Elsevier

Journal of Membrane Science

Volume 569, 1 January 2019, Pages 200-208
Journal of Membrane Science

Algal fouling control in a hollow fiber module during ultrafiltration by angular vibrations

https://doi.org/10.1016/j.memsci.2018.10.029Get rights and content

Highlights

  • An angular vibrating hollow-fiber membrane system was developed in a lab scale.

  • Effect of angular vibration on fouling control was: algal cells > debris > IOM > EOM.

  • Angular vibrations enhanced the rejection of EOM and IOM, especially for IOM.

  • Energy for algal fouling control by angular vibration was less than by axial vibration.

Abstract

In this work, a lab-scale membrane system was developed to accommodate angular vibrations with a hollow fiber module. A series of filtration experiments were designed and implemented to investigate the effects of angular vibrations on fouling by various algae-derived foulants, including algal cells, debris, extracellular organic matter (EOM), and intracellular organic matter (IOM). The experimental results indicated the effects of angular vibrations on fouling mitigation at the frequency of 2 Hz in the following order: algal cells (~ 97.4%) > debris (~ 93.6%) > IOM (~ 81.8%) > EOM (~ 52.3%). The vibrations mainly targeted the cake layer formed by these foulants, but was ineffective for pore blocking which constituted a large portion of EOM fouling. Although the fouling rates were accelerated during the normal ultrafiltration of the combined foulants, the effects of angular vibrations continued to be impressive (67.8–92.6%). This was because the cake layer formation was the dominant fouling mechanism. Furthermore, the vibrations were found to improve the organic removal by alleviating concentration polarization. In this paper, quantitative analysis is presented to demonstrate the potential of angular vibrations to control algal fouling with the relatively low energy consumption of 0.034 mW.

Introduction

Algal blooms impose several adverse effects on aquatic ecosystems and can occasionally be a severe threat to human health. Although flocculation is widely considered as the most cost-effective method for algae removal, excessive dosing may cause additional problems [1], [2], [3]. Recently, ultrafiltration (UF) has been playing an increasingly important role in algal harvesting, since it can completely retain algal cells and is less affected by the feed water quality [2], [4], [5]. Furthermore, with recent significant improvements in membrane properties, the cost of UF membrane processes has considerably reduced [5]. However, membrane fouling due to the accumulation of algal cells, debris, and metabolic matters on the membrane surface and within the membrane pores, is an inevitable obstacle during operation [6], [7]. Thus, controlling algal fouling is of critical importance for enhancing the membrane filtration performance.

Several investigations have focused on algal fouling mitigation strategies by improving the hydrodynamic conditions on the membrane surface. For example, the shear enhancement at the membrane–liquid interface in cross-flow filtration can be easily implemented by increasing tangential fluid velocity along the membrane [8], [9]. However, increases in the cross-flow rate are always coupled with decreases in the transmembrane pressure (TMP), which causes a dramatic reduction in permeate flux. Although aeration is also a common unsteady-state hydrodynamic method used to prevent algal fouling [8], [10], it is difficult to maintain a homogeneous distribution of bubbles across the membrane surface using this method, especially in highly packed hollow fiber modules (HFMs).

According to Jaffrin et al. [11], since the 1980s, dynamic filtration systems have been gradually employed for the control of concentration polarization (CP) and membrane fouling. The mechanism of these systems relies on a high shear rate near the membrane surface that is induced by the relative motion between the membrane and feed solution. Early designs were based on the rotation of a disk near a fixed circular membrane [12] or of membranes in a stationary housing [13]. Vibration is the other dynamic design. An original concept, known as vibratory shear enhanced process (VSEP), incorporates circular membrane stacks with torsional vibrations around a perpendicular axis [14]. With further development, vibration in the axial and transverse directions can be applied to both hollow fiber [15], [16], [17], [18] and flat sheet membranes [19]. The rotating systems and VSEP have been successfully applied to microfiltration (MF) and UF of algae-rich water [20], [21], [22], [23]. However, both systems are confined to flat sheet membranes, which limits their application due to the low membrane area and/or complex structural designs [11]. HFMs can accommodate membranes with relatively large areas while maintaining a compact configuration. Thus, they are preferred for industrial applications in low-pressure filtration. In this study, we focused on the application of vibration in submerged HFMs during the UF of algae-rich water.

