Fabrication of conductive ceramic membranes for electrically assisted fouling control during membrane filtration for wastewater treatment

Highlights

• Coating CNTs onto flat-sheet ceramic membrane surface creates a conductive membrane.

• Conductive ceramic membrane enables electrically enhanced filtration of wastewater.

• Electrostatic force rejects foulants from negatively charged membrane for fouling control.

Abstract

Membrane technology is widely used in water and wastewater treatment. However, membrane fouling remains one of the biggest challenges for membrane applications. In this study, an electrically assisted technique was developed for the control of fouling on flat-sheet ceramic membranes. The novel conductive membrane was fabricated by coating dopamine and carbon nanotubes (CNTs) onto the surface of an α-alumina membrane support to form a conductive CNT coating. The resulting flat-sheet conductive ceramic membrane (FSCCM) exhibited excellent electric conductivity and stability, which performed well in filtration of the synthetic wastewater containing inorganic matter (kaolin solution) or organic pollutants (oil emulsion). By applying a negative charge on the FSCCM with a DC voltage of 2.0 V, the membrane fouling rate was reduced by approximately 50%. The energy consumption rate for the electrically assisted membrane fouling control was only 22.2 × 10⁻³ kWh/m³ in paused-charge mode, with a pause duration of 15 s. A fouling-layer analysis indicted that the imposed electric field greatly reduced the amount of strongly attached foulants on the membrane surface and in the membrane pores. It is believed that the electric field exerted an electrostatic force on the negatively charged pollutants, such as particles and oil droplets, which prevented the foulants from attaching to the membrane surface. This FSCCM-based method provides a clean, effective, and energy-efficient technique for membrane fouling control, thereby enabling high-rate membrane filtration.

Introduction

Membrane technology has been increasingly used in wastewater treatment and reuse, due to its many advantages, such as its maintaining stable and excellent effluent quality, ease of operation, and small required footprint (Padaki et al., 2015). Inorganic membranes, such as ceramic membranes (CMs), have advantages over organic membranes, namely their better physical and chemical stability, higher mechanical strength, greater durability under chemical and high-temperature conditions, and longer service life (Zhao et al., 2019). The rapid decrease in the price of CMs has led to their increased use in recent years. However, membrane fouling remains the biggest challenge to membrane applications, including those of CMs. Numerous efforts have been made to develop new methods and strategies for the effective prevention of membrane fouling during the filtration process. For example, Wu et al. (2006) dosed inorganic coagulants into a submerged membrane bioreactor to mitigate a fouling problem. Wang et al. (2013) used hydraulic flushing to minimize the fouling of ultrafiltration membranes. In addition to modified operations, forward flushing, backward flushing, air flushing, and frequent chemical cleaning, electrical technologies involving electrostatic forces or electrochemical reactions have also been used for membrane fouling control (Xu et al., 2021).

Electrically facilitated membrane fouling control is an innovative technique for fouling mitigation during membrane applications (Trellu et al., 2018). Most potential foulants, i.e., pollutants (e.g., particles, microbial cells, organic macromolecules), in water and wastewater are negatively charged (Leite et al., 2019). Thus, by negatively charging a membrane using electricity, foulant materials that approach the charged membrane will be repelled, resulting in effective fouling minimization. However, the application of this method to CMs (e.g., alumina (Al₂O₃) and titania (TiO₂) membranes) is difficult, because of their non-conductive nature. Previous studies have used attached conductive metals (e.g., stainless steel or titanium (Ti) mesh) to CMs to form conductive membrane modules (Fu et al., 2019). However, such modules are rather complex and their power efficiency is low, due to power lost from the metal surface (Yuan et al., 2019). Conductive membranes may also be made directly from conductive materials, such as Ti₄O₇, a sub-stoichiometric Ti oxide species (Hua et al., 2020; Zaky and Chaplin, 2013). However, such membranes are too expensive to be used for wastewater treatment. Thus, more cost-effective methods need to be developed to fabricate conductive CMs.

Coating a conductive layer onto a membrane surface is a more practical and affordable way to make conductive CMs. Carbon materials, such as graphene, carbon nanotubes (CNTs), and carbon black, have been used to form conductive coating layers (Anis et al., 2021; Chen et al., 2020). However, there are several problems with this method. Specifically, it is difficult to coat carbon materials onto ceramic surfaces; a graphene layer greatly reduces the permeability of a coated membrane; and a coating of carbon black may not afford sufficient electrical conductivity (Pan et al., 2019; Yang et al., 2018). CNTs have a high conductivity and porosity, which can be more suitable for use as a conductive coating material. Zhang et al. (2011) attempted to coat CNTs onto a ceramic membrane by pyrolysis, to generate a CNT-coated membrane for organic wastewater treatment. However, it is difficult to add CNTs to a ceramic surface to create a coating of sufficient bond strength and stability (Ji et al., 2016). There is thus a need for an innovative and more effective coating technique to make conductive CMs, and hence enable electrically assisted membrane fouling control.

In this study, CNTs were used as the coating material on a flat-sheet CM to produce conductive membranes. Instead of directly coating CNTs onto the ceramic surface, polydopamine (PDA) was first coated on the surface of a CM support to create an intermediate adhesive layer, prior to the conductive CNT coating. An DC electric current was applied to the flat-sheet conductive CM (FSCCM) to create an electric field for membrane fouling control during the filtration process. The performance and effectiveness of this electrically assisted method of membrane fouling control were evaluated for the treatment of synthetic wastewaters, such as particle suspensions and oily wastewater. The key factors were investigated and the components of the fouling resistance to filtration were analyzed, thereby revealing the mechanism of fouling mitigation. The energy consumption of electrically assisted fouling control during a high-rate CM filtration process was also estimated.