Membrane bioreactors: Engineering aspects

Membrane bioreactors have in-situ separation capability lacking in other types of immobilized cell reactors. This makes them very useful for certain systems. Enzyme reactions utilizing cofactors and hydrolysis of macromolecules are advantageous in membrane reactors. Anaerobic cell culture may be efficiently carried out in membrane cell recycle systems, while aerobic cultures work well in dual hollow fiber reactors. Animal and plant cells have much a better chance of success in membrane reactors because of the protective environment of the reactor and the small oxygen uptake rate of these cells.     

Construction of mini-chemostats for high-throughput strain characterization

To achieve large-scale, high-throughput experiments for systems biology research of microorganisms, reliable data from robust cultivation systems are needed. Chemostats are such systems, ensuring reproducibility and quality by providing a stable, well-controlled environment for the cells. However, many of the available chemostat systems require large amounts of media and are complex to set up and expensive to purchase and maintain. To address these concerns, we developed a mini-chemostat (MC) system with 16 reactors, each at a working volume of 40 ml. Sensors measure dissolved oxygen in the reactor, while OD600 is measured in the outflow. We further developed a CO₂ and pH sensor array that can be plugged into the outflow of the reactors. The system was used to characterize yeast physiology at four metabolically different conditions: limitations of glucose, both aerobic and anaerobic, nitrogen, and ethanol. The physiology of yeast cells grown at the four different conditions in the MC system was compared with the yeast cells grown in a DASGIP 1 L system using RNAseq analysis. The results show that the MC system provides the same environmental conditions as the DASGIP system and that the MC system is reproducible between different runs. The system is built to be easily scalable with more reactors and to include more sensors, if available. Our study shows that a robust, reproducible chemostat system for high-throughput and large-scale experiments can be built at low costs.     

Biotechnology of algal biomass production: a review of systems for outdoor mass culture

Microalgae are very efficient solar energy converters and they can produce a great variety of metabolites. Man has always tried to take advantage of these proporties through algal mass culture. Despite the fact that many applications for microalgae have been described in the literature, these micro-organisms are still of minor economic importance. Industrial reactors for algal culture are at present, all designed as open race-ways (shallow open ponds where culture is circulated by a paddle-wheel). Technical and biological limitations of these open systems have given rise to the development of enclosed photoreactors (made of transparent tubes, sleeves or containers and where light source may be natural or artificial). The present review surveys advances in these two technologies for cultivation of microalgae. Starting from published results, the advantages and disadvantages of open systems and closed photobioreactors are discussed. A few open systems are presented for which particularly reliable results are available. Emphasis is then put on closed systems, which have been considered as capital intensive and are justified only when a fine chemical is to be produced.     

Which are the polyphosphate accumulating organisms in full-scale activated sludge enhanced biological phosphate removal systems in Australia?

AIMS: To see if the compositions of the microbial communities in full scale enhanced biological phosphorus removal activated sludge systems were the same as those from laboratory scale sequencing batch reactors fed a synthetic sewage. METHODS: Biomass samples taken from nine full scale enhanced biological phosphate removal (EBPR) activated sludge plants in the eastern states of Australia were analysed for their populations of polyphosphate (polyP)-accumulating organisms (PAO) using semi-quantitative fluorescence in situ hybridization (FISH) in combination with DAPI (4'-6-diamidino-2-phenylindole) staining for polyP. RESULTS: Very few betaproteobacterial Rhodocyclus related organisms could be detected by FISH in most of the plants examined, and even where present, not all these cells even within a single cluster, stained positively for polyP with DAPI. In some plants in samples from aerobic reactors the Actinobacteria dominated populations containing polyP. CONCLUSIONS: The PAO populations in full-scale EBPR systems often differ to those seen in laboratory scale reactors fed artificial sewage, and Rhodocyclus related organisms, dominating these latter communities may not be as important in full-scale systems. Instead Actinobacteria may be the major PAO. SIGNIFICANCE AND IMPACT OF THE STUDY: These findings illustrate how little is still known about the microbial ecology of EBPR processes and that more emphasis should now be placed on analysis of full-scale plants if microbiological methods are to be applied to monitoring their performances.     

