Antibiofouling performance and mechanisms of a modified polyvinylidene fluoride membrane in an MBR for wastewater treatment: Role of silver@silica nanopollens

Highlights

• Silica nanopollens (SNP) were used as AgNPs nanocarriers for membrane modification.

• Ag@SNP improved Ag delivery efficacy, avoided agglomeration and control Ag⁺ release.

• Long-term MBR tests showed lasting antibiofouling property of the modified membrane.

• The modified membrane had thin fouling layer that was comprised of mainly dead cells.

Abstract

Biofouling remains to be one of major obstacles in membrane bioreactors (MBRs), calling for the development of antibiofouling membranes. Silver nanoparticles (AgNPs), being a kind of broad spectrum bactericidal agent, have been widely used for modifying membrane; however, uncontrollable release of AgNPs and thus a short lifetime of modified membranes are thorny issues for the AgNPs-modified membranes. In this study, silica nanopollens were used as AgNPs nanocarriers for membrane modification (ASNP-M), which could improve silver delivery efficacy, avoid agglomeration and control Ag⁺ release towards bacteria. At a silver loading of 107.7 ± 10.9 μg Ag/cm², ASNP-M effectively inhibited growth of Escherichia coli and Staphylococcus aureus, with an Ag⁺ release rate of 0.5 μg/(cm² d). Long-term MBR tests showed that ASNP-M exhibited a significantly reduced transmembrane pressure increase rate of 0.88 ± 0.34 kPa/d which was much lower than that of two control membranes, i.e., pristine membrane (M0) (2.32 ± 0.86 kPa/d) and Ag@silica nanospheres (without spikes) modified membrane (ASNS-M) (2.25 ± 1.28 kPa/d). No significant adverse influences on the pollutant removal were also observed in the reactor. Foulants analysis revealed that biofilm of ASNP-M was thinner and comprised of mainly dead cells, and only organic matter with strong adhesion properties was allowed to attach onto the membrane surface. Bacterial community analysis suggested that the incorporation of Ag@silica nanopollens inhibited colonization of bacteria which are capable of causing membrane biofouling (e.g., Proteobacteria and Actinobacteria). These findings highlight the potential of the antibiofouling membrane to be used in MBRs for wastewater treatment and reclamation.

Introduction

Membrane bioreactors (MBRs) have been widely used in municipal and industrial wastewater treatment, which have prominent advantages over conventional activated sludge (CAS) systems including smaller footprint, less sludge production and improved effluent quality (Ma et al., 2018; Meng et al., 2017). However, membrane fouling, which causes frequent chemical cleaning and ultimately shortens membrane lifespan, remains as a challenge in MBRs (Matin et al., 2011; Le-Clech et al., 2006). Biofouling, due to unwanted deposition, growth and metabolism of microorganisms on membrane surfaces and/or inside membrane pores, is considered as a thorny issue limiting applications of MBRs (Deng et al., 2016; Zhang et al., 2016a).

Membrane surface properties have a great impact on biofouling behaviors, and an attractive strategy for biofouling control is to construct antibiofouling membrane surface that can inhibit bacterial proliferation and retard biofilm formation (Zhang et al., 2016a; Zodrow et al., 2009). Recently, the combination of polymeric materials with silver nanoparticles (AgNPs) has attracted much attention in membrane fabrication due to their strong disinfection capability (Yang et al., 2016; Zhang et al., 2012a). The membranes are usually fabricated by incorporating AgNPs in the membrane directly or immobilizing AgNPs on membrane surfaces (Ben-Sasson et al., 2014; Yin et al., 2013; He et al., 2017; Huang et al., 2016). However, the uncontrollable release of AgNPs often leads to a fast loss of antibacterial ability and thus a short lifetime of AgNPs (Yin and Dang, 2015; Huang et al., 2014). To tackle these problems, extensive efforts have been dedicated to developing nanomaterials to immobilize AgNPs and improve antimicrobial efficiency, including nanozeolite (Wu et al., 2015), silica nanospheres (Huang et al., 2014), graphene oxide (Mahmoudi et al., 2015), and biocarrier (Zhang et al., 2012a).

The initial loading mass of Ag and the rate of Ag⁺ release hold the key to the efficacy and life-time of the biofouling resistance of Ag-modified membrane (Zhang et al., 2012a). Sufficient Ag loading as well as stable Ag release are both of great significance for long-term performance. Inspired by nanopollens which can easily adhere to the hairy legs of a honey bee for pollination (Song et al., 2016; Barrier et al., 2011; Thorp, 2000) due to their large specific surface area and enhanced interaction with microbes, we developed biomimetic materials, silica nanopollens, as an AgNPs nanocarrier for membrane preparation. Silica nanopollens, which have a structure of nanoscale channels as well as numerous spikes on the outer shell, were supposed to improve silver delivery efficacy, avoid agglomeration and control Ag⁺ release towards bacteria.

In this study, silica nanopollens loaded with AgNPs were synthesized and used for microfiltration membrane fabrication during phase-inversion process. We systematically investigated the antibiofouling performance and mechanisms in a long-term operated MBR for treating municipal wastewater. The evolution of transmembrane pressure (TMP) and effluent quality was monitored, and the antibiofouling mechanisms were examined by confocal laser scanning microscope (CLSM), real-time PCR and quartz crystal microbalance with dissipation monitoring (QCM-D). Furthermore, Illumina Miseq was employed to analyze the potential influences on microbial communities of the biofilm formed on modified membranes.