Now showing 1 - 9 of 9
  • Publication
    Biofilm Development in a Membrane Aerated Biofilm Reactor: Effect of Flow Velocity on Performance
    The effect of liquid flow velocity on biofilm development in a membrane-aerated biofilm reactor was investigated both by mathematical modeling and by experiment, using Vibrio natriegens as a test organism and acetate as carbon substrate. It was shown that velocity influenced mass transfer in the diffusion boundary layer, the biomass detachment rate from the biofilm, and the maximum biofilm thickness attained. Values of the overall mass transfer coefficient of a tracer through the diffusion boundary layer, the biofilm, and the membrane were shown to be identical during different experiments at the maximum biofilm thickness. Comparison of the results with published values of this parameter in membrane attached biofilms showed a similar trend. Therefore, it was postulated that this result might indicate the mechanism that determines the maximum biofilm thickness in membrane attached biofilms. In a series of experiments, where conditions were set so that the active layer of the membrane attached biofilm was located close to the membrane biofilm interface, it was shown that the most critical effect on process performance was the effect of velocity on biofilm structure. Biofilm thickness and effective diffusivity influenced reaction and diffusion in a complex manner such that the yield of biomass on acetate was highly variable. Consideration of endogenous respiration in the mathematical model was validated by direct experimental measurements of yield coefficients. Good agreement between experimental measurements of acetate and oxygen uptake rates and their prediction by the mathematical model was achieved.
      735Scopus© Citations 65
  • Publication
    Studies on the effect of concentration of a self-inhibitory substrate on biofilm reaction rate under co-diffusion and counter diffusion configurations
    (Elsevier, 2009-06-15) ; ;
    A simple mathematical model was developed to investigate the utilization rate of a self-inhibitory substrate in idealised biofilm reactors operating with either counter-diffusion or co-diffusion of oxygen and phenol. This study has implications for the development of membrane-supported biofilm technologies, such as the membrane-aerated biofilm reactor. An unsteady-state formulation of the model was used to investigate the effect of shock loads of phenol on biofilm performance. It was found that the counter-diffusion configuration may be advantageous under high phenol concentrations provided the biofilm thickness is above a critical value. The performance advantage of the counter-diffusion configuration is gained by the presence of an oxygen depleted layer, adjacent to the liquid–biofilm interface which acts as a diffusive barrier to phenol transport to the region of respiratory activity.
      380Scopus© Citations 12
  • Publication
    Model-based comparative performance analysis of membrane aerated biofilm reactor configurations
    (Wiley, 2008-04-15) ;
    The potential of the membrane aerated biofilm reactor (MABR) for high-rate bio-oxidation was investigated. A reaction-diffusion model was combined with a preliminary hollow-fiber MABR process model to investigate reaction rate-limiting regime and to perform comparative analysis on prospective designs and operational parameters. High oxidation fluxes can be attained in the MABR if the intra-membrane oxygen pressure is sufficiently high, however the volumetric oxidation rate is highly dependent on the membrane specific surface area and therefore the maximum performance, in volumetric terms, was achieved in MABRs with relatively thin fibers. The results show that unless the carbon substrate concentration is particularly high, there does not appear to be an advantage to be gained by designing MABRs on the basis of thick biofilms even if oxygen limitations can be overcome.
      788Scopus© Citations 37
  • Publication
    Bacterial adhesion onto nanofiltration and reverse osmosis membranes: Effect of permeate flux
    The influence of permeate flux on bacterial adhesion to NF and RO membranes was examined using two model Pseudomonas species, namely Pseudomonas fluorescens and Pseudomonas putida. To better understand the initial biofouling profile during NF/RO processes, deposition experiments were conducted in cross flow under permeate flux varying from 0.5 up to 120 L/(h m2), using six NF and RO membranes each having different surface properties. All experiments were performed at a Reynolds number of 579. Complementary adhesion experiments were performed using Pseudomonas cells grown to early-, mid- and late-exponential growth phases to evaluate the effect of bacterial cell surface properties during cell adhesion under permeate flux conditions. Results from this study show that initial bacterial adhesion is strongly dependent on the permeate flux conditions, where increased adhesion was obtained with increased permeate flux, until a maximum of 40% coverage was reached. Membrane surface properties or bacterial growth stages was further found to have little impact on bacterial adhesion to NF and RO membrane surfaces under the conditions tested. These results emphasise the importance of conducting adhesion and biofouling experiments under realistic permeate flux conditions, and raises questions about the efficacy of the methods for the evaluation of antifouling membranes in which bacterial adhesion is commonly assessed under zero-flux or low flux conditions, unrepresentative of full-scale NF/RO processes.
      488Scopus© Citations 20
  • Publication
    Performance analysis of a pilot-scale membrane aerated biofilm reactor for the treatment of landfill leachate
    A 60 L membrane aerated biofilm reactor (MABR) was successfully employed to treat landfill leachate, which contained very high concentrations of refractory chemical oxygen demand (COD) and ammonium. Air or pure oxygen was supplied to the bioreactor through polydimethyl siloxane hollow fibre membranes. Over a year of operation with an average hydraulic retention time of about 5 days, and influent ammonium concentrations ranging from 500 to 2500 mg/L, the MABR achieved 80–99% nitrification. Simultaneously, the influent COD concentrations which ranged from 1000 to 3000 mg/L were reduced by approximately 200–500 mg/L. Oxygen transfer rates as high as 35 g O2/m2-day were achieved during the study. By operating at low gas flowrates, high oxygen transfer efficiencies were achieved without any negative impact on oxygen transfer rates. This suggested that the biofilm was not oxygen limited during this study. The very low gas flowrates and the low pressure losses required to move air through the membranes resulted in very high standard aeration efficiencies that exceeded 10 kg O2/kW h. The results indicate that mixing energy far exceeded that required for aeration in this study. Our results suggest that with process optimisation, MABR technology offers a low energy option for effective leachate treatment.
