Nevertheless, there shall be a balance between the manufacturing cost of ceramic membranes derived from those low-cost materials and their performance typically characterized by key properties, such as mechanical strength and chemical resistance.

On the other hand, the one-time manufacturing cost of ceramic membranes can be largely offset, with improved performance in water and wastewater treatment, longer life span and lower maintenance requirement [17]. In this connection, advanced ceramic membranes can become comparable to the polymer counterparts, in terms of the overall cost for water production [18]. The overall tech-economy, for example, based on an operating period of 20 years for ceramic membranes, can even outperform the polymeric counterparts, owing to the much-reduced frequency in membrane replacement [19], [20]. Ceramic membranes show generally higher tolerance to total suspended solids (TSS) and aggressive chemical/hydraulic cleaning, and their risk of irreversible fouling is therefore lower compared to the polymeric counterparts. Due to the hydrophilic surfaces and high permeability, advanced ceramic membranes can operate at a relatively higher flux and lower fouling than those of a polymeric membrane, which will contribute to further offset in the overall water production cost.

In this regard, alternative approaches have been explored to reduce the overall cost of the produced water, for example, by means of raising the overall filtration performance (i.e., high permeate flux and antifouling), reducing the maintenance cost and elongating the membrane lifetime [21]. Among them, mitigation of membrane fouling has drawn great attention in the past decades, and considerable efforts have been devoted to retarding the membrane fouling, easing the fouling migration and/or understanding the fouling evolution. In membrane-based water and wastewater treatment, membrane fouling will greatly deteriorate the filtration efficiency, by either reducing the permeate flux or raising the transmembrane pressure. In particular, the irreversible membrane fouling requires an aggressive cleaning process, which could increase the maintenance cost and potentially shorten the lifetime. Recently, Li et al. [22] presented a review on ceramic nanocomposite membranes, with focuses on membrane fouling and efforts towards membrane fouling migration. More recently, Zhang et al. [23] reviewed the application of ceramic membranes in water and wastewater treatment, with emphasis on membrane fouling and prevention. Since the membrane filtration process in water and wastewater treatment is largely governed by the interaction between membrane surface and water/wastewater, both the surface chemistry and porous microstructure of ceramic membranes play crucial roles in the membrane filtration process [24]. This topic, however, has rarely been examined in previous review articles.

In the pressure-driven membrane filtration process (i.e., dead-end filtration and cross-flow filtration), the water flux is intrinsically affected by the whole membrane structure. According to the Hagen-Poiseuille (H-P) equation (1) and Darcy equation (2) [25],

$$J=\frac{\mathrm{\Delta }p{d}_m^2\epsilon }{32\mu L\tau }$$

$$J=\frac{\mathrm{\Delta }p}{\mu R_m}$$

where J is the water flux, △p is the transmembrane pressure, d_m is the pore size of the membrane layer, ε is the assumed porosity of the membrane, μ is the viscosity of pure water, L is the membrane thickness, τ is the tortuosity factor and R_m is the resistance of the membrane (m⁻¹), the structural features such as the pore size [26], pore shape [27], level of porosity [28], membrane thickness [29], [30], [31] and tortuosity [32] thus govern the overall water flux. In general, large pore size, high level of porosity and a thin membrane layer with straight pore alignment can ensure a high water flux. However, the increase in surface pore size is meaningless, which will reduce the selectivity and separation efficiency in the membrane filtration process. Therefore, research efforts shall be devoted to increase the level of porosity and/or reduce the membrane thickness and control the surface pore size/configuration. These structural parameters govern the fluid flow behavior in the porous matrix and on the membrane surface. In this regard, the build-up of contaminants on the membrane surface and/or inside the membrane pore channels shall also be deliberately regulated through the structural optimization.

As reported recently [33], surface patterned polymeric membranes show much-enhanced water flux and fouling resistance, because of the enlarged filtration surface area and the induced flow turbulence on the surface. However, due to the inherent hardness and brittleness of ceramic materials, it is challenging to prepare the surface patterned ceramic membranes by traditional template micro-molding methods. The advances in additive manufacture enable the fabrication of ceramic bulks with complex structures [34], providing an unprecedented opportunity to design and fabricate surface patterns on ceramic membranes [35].

In the filtration process, both water and contaminants in the water/wastewater will be in contact with the membrane and pore surface. Therefore, the surface chemistry of ceramic membranes will significantly affect the interactions between membrane surface and water/pollutant, and hence the filtration performance [36]. Surface modification has been adopted as an effective strategy to regulate the chemical and surface properties (e.g., hydrophilicity and surface charge) of ceramic membranes. Typically, a hydrophilic membrane surface would facilitate the water molecules passing through the membrane and increase the water flux. The hydration layer thus formed on the membrane surface can further prevent the attachment of hydrophobic contaminants and benefit the fouling resistance [37]. By minimizing the attraction between the membrane and contaminants, the built-up of the fouling layer on the membrane surface can be slowed down to some extent [38], [39]. Given most contaminants in wastewater are negatively charged [40], the introduction of negative charging on the membrane surface will also slow down the fouling process due to the electrostatic repelling. However, the traditional surface modification usually involves the deposition of a secondary phase on the membrane surface, which inevitably reduces the surface pore size and the level of porosity. As a consequence, the permeate flux will be decreased [26]. Therefore, an innovation in the preparation process is urgently needed to effectively improve the antifouling performance and at the same time enhance the water flux of ceramic membranes.

Besides, the static interactions between the membrane surface and contaminant molecules are relatively weak [40], especially for small-sized molecules in low concentrations. Therefore, to effectively reduce the membrane fouling, ceramic membranes shall be functionalized. As previously demonstrated, certain external stimuli such as electrical field [41] will strengthen the filtration process, and the integration of membrane filtration with catalytic degradation would be able to mitigate fouling and at the same time enhance the filtration performance [42], [43].

In this overview, we will examine emerging approaches in manipulating the microstructure and chemistry of ceramic membrane layers on top of the macro-porous support, aiming at better ceramic membranes for water and wastewater treatment, as shown in Fig. 1. Firstly, recent advances in structural engineering for high permeable ceramic membranes are looked into, including reforming the intermediate layer(s) in the thickness and level of porosity, developing a gradient porous membrane layer and surface-patterning. For the surface chemistry of ceramic membranes in terms of hydrophilicity and charge, several new strategies are highlighted, where the objectives are to reduce the fouling and simultaneously enhance/endure the flux. As an emerging aspect, functionalized ceramic membranes coupled with external stimuli and/or catalytic capability are reviewed, with emphasis on structural optimization and membrane design for better filtration performance. Finally, a summary and perspectives are presented, in terms of the integration between microstructure design and surface chemistry, aiming at further fueling the applications of ceramic membranes in water and wastewater treatment.