Anaerobic Reactor Design Concepts for the Treatment of Domestic Wastewater

Since the earlier anaerobic treatment systems, the design concepts were improved from classic reactors like septic tanks and anaerobic ponds, to modern high rate reactor configurations like anaerobic filters, UASB, EGSB, fixed film fluidized bed and expanded bed reactors, and others. In this paper, anaerobic reactors are evaluated considering the historical evolution and types of wastewaters. The emphasis is on the potential for application in domestic sewage treatment, particularly in regions with a hot climate. Proper design and operation can result in a high capacity and efficiency of organic matter removal using single anaerobic reactors. Performance comparison of anaerobic treatment systems is presented based mostly on a single but practical parameter, the hydraulic retention time. Combined anaerobic reactor systems as well as combined anaerobic and non-anaerobic systems are also presented.     

Characteristics of immobilized yeast reactors producing ethanol from glucose

The evolutionary performance of up flow reactors are affected by the cell immobilization matrix and the matrix particle size distribution. Higher productivities are obtained using a low-density brick with a particle size of ca. 400-1400 mum. A medium condition favoring growth quickly leads to large biomass gradients within the reactors and, eventually, reductions in average productivities due to bed plugging. These systems can be accurately modelled using Monod kinetics when dispersion and the biomass gradient are considered. The productivity was apparently not controlled by substrate diffusion in these cases.     

Nuclear power: levels of safety

The rise and fall of the nuclear power industry in the United States is a well-documented story with enough socio-technological conflict to fill dozens of scholarly, and not so scholarly, books. Whatever the reasons for the situation we are now in, and no matter how we apportion the blame, the ultimate choice of whether to use nuclear power in this country is made by the utilities and by the public. Their choices are, finally, based on some form of risk-benefit analysis. Such analysis is done in well-documented and apparently logical form by the utilities and in a rather more inchoate but not necessarily less accurate form by the public. Nuclear power has failed in the United States because both the real and perceived risks outweigh the potential benefits. The national decision not to rely upon nuclear power in its present form is not an irrational one. A wide ranging public balancing of risk and benefit requires a classification of risk which is clear and believable for the public to be able to assess the risks associated with given technological structures. The qualitative four-level safety ladder provides such a framework. Nuclear reactors have been designed which fit clearly and demonstrably into each of the possible qualitative safety levels. Surprisingly, it appears that safer may also mean cheaper. The intellectual and technical prerequisites are in hand for an important national decision. Deployment of a qualitatively different second generation of nuclear reactors can have important benefits for the United States. Surprisingly, it may well be the "nuclear establishment" itself, with enormous investments of money and pride in the existing nuclear systems, that rejects second generation reactors. It may be that we will not have a second generation of reactors until the first generation of nuclear engineers and nuclear power advocates has retired.     

A review of advanced small-scale parallel bioreactor technology for accelerated process development: current state and future need

The pharmaceutical and biotech industries face continued pressure to reduce development costs and accelerate process development. This challenge occurs alongside the need for increased upstream experimentation to support quality by design initiatives and the pursuit of predictive models from systems biology. A small scale system enabling multiple reactions in parallel (n ≥ 20), with automated sampling and integrated to purification, would provide significant improvement (four to fivefold) to development timelines. State of the art attempts to pursue high throughput process development include shake flasks, microfluidic reactors, microtiter plates and small-scale stirred reactors. The limitations of these systems are compared to desired criteria to mimic large scale commercial processes. The comparison shows that significant technological improvement is still required to provide automated solutions that can speed upstream process development.     

Evaluation of intrinsic immobilized kinetics in hollow fiber reactor systems.

Immobilized cell and enzyme hollow fiber reactors have been developed for a variety of biochemical and biomedical applications. Reported mathematical models for predicting substrate conversion in these reactors have been limited in accuracy because of the use of free-solution kinetic parameters. This paper describes a method for determining the intrinsic kinetics of enzymes immobilized in hollow fiber reactor systems using a mathematical model for diffusion and reaction in porous media and an optimization procedure to fit intrinsic kinetic parameters to experimental data. Two enzymes, a thermophilic beta-galactosidase that exhibits product inhibition and L-lysine alpha-oxidase, were used in the analysis. The intrinsic kinetic parameters show that immobilization enhanced the activity of the beta-galactosidase while decreasing the activity of L-lysine alpha-oxidase. Both immobilized enzymes had higher Km values than did the soluble enzyme, indicating less affinity for the substrate. These results are used to illustrate the significant improvement in the ability to predict substrate conversion in hollow fiber reactors.     

