G. Hamer

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.

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.

Aerobic thermophilic waste sludge biotreatment : carboxylic acid production and utilization during biodegradation of bacterial cells under oxygen limitation

The production and utilization of carboxylic acids during aerobic thermophilic treatment of a model sludge composed of bacterial cells were examined in a laboratory treatment system. Operation under a limited supply of O2, typical for such treatment systems, resulted in a distinct pattern of production and simultaneous utilization of low molecular weight carboxylic acids. Pulse-addition of a mixture of carboxylic acids at the end of a fed-batch cycle indicated that simultaneous utilization of acetate, propionate, isobutyrate, n-butyrate and isovalerate could occur, but only after a lag phase during which only acetate was utilized. In an attempt to differentiate between production and utilization of the carboxylic acids, a series of pulse experiments were performed using 14C-labelled acetate. The results indicated that production continued late into the fed-batch cycle whereas utilization could occur during the entire cycle. When acetate was pulsed to the process, only 11% of the acetate carbon was incorporated into new biomass, whereas 75% was converted into CO2. However, 14% of the original radioactivity persisted in the supernatant despite complete acetate utilization. This suggested that some of the acetate was metabolized into more slowly biodegradable products.

The effect of lithium chloride on the biooxidation of aqueous methanol/acetone mixtures.

Abstract. Lithium chloride, more specifically the lithium cation, has been implicated in interference in biological systems. In the case of Escherichia coli, interference involves the Na+(Li+)/H+ antiporter transport system. The study reported here concerns the effects of LiCl on a mixed enrichment culture that is able to biodegrade both methanol and acetone under aerobic conditions. The results obtained using unsteady state continuous flow culture techniques demonstrate a significant disruptive effect of LiCl on culture performance. In addition, a reduction in the substrate-based biomass yield coefficient, which is a clear advantage as far as biotreatment process performance is concerned, also occurs. The ultimate fate of the LiCl was not determined.

Thermophilic Aerobic Processes for Waste Sewage Sludge Treatment

Waste sewage sludge, the major by-product from municipal sewage treatment, is the most widely distributed noxious slurry produced by man. It comprizes a relatively dilute aqueous suspension derived from physicomechanical primary and biological secondary sewage treatment and is unsuitable for direct incineration and expensive to dewater. The noxious nature of waste sewage sludge results from the partitioning of certain apparently recalcitrant toxic chemicals and pathogenic organisms, originally present in municipal sewage, into sludge solids during conventional sewage treatment with variants of the activated sludge process.