      914Scopus© Citations 72
  • Publication
    Comparative economic analysis of full scale MABR configurations
    The membrane-aerated biofilm reactor (MABR) is a technology that can deliver oxygen at high rates and transfer efficiencies. This paper provides a comparative cost analysis of the MABR compared to the activated sludge process. Membrane cost and electricity cost were found to be the critical parameters determining the relative feasibility of the conventional process to the membrane based process. The general downward trend in the market price of membranes and the steady increase in energy costs in recent years may prove to be a strong driver for the further development of this technology.
  • Publication
    A physical impact of organic fouling layers on bacterial adhesion during nanofiltration
    Organic conditioning films have been shown to alter properties of surfaces, such as hydrophobicity and surface free energy. Furthermore, initial bacterial adhesion has been shown to depend on the conditioning film surface properties as opposed to the properties of the virgin surface. For the particular case of nanofiltration membranes under permeate flux conditions, however, the conditioning film thickens to form a thin fouling layer. This study hence sought to determine if a thin fouling layer deposited on a nanofiltration membrane under permeate flux conditions governed bacterial adhesion in the same manner as a conditioning film on a surface. Thin fouling layers (less than 50 μm thick) of humic acid or alginic acid were formed on Dow Filmtec NF90 membranes and analysed using Atomic Force Microscopy (AFM), confocal microscopy and surface energy techniques. Fluorescent microscopy was then used to quantify adhesion of Pseudomonas fluorescens bacterial cells onto virgin or fouled membranes under filtration conditions.It was found that instead of adhering on or into the organic fouling layer, the bacterial cells penetrated the thin fouling layer and adhered directly to the membrane surface underneath. Contrary to what surface energy measurements of the fouling layer would indicate, bacteria adhered to a greater extent onto clean membranes (24 ± 3% surface coverage) than onto those fouled with humic acid (9.8 ± 4%) or alginic acid (7.5 ± 4%). These results were confirmed by AFM measurements which indicated that a considerable amount of energy (10−7 J/μm) was dissipated when attempting to penetrate the fouling layers compared to adhering onto clean NF90 membranes (10−15 J/μm). The added resistance of this fouling layer was thusly seen to reduce the number of bacterial cells which could reach the membrane surface under permeate conditions. This research has highlighted an important difference between fouling layers for the particular case of nanofiltration membranes under permeate flux conditions and surface conditioning films which should be considered when conducting adhesion experiments under filtration conditions. It has also shown AFM to be an integral tool for such experiments.
      329Scopus© Citations 20
  • Publication
    Factors influencing 4-fluorobenzoate degradation in biofilm cultures of Pseudomonas knackmussii B13
    Membrane aerated biofilm reactors (MABRs) have potential in wastewater treatment as they permit simultaneous COD minimisation, nitrification and denitrification. Here we report on the application of the MABR to the removal of fluorinated xenobiotics from wastewater, employing a Pseudomonas knackmussii monoculture to degrade the model compound 4-fluorobenzoate. Growth of biofilm in the MABR using the fluorinated compound as the sole carbon source occurred in two distinct phases, with early rapid growth (up to 0.007 h−1) followed by ten-fold slower growth after 200 h operation. Furthermore, the specific 4-fluorobenzoate degradation rate decreased from 1.2 g g−1 h−1 to 0.2 g g−1 h−1, indicating a diminishing effectiveness of the biofilm as thickness increased. In planktonic cultures stoichiometric conversion of substrate to the fluoride ion was observed, however in the MABR, approximately 85% of the fluorine added was recovered as fluoride, suggesting accumulation of ‘fluorine’ in the biofilm might account for the decreasing efficiency. This was investigated by culturing the bacterium in a tubular biofilm reactor (TBR), revealing that there was significant fluoride accumulation within the biofilm (0.25 M), which might be responsible for inhibition of 4-fluorobenzoate degradation. This contention was supported by the observation of the inhibition of biofilm accumulation on glass cover slips in the presence of 40 mM fluoride. These experiments highlight the importance of fluoride ion accumulation on biofilm performance when applied to organofluorine remediation.
      975Scopus© Citations 31
  • Publication
    Membrane aerated biofilms for high rate biotreatment : performance appraisal, engineering principles, scale-up and development requirements
    (ACS, 2008-03-15) ;
    Diffusion of the electron acceptor is the rate controlling step in virtually all biofilm reactors employed for aerobic wastewater treatment. The membrane-aerated biofilm reactor (MABR) is a technology that can deliver oxygen at high rates and transfer efficiencies, thereby enhancing the biofilm activity. This paper provides a comparative performance rate analysis of the MABR in terms of its application for carbonaceous pollutant removal, nitrification/denitrification and xenobiotic biotreatment. We also describe the mechanisms influencing process performance in the MABR and the inter-relationships between these factors. The challenges involved in scaling-up the process are discussed with recommendations for prioritization of research needs.
      1982Scopus© Citations 212