Now showing 1 - 5 of 5
  • Publication
    Understanding particle deposition kinetics on NF membranes: A focus on micro-beads & membrane interactions at different environmental conditions
    The significance of nanofiltration membrane surface properties when interacting with microbeads with and without permeate flux was investigated. This was achieved by characterising the surface tension and zeta potential of micro-beads and NF90 membranes to determine the colloid–membrane interaction forces. Dynamic adhesion assays under different ionic strengths (0.1 M and 0.01 M) and pH (5, 7, and 9) were conducted. Experimental results showed that at high ionic strength, pH does not have a significant effect on adhesion rates, while at low ionic strength the adhesion rate increased at pH 7 (4.56 s−1 cm−2) compared to pH 5 and pH 9, with rates of 2.69 and 3.66 s−1 cm−2 respectively. A model was devised to predict colloidal adhesion onto membranes under increasing permeate flux conditions, taking into account all interaction forces. Model predictions indicate that drag force overwhelms all other colloid–membrane interaction forces when the permeate flux increases to 7.2 L h−1 m−2. This study suggests that altering membrane surface properties for the prevention of fouling may be limited in its success as an antifouling strategy.
      248Scopus© Citations 12
  • Publication
    Materials Analysis of Bacterial Adhesion and Early-Stage Biofilm Development
    (University College Dublin. School of Chemical and Bioprocess Engineering, 2017) ;
    Bacterial adhesion and the subsequent biofilm formation is a complex phenomenon which has many consequences in water filtration. This aggregation of microorganisms can be difficult to remove from nanofiltration and reverse osmosis membrane surfaces, causing damage and eventual replacement of the membrane. In order to elucidate the cause of this biofilm formation, three influential factors were studied: surface topography, nutrient concentration and shear stress. Analysis was performed on the surface topographical heterogeneities in order to examine the influence of surface topography. Image analysis of the adhesion of Pseudomonas fluorescens (Ps. fluorescens) and Staphylococcus epidermidis (S. epidermidis) to the surface topographical heterogeneities was determined for two commercial membranes, NF270 and BW30, using a flow-cell system. Membrane area analysis, using AFM and SEM, showed up to 13% of topographical heterogeneities on the membrane surface with up to 30% of total adhered cells that were discovered within these topographical heterogeneities. For the analysis of the nutrient availability and shear stress on the structural formation of Ps. fluorescens biofilm under two different dynamic conditions, an air-liquid interface biofilm and a flow cell grown biofilm were assessed by confocal scanning laser microscopy (CLSM). The analysis showed a three-fold increase in the EPS biovolume of the high nutrient air-liquid interface grown biofilm. However, the flow cell biofilm increased the biovolume for low nutrient and higher shear stress conditions, suggesting harsher growth conditions of the biofilm results in greater biofilm development. Finally, the adhesive and viscoelastic properties of the Ps. fluorescens air-interface grown biofilm for two different nutrient dilution factors was determined by nanoindentation. The low nutrient availability showed higher adhesion force and work of adhesion with distributed colonies across the surface, while the high nutrient grown biofilm led to a reduction in the adhesive and elastic nature of the biofilm.
  • Publication
    The effects of extrinsic factors on the structural and mechanical properties of Pseudomonas fluorescens biofilms: A combined study of nutrient concentrations and shear conditions
    The growth of biofilms on surfaces is a complicated process influenced by several environmental factors such as nutrient availability and fluid shear. In this study, combinations of growth conditions were selected for the study of Pseudomonas fluorescens biofilms including as cultivation time (24- or 48 h), nutrient levels (1:1 or 1:10 King B medium), and shear conditions (75 RPM shaking, 0.4 mL min −1 or 0.7 mL min −1). The use of Confocal Laser Scanning Microscopy (CLSM) determined biofilm structure, while liquid-phase Atomic Force Microscopy (AFM) techniques resolved the mechanical properties of biofilms. Under semi-static conditions, high nutrient environments led to more abundant biofilms with three times higher EPS content compared to biofilms grown under low nutrient conditions. AFM results revealed that biofilms formed under these conditions were less stiff, as shown by their Young's modulus values of 2.35 ± 0.08 kPa, compared to 4.98 ± 0.02 kPa for that of biofilms formed under low nutrient conditions. Under dynamic conditions, however, biofilms exposed to low nutrient conditions and high shear rates led to more developed biofilms compared to other tested dynamic conditions. These biofilms were also found to be significantly more adhesive compared to their counterparts grown at higher nutrient conditions.
      4Scopus© Citations 26
  • Publication
    Nanofiltration and reverse osmosis surface topographical heterogeneities: do they matter for initial bacterial adhesion?
    The role of the physicochemical and surface properties of NF/RO membranes influencing bacterial adhesion has been widely studied. However, there exists a poor understanding of the potential role membrane topographical heterogeneities can have on bacterial adhesion. Heterogeneities on material surfaces have been shown to influence bacterial adhesion and biofilm development. The purpose of this study was therefore to investigate whether the presence of membrane topographical heterogeneities had a significant role during bacterial adhesion as this could significantly impact on how biofouling develops on membranes during NF/RO operation. An extensive study was devised in which surface topographical heterogeneities from two commercial membranes, NF270 and BW30, were assessed for their role in the adhesion of two model organisms of different geometrical shapes, Pseudomonas fluorescens and Staphylococcus epidermidis. The influence of cross-flow velocity and permeate flux was also tested, as well as the angle to which bacteria adhered compared to the flow direction. Bacterial adhesion onto the membranes and in their surface topographical heterogeneities was assessed using Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), fluorescence microscopy and image analysis. Results showed that up to 30% of total adhered cells were found in membrane defect areas when defect areas only covered up to 13% of the membrane surface area. This suggests that topographical heterogeneities may play a significant role in establishing environmental niches during the early stages of biofilm development. Furthermore, no noticeable difference between the angle of cell attachment in defect areas compared to the rest of the membrane surface was found.
      386Scopus© Citations 16
  • Publication
    The significance of calcium ions on Pseudomonas fluorescens biofilms – a structural and mechanical study
    The purpose of this study was to investigate the effects of calcium ions on the structural and mechanical properties of Pseudomonas fluorescens biofilms grown for 48 h. Advanced investigative techniques such as confocal laser scanning microscopy and atomic force spectroscopy were employed to characterize biofilm structure as well as biofilm mechanical properties following growth at different calcium concentrations. The presence of calcium during biofilm development led to higher surface coverage with distinct structural phenotypes in the form of a granular and heterogeneous surface, compared with the smoother and homogeneous biofilm surface in the absence of calcium. The presence of calcium also increased the adhesive nature of the biofilm, while reducing its elastic properties. These results suggest that calcium ions could have a functional role in biofilm development and have practical implications, for example, in analysis of biofouling in membrane-based water-treatment processes such as nanofiltration or reverse osmosis where elevated calcium concentrations may occur at the solid–liquid interface.
      369Scopus© Citations 32