Assessment of microbial community structure changes by amplified ribosomal DNA restriction analysis (ARDRA).

Amplified ribosomal DNA restriction analysis (ARDRA) is a simple method based on restriction endonuclease digestion of the amplified bacterial 16S rDNA. In this study we have evaluated the suitability of this method to detect differences in activated sludge bacterial communities fed on domestic or industrial wastewater, and subject to different operational conditions. The ability of ARDRA to detect these differences has been tested in modified Ludzack-Ettinger (MLE) configurations. Samples from three activated sludge wastewater treatment plants (WWTPs) with the MLE configuration were collected for both oxic and anoxic reactors, and ARDRA patterns using double enzyme digestions AluI+MspI were obtained. A matrix of Dice similarity coefficients was calculated and used to compare these restriction patterns. Differences in the community structure due to influent characteristics and temperature could be observed, but not between the oxic and anoxic reactors of each of the three MLE configurations. Other possible applications of ARDRA for detecting and monitoring changes in activated sludge systems are also discussed.     

Growth dynamics of Artemisia annua hairy roots in three culture systems

The transient growth of Artemisia annua hairy roots was compared for cultures grown in shake flasks and in bubble column and mist reactors. Instantaneous growth rates were obtained by numerically differentiating the transient biomass measurements. Specific sugar consumption rates showed good agreement with literature values. From the growth rate and sugar consumption rate, the specific yield and maintenance coefficient for sugar were determined for all three culture systems. These values were statistically indistinguishable for roots grown in shake flasks and bubble columns. In contrast, the values for roots grown in bubble columns and mist reactors were statistically different, suggesting that sugar utilization by roots grown in these two systems may be different. By measuring respiration rates in the bubble column reactor we also determined the actual biomass yield and maintenance coefficient for O(2) and CO(2). Together with an elemental analysis of the roots, this allowed us to obtain a reasonable carbon balance.     

Carbon balance of anaerobic granulation process: carbon credit

The concept of carbon credit arose out of increasing awareness of the need to reduce emissions of greenhouse gases to combat global warming which was formalized in the Kyoto protocol. In addition to contribution to sustainable development with energy recovery in the form of methane, carbon credits can be claimed by application of advanced anaerobic processes in wastewater treatment for reducing emissions of greenhouse gases. As anaerobic granular systems are capable of handling high organic loadings concomitant with high strength wastewater and short hydraulic retention time, they could render much more carbon credits than other conventional anaerobic systems. This study investigated the potential carbon credit derived from laboratory-scale upflow anaerobic sludge blanket (UASB) reactors based on a carbon balance analysis. Methane emission reduction could be calculated by calculating the difference of UASB reactors and open lagoon treatment systems. Based on the 2.5l bench-scale reactor, the total CH(4) emissions reduction was calculated as 29 kg CO(2)/year. On scaling up to a typical full-scale anaerobic digester, the total CH(4) emissions reduction could achieve 46,420 tons CO(2) reduction/year. The estimated carbon credits would amount to 278,500 US$ per year by assuming a carbon price of 6 US$ per metric ton CO(2) reduction. The analysis postulated that it is financially viable to invest in advanced anaerobic granular treatment system from the revenue generated from carbon credits.     

A methodology for the design of scale-down bioreactors by the use of mixing and circulation stochastic models

Scale-up is traduced in practice by an increase of the dimensions of the bioreactors, leading to a modification of the time scale and thus of the process dynamics. In the present work, a methodology to study the effect of scale-up on bioreactors hydrodynamics and to put in place scale-down reactors representative of the flow properties encountered in real scales bioreactors is detailed.In order to simplify the analysis, we have proposed the use of a stochastic model which is directly affected by the time scale. Indeed, to run simulations with such models, we have to specify the time taken to achieve a transition Δt. Stochastic models are thus reliable to study scale-up effect on stirred reactors hydrodynamics. In addition, these models permit to have an insight on the internal dynamic of the process.In the case of the circulation process, qualitative aspects have to be taken into account and induce a modification of the flow regions arrangement of the model. The stochastic analysis of large-scale bioreactors permits to propose a translating methodology into a scale-down context. Optimised scale-down reactors can be used further to carry out fermentation tests with the hydrodynamic conditions of the industrial scale. In a general rule, the performances of stochastic model allow to facilitate greatly the analysis of the scale-up effect and the hydrodynamic