Eoin Casey

Review of membrane aerated biofilm reactors

Membrane aerated biofilm reactors (MABRs) represent a new technology for aerobic wastewater treatment. Oxygen diffuses through a gas permeable membrane into the biofilm where oxidation of pollutants, supplied on the biofilm side of the membrane, takes place. The potential of this system for various wastewater treatment applications is reviewed in the context of recent developments in the understanding of the fundamentals of such systems. The difference between MABRs and conventional biofilm reactors is highlighted by the concept of active layers within biofilms and the importance of active layer location. This review also discusses the choice of membrane in such reactors and the development of mathematical models that describe MABR performance. All published studies on the use of MABRs are discussed under pollutant species headings: organic carbonaceous pollutant biodegradation, volatile organic pollutant removal, and nitrification/denitrification processes.

Oxygen mass transfer characteristics in a membrane-aerated biofilm reactor.

Immobilization of pollutant-degrading microorganisms on oxygen-permeable membranes provides a novel method of increasing the oxidation capacity of wastewater treatment bioreactors. Oxygen mass transfer characteristics during continuous-flow steady-state experiments were investigated for biofilms supported on tubular silicone membranes. An analysis of oxygen mass transport and reaction using an established mathematical model for dual-substrate limitation supported the experimental results reported. In thick biofilms, an active layer of biomass where both carbon substrate and oxygen are available was found to exist. The location of this active layer varies depending on the ratio of the carbon substrate loading rate to the intramembrane oxygen pressure. The thickness of a carbon-substrate-starved layer was found to greatly influence the mass transport of oxygen into the active biomass layer, which was located close to, but not in contact with, the biofilm-liquid interface. The experimental results demonstrated that oxygen uptake rates as high as 20 g m-2 d-1 bar-1 can be achieved, and the model predicts that, for an optimized biofilm thickness, oxygen uptake rates of more than 30 g m-2 d-1 bar-1 should be possible. This would allow membrane-aerated biofilm reactors to operate with much greater thicknesses of active biomass than can conventional biofilm reactors as well as offering the further advantage of close to 100% oxygen conversion efficiencies for the treatment of high-strength wastewaters. In the case of dual- substrate-limited biofilms, the potential to increase the oxygen flux does not necessarily increase the substrate (acetate) removal rate.

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.

The effect of salt and fibre direction on water dynamics, distribution and mobility in pork muscle: A low field NMR study

The effect of salt concentration and fibre orientation on water within the meat matrix was investigated by low-field nuclear magnetic resonance (LF-NMR), water-binding capacity (WBC), diffusion studies and histological analysis. Pork M. longissimus thoracis et lumborum samples were cured with 5.7, 15.3 or 26.3% w/w NaCl at a parallel or perpendicular fibre direction. NMR transverse (T2) relaxation identified three water components (T2b, T21 and T22) which all exhibited characteristics correlated to WBC. Results indicated that T2b increases with increasing NaCl concentration. Increasing intra-myofibrillar water and decreasing extra-myofibrillar water resulted in the highest WBC. Water diffused more quickly into the extra-myofibrillar space in samples cured at a parallel fibre direction. This water remained loosely bound in samples cured with the saturated solution (26.3% w/w NaCl) leading to decreased WBC. This study provides further information on water binding within the meat matrix by applying the results of LF-NMR to traditional water-binding theories.

Conversion of grass biomass into fermentable sugars and its utilization for medium chain length polyhydroxyalkanoate (mcl-PHA) production by Pseudomonas strains

This study investigated the potential of grass biomass as a feedstock for mcl-PHA production. Pretreatments (2% NaOH at 120°C or hot water at 120°C) of perennial ryegrass were employed alone or in combination with sodium chlorite/acetic acid (SC/AA) delignification to evaluate the enzymatic digestibility and subsequent utilization of resultant sugars by Pseudomonas strains. NaOH pretreated sample had better digestibility than raw and hot water treated samples and this hydrolysate supported good growth of all tested strains with limited mcl-PHA (6-17% of cell dry mass (CDM)) accumulation. Digestibility of both untreated and pretreated samples was improved after SC/AA delignification and produced glucose (74-77%) rich hydrolysates. Tested strains accumulated 20-34% of CDM as PHA when these hydrolysates were used as sole carbon and energy source. CDM and PHA yields obtained for these strains when tested with laboratory grade sugars was similar to that achieved with grass derived sugars.

Cicada Wing Surface Topography: An Investigation into the Bactericidal Properties of Nanostructural Features.

Recently, the surface of the wings of the Psaltoda claripennis cicada species has been shown to possess bactericidal properties and it has been suggested that the nanostructure present on the wings was responsible for the bacterial death. We have studied the surface-based nanostructure and bactericidal activity of the wings of three different cicadas (Megapomponia intermedia, Ayuthia spectabile and Cryptotympana aguila) in order to correlate the relationship between the observed surface topographical features and their bactericidal properties. Atomic force microscopy and scanning electron microscopy performed in this study revealed that the tested wing species contained a highly uniform, nanopillar structure on the surface. The bactericidal properties of the cicada wings were investigated by assessing the viability of autofluorescent Pseudomonas fluorescens cells following static adhesion assays and targeted dead/live fluorescence staining through direct microscopic counting methods. These experiments revealed a 20-25% bacterial surface coverage on all tested wing species; however, significant bactericidal properties were observed in the M. intermedia and C. aguila species as revealed by the high dead:live cell ratio on their surfaces. The combined results suggest a strong correlation between the bactericidal properties of the wings and the scale of the nanotopography present on the different wing surfaces.

