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Journal ArticleDOI

A review of the innovative gas separation membrane bioreactor with mechanisms for integrated production and purification of biohydrogen.

TL;DR: An assessment of the innovative Gas Separation Membrane Bioreactor (GS-MBR), which is an emerging technology because of its potential for in-situ biohydrogen production and separation, finds that its features make it worthy of study.
About: This article is published in Bioresource Technology.The article was published on 2018-09-05 and is currently open access. It has received 31 citations till now. The article focuses on the topics: Membrane bioreactor & Biohydrogen.

Summary (4 min read)

Introduction

  • The scope of the review is to discuss the current state of knowledge and lessons learned on biofouling of membrane separators being used for microbial electrochemical technologies (MET).
  • It is illustrated what crucial membrane features have to be considered and how these affect the MET performance, paying particular attention to membrane biofouling.
  • It seems evident that materials are not similarly suitable for MFC and FC.
  • The transport of ionic substances between the electrode compartments is hindered.

3.1. Membrane-associated aspects

  • As summarized by Williams (2014), this membrane feature is linked to the measure of surface texture and signifies to what extent the membrane surface deviates from an ideally smooth one.
  • As a matter of fact, pores could be obvious places for microorganisms to settle and reproduce themselves, leading to clogging (Lim et al., 2012).
  • Moreover, among membrane qualities, the wettability also deserves attention.
  • Obviously, explained by the findings of research studies.
  • As reported, highly negative surface charges are considered advantageous to counteract biofouling (Kim et al., 2014).

3.2. Microbiology-related considerations

  • Apart from the inherent membrane traits assessed in Section 3.1., a range of microbiology-associated factors also governs biofouling.
  • Accordingly, a membrane with higher dissolved oxygen flux can select more aerobic microorganisms on the membrane (Leong et al., 2013).
  • Thus, studying the impacts of various membrane separators in MET could be suggested to better elaborate how much the actual type of membrane affects the system behavior both in electrochemical and microbiological terms (Sotres et al., 2015).
  • Feasible techniques possibly used for this purpose will be interpreted in the following section.

4. Methods for the evaluation of membrane biofouling in microbial

  • This section introduces methods that could be used to investigate membrane properties, particularly those that are considered to influence its biofouling resistivity/sensitivity (as detailed in Sections 2 and 3).
  • The surface typology/morphology of a given membrane can be studied by scanning electron microscopy (SEM).
  • It reflects the ability of a liquid to wet a surface (Gugliuzza, 2015) and can be associated with the surface tension.
  • The amount of water absorbed by a membrane separator in the course of MET operation can result in swelling, consequently, increasing the thickness and hence the membrane resistance.
  • Water uptake also influences proton conductivity of the membrane, since water molecules can participate in conveying them across the membrane if it is properly hydrated (Daud et al., 2019).

4.3. Methods for assessing the membrane resistance

  • Technical solutions to determine the membrane resistance are based on electrochemical impedance measurements (EIS).
  • Mostly, EIS is done with a potentiostat equipped with a frequency response analyzer.
  • Fig. 1 shows an exemplary Nyquist Plot for illustration.
  • By analyzing the obtained EIS plots, the total internal resistance can be separated to solid + liquid electrolyte, charge transfer and diffusion resistance components (Nam et al. 2010).

4.4. Proton conductivity of membranes

  • The efficient charge balancing (ion and proton transfer) of the membrane is one of the most essential requirements to achieve good operational performance in MET (Bakonyi et al., 2018).
  • In real applications, the widely used PEMs are not ideally selective to proton as they allow the passage of various ions at the same time.
  • Once the amount of cations in the cathode chamber is high enough, transport of protons becomes energetically more favorable (Sleutels et al., 2017).
  • In case of the occurrence of membrane biofouling, meaning that the lose some of its attractiveness (Kim et al., 2016).
  • Specific ionic conductivity measurements can be performed by experimental arrangement following Eq. 2 (Xu et al., 2012): σ = L (R x S)-1 (2) where σ, L, R and S are assigned for the conductivity (S cm-1), the distance between the electrodes (cm), the membrane resistance (Ω) and the area of the membrane (cm2), respectively.