The vibration of HFMs can be axial (parallel to the module axis) [17] and transverse (perpendicular to [15], [16], [17], [18] or azimuthal around [24] the module axis). The feasibility of mitigating algal fouling in an HFM by axial vibrations has been explored by Zhao et al. [25], [26], [27]. According to these investigators, the flux decline rate caused by algal cells and their extracellular organic matter (EOM) can be reduced by over 90% at a vibration frequency of 10 Hz. Unlike the random bubble motion, the axial vibration motion occurs only at the liquid–membrane interface, with the induced shear playing a role between 0 and 0.5 mm from the membrane surface [25]. This laminar flow can certainly prevent damage to the integrity of the algal cells [27]. However, it may also limit the improvement of mass transfer at the membrane surface. Compared to axial vibrations, transverse vibrations prevent the vertical lifting of membrane modules, while they potentially increase secondary flows around the hollow fibers, even at the low vibration frequency of 1 Hz [15], [17]. Most of the literature on transverse vibration systems only focus on the filtrations of yeast, bentonite, alginate, and anaerobic bioreactor effluents, where the HFM vibrations are perpendicular to the fiber length [15], [16], [17], [18]. Few studies explore the effects of transverse vibrations in azimuthal directions on membrane fouling in an HFM [24], although the space requirement for torsional oscillations of an HFM around the module axis is smaller than vibrations perpendicular to the axis. Moreover, the application of angular vibrations to the filtration of algae-rich water has not yet been evaluated.

Compared to model foulants, algal culture contains more complex components with specific properties. This may lead to varying influences of vibrations on filtration performance, since the function of shear force is highly dependent on foulant contents [18], [19] and properties such as size/molecular weight (MW) distribution [17], [18] and electronic charge [28]. Zhao et al. [26] compared the effects of axial vibrations on algal cell and EOM fouling mitigation. However, the components of algal culture are not limited to intact cells and EOM. Algal cells may disintegrate into inhomogeneous pieces, thereby releasing intracellular organic matter (IOM) in the death growth phase or under certain adverse circumstances (e.g., high pump shear) [29], [30]. However, there are still no comparative studies of the effects of vibrations on fouling by these components. Furthermore, when these foulants co-deposit on the membrane surface, the interplays between them can alter the original membrane–foulant and foulant–foulant interactions [7], [31]. The consequent change in vibration performance is unclear.

This work aimed at investigating the effects of angular vibrations on the control of fouling by algal cells, debris, EOM, IOM and their mixtures during UF in an HFM. The influence of angular vibrations on foulant removal was also investigated for these various feed solutions. Moreover, the energy consumption of angular vibrations was calculated and compared with that of axial vibrations.

Section snippets

Cultivation of algae

A stock culture of Microcystis aeruginosa (M. aeruginosa) was purchased from the Wuhan Institute of Hydrobiology, Chinese Academy of Sciences. M. aeruginosa was cultivated in a BG11 medium, under a light regime of 12 h fluorescent light/12 h dark at 25 °C [32]. Cell population was monitored in a blood counting chamber with a microscope.