Oxygen-Mediated Regulation of Biofilm Development Is Controlled by the Alternative Sigma Factor σB in Staphylococcus epidermidis

ABSTRACT Using a modified rotating-disk reactor to sparge oxygen to Staphylococcus epidermidis cultures, we found that oxygen negatively regulates biofilm development by influencing the activity of σB. Under anaerobic conditions, increased σB activity activates icaADBC, which encodes enzymes responsible for polysaccharide intercellular adhesin synthesis, by repressing transcription of the negative regulator icaR.

Characteristics of a Methanotrophic Culture in a Membrane-Aerated Biofilm Reactor

The membrane‐aerated biofilm reactor (MABR) shows considerable potential as a bioprocess that can exploit methanotrophic biodegradation and offers several advantages over both conventional biofilm reactors and suspended‐cell processes. This work seeks primarily to investigate the oxidation efficiency in a methanotrophic MABR. A mixed methanotrophic biofilm was immobilized on an oxygen‐permeable silicone membrane in a single tube hollow fiber configuration. Under the conditions used the maximum oxygen uptake rate reached values of 16 g/m2·d, and the rate of biofilm growth achieved was 300 μm/d. Both indicators reflect a very high metabolic rate. It was shown that the biofilm was predominantly in a dual‐substrate limitation regime but below about 250 μm was fully penetrated by both substrates. Oxygen limitation was not observed. Analysis indicated that microbial activity stratification was evident and the location of stratified layers of oxygen‐consuming components of the consortium could be manipulated via the intramembrane oxygen pressure. The results confirm that an MABR can be employed to minimize substrate diffusion limitations in thick biofilms.

The importance of laboratory water quality for studying initial bacterial adhesion during NF filtration processes

Biofouling of nanofiltration (NF) and reverse osmosis (RO) membranes for water treatment has been the subject of increased research effort in recent years. A prerequisite for undertaking fundamental experimental investigation on NF and RO processes is a procedure called compaction. This involves an initial phase of clean water permeation at high pressures until a stable permeate flux is reached. However water quality used during the compaction process may vary from one laboratory to another. The aim of this study was to investigate the impact of laboratory water quality during compaction of NF membranes. A second objective was to investigate if the water quality used during compaction influences initial bacterial adhesion. Experiments were undertaken with NF 270 membranes at 15 bar for permeate volumes of 0.5 L, 2 L, and 5 L using MilliQ, deionized or tap water. Membrane autopsies were performed at each permeation point for membrane surface characterisation by contact angle measurements, profilometry, and scanning electron microscopy. The biological content of compacted membranes was assessed by direct epi-fluorescence observation following nucleic acid staining. The compacted membranes were also employed as substrata for monitoring the initial adhesion of Ps. fluorescens under dynamic flow conditions for 30 min at 5 min intervals. Compared to MilliQ water, membrane compaction using deionized and tap water led to decreases in permeate flux, increase in surface hydrophobicity and led to significant build-up of a homogeneous fouling layer composed of both living and dead organisms (>10(6) cells cm(-2)). Subsequent measurements of bacterial adhesion resulted in cell loadings of 0.2 × 10(5), 1.0 × 10(5) cells cm(-2) and 2.6 × 10(5) cells cm(-2) for deionized, tap water and MilliQ water, respectively. These differences in initial cell adhesion rates demonstrate that choice of laboratory water can significantly impact the results of bacterial adhesion on NF membranes. Standardized protocols are therefore needed for the fundamental studies of bacterial adhesion and biofouling formation on NF and RO membrane. This can be implemented by first employing pure water during all membrane compaction procedures and for the modelled feed solutions used in the experiment.

Rapid depletion of dissolved oxygen in 96‐well microtiter plate Staphylococcus epidermidis biofilm assays promotes biofilm development and is influenced by inoculum cell concentration

Biofilm-related research using 96-well microtiter plates involves static incubation of plates indiscriminate of environmental conditions, making oxygen availability an important variable which has not been considered to date. By directly measuring dissolved oxygen concentration over time we report here that dissolved oxygen is rapidly consumed in Staphylococcus epidermidis biofilm cultures grown in 96-well plates irrespective of the oxygen concentration in the gaseous environment in which the plates are incubated. These data indicate that depletion of dissolved oxygen during growth of bacterial biofilm cultures in 96-well plates may significantly influence biofilm production. Furthermore higher inoculum cell concentrations are associated with more rapid consumption of dissolved oxygen and higher levels of S. epidermidis biofilm production. Our data reveal that oxygen depletion during bacterial growth in 96-well plates may significantly influence biofilm production and should be considered in the interpretation of experimental data using this biofilm model.