4.5.1. Proton mass transfer

  • It is important to complement the characterization of a membrane with proton mass transfer properties, since these are significantly related to the pH-splitting phenomena.
  • The pH-splitting does refer to the accumulation of protons in the anode chamber and hydroxides in the cathode chamber, as charge balancing ion transfer is realized by other ions than H+/ OH- (Li et al., 2011; Sleutels et al., 2017).
  • After multiplying kH with the membrane thickness, the proton diffusion coefficient of a particular membrane can be displayed (Bakonyi et al., 2018).
  • Alternative to Eq. 3., the so-called transport number (or transference number) can be derived (Harnisch et al., 2008), which shows the “the part of the current that is transported through the electrolyte due to the motion of the ionic species” according to Oliot et al. (2016b).
  • Such an approach was applied by Park et al. (2017) to characterize the proton transport through membrane in twochambered bioelectrochemical reactors.

4.5.2. Bulk ions and substrates

  • The characterization of ions that also contribute to the electro-neutralizing charge transfer can be done by calculating their mass transfer coefficients (𝑘𝐼/𝑆) based on the ion concentration in the compartment with high ion activity (CS,0) and in the receiving compartment with low ion activity (CS), according to Eq. 4 (Xu et al., 2012).
  • Ping et al. (2013) investigated microbial desalination cells (containing a pair of AEM and CEM as well as seawater in between those) during long-time operation (8 months).
  • Therefore, membranes being more permeable to oxygen will promote the growth of aerobic strains on its surface.
  • Hence, it might be assumed that membranes leading to larger oxygen fluxes (from the aerated cathode to the nonaerated anode) are more prone to biofouling and accumulate more biomass on the surface.
  • To determine the oxygen mass transfer coefficient (kO) of a separator, the procedures suggested by Kim et al. (2007) and Chae et al. (2008) could be followed.

4.6.2. Quantification of EPS

  • As noted above, EPS play a key-role in the membrane biofouling process.
  • EPS, in accordance with Jiang et al. (2010), can cover a range of constituents, in particular high molecular-weight polymers, e.g. saccharides and proteins.
  • Therefore, the carbohydrate and protein portions of EPS are distinguished and analyzed separately.
  • Determination of EPS on the membrane requires their extraction first (Kim et al., 2014).
  • Thereafter, carbohydrates can be quantified by phenol/sulfuric-acid method, while proteins can be measured for example via the Lowry method based on protein assay (Jiang et al., 2010; Kim et al., 2014).

4.6.3. Evaluation of the microbial community

  • Heidrich et al. (2016) underlined that methods for quantification of selected microbes include (i) quantitative polymerase chain reaction (qPCR) as well as (ii) fluorescence in situ hybridization (FISH).
  • In addition, Kim et al. (2014) did total cell counting in MFC with Confocal Laser Scanning Microscopy (CLSM) and DAPI (4'6- diamidino-2-phenylindole) staining.
  • As reported by Qiao et al. (2017), the procedure to determine proteins can rely on the colorimetric Bradford Protein Assay and the Lowry method (Xu et al., 2012).
  • Mainly, it can characterize samples (wastewaters, electrode-surface biofilms) in the aspects of evaluates the microbial community structures and activity (both qualitatively and quantitatively) in using almost real-time.
  • The summary of the mentioned methods for (bio)fouled membrane as well as fouling layer characterization is provided in form of Fig. 2.

5. Biofouling experiences with membrane separators

  • Biofouling normally occurs on the side of the membrane that is in touch with the compartment containing the biological catalysts.
  • Accordingly, research strategies should focus on the (i) development of new materials and/or (ii) the improvement of existing ones to withstand against such effects thanks to better surface properties.
  • As concluded by Venkatesan and Dharmalingam (2015b), hydrophilic inorganic composite particles, e.g. TiO2 incorporated in PEM could be useful to improve resistance of the membranes against biofouling.
  • As it was elaborated, the silica-based, functionalized fillers added to Nafion helped to achieve more negative membrane surface charges and hence the composite membranes were more successful to tackle the biofouling phenomena in MFC.
  • Establishment of further concepts can be suggested to broaden this field and provide more options for anti-fouling membrane construction.

7. Conclusions

  • It was overviewed what membrane properties (e.g. mechanical stability or mass transfer) membrane features (mainly surface morphology, charge, structure and hydrophilicity) affect the formation of biofouling layers.
  • Accordingly, two-sided approach – considering the interrelation of membrane properties and biofouling – was suggested to address its complexity.
  • Biofouling monitoring methods (in terms of fouling layer chemical composition, microbial community) were presented and possible directions in membrane development (such as the promising employment of ionic liquids) to counteract biofouling were demonstrated.

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Q1. What are the contributions in this paper?

In this paper, a two-sided approach considering the interrelation of membrane properties and biofouling was suggested to address its complexity.