Preparation of feed solution

To separate algal cells and EOM, the algae culture in the stationary growth phase with a cell concentration of 0.9–1.2 × 1010 cells/L was centrifuged at 4000 rpm

Characteristics of algae-derived foulants

As shown in Fig. S2, the particle size distributions of algal bodies, including cells and debris, displayed high diversity between different feed solutions. Algal cells had a narrow particle size distribution with a small average particle size of ~ 5 µm. However, the debris had a wide distribution from fine particles (~ 5 µm) to large aggregates (~ 120 µm) due to self-aggregation caused by IOM. The result is consistent with those reported by Liu et al. [33]. Part of the IOM adhered to the

Conclusions

A lab-scale angular vibrating HFM system was used to investigate the UF fouling behavior of various algae-derived foulants. The following conclusions can be drawn:

  • (1)

    Applying angular vibrations to an HFM considerably retarded the cake resistances in algal fouling. The efficiency of the fouling control via angular vibrations was in order of algal cells > debris > IOM > EOM. The vibration effect was closely related to the MW distribution and the hydrophilicity of AOM, and the size of algal bodies,

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (Grant no. 51508075), China Postdoctoral Science Foundation (Grant no. 2016M591503), and the Natural Science Foundation of the Heilongjiang Province of China (Grant no. LC2016016).

References (50)

  • M. Frappart et al.

    Reverse osmosis of diluted skim milk: comparison of results obtained from vibratory and rotating disk modules

    Sep. Purif. Technol.

    (2008)
  • S. Lee et al.

    Rotating reverse osmosis: a dynamic model for flux and rejection

    J. Membr. Sci.

    (2001)
  • G. Genkin et al.

    The effect of vibration and coagulant addition on the filtration performance of submerged hollow fibre membranes

    J. Membr. Sci.

    (2006)
  • A. Kola et al.

    Application of low frequency transverse vibration on fouling limitation in submerged hollow fibre membranes

    J. Membr. Sci.

    (2012)
  • T. Li et al.

    Submerged hollow fibre membrane filtration with transverse and longitudinal vibrations

    J. Membr. Sci.

    (2014)
  • A. Kola et al.

    Transverse vibration as novel membrane fouling mitigation strategy in anaerobic membrane bioreactor applications

    J. Membr. Sci.

    (2014)
  • M.R. Bilad et al.

    Harvesting microalgal biomass using a magnetically induced membrane vibration (MMV) system: filtration performance and energy consumption

    Bioresour. Technol.

    (2013)
  • K.J. Hwang et al.

    Filtration flux-shear stress-cake mass relationships in microalgae rotating-disk dynamic microfiltration

    Chem. Eng. J.

    (2014)
  • K. Kim et al.

    Dynamic microfiltration with a perforated disk for effective harvesting of microalgae

    J. Membr. Sci.

    (2015)
  • S.D. Rios et al.

    Antifouling microfiltration strategies to harvest microalgae for biofuel

    Bioresour. Technol.

    (2012)
  • C. Nurra et al.

    Vibrating membrane filtration as improved technology for microalgae dewatering

    Bioresour. Technol.

    (2014)
  • S.C. Low et al.

    A combined VSEP and membrane bioreactor system

    Desalination

    (2005)
  • F.C. Zhao et al.

    Comparison of axial vibration membrane and submerged aeration membrane in microalgae harvesting

    Bioresour. Technol.

    (2016)
  • F.C. Zhao et al.

    Increasing the vibration frequency to mitigate reversible and irreversible membrane fouling using an axial vibration membrane in microalgae harvesting

    J. Membr. Sci.

    (2017)
  • F.C. Zhao et al.

    Using axial vibration membrane process to mitigate membrane fouling and reject extracellular organic matter in microalgae harvesting

    J. Membr. Sci.

    (2016)
  • Cited by (23)

    • Energy-efficient membranes for microalgae dewatering: Fouling challenges and mitigation strategies

      2022, Separation and Purification Technology
      Citation Excerpt :

      Microalgae are of great interest in treating wastewater and manufacturing various high-value products, such as biofuels, vitamin tablets, and cosmetics [1–3], and they have been proposed to capture CO2 from fossil fuel-derived flue gas or from the air by directly converting CO2 into valuable products, enabling a low-cost approach for carbon capture and utilization [4,5].

    View all citing articles on Scopus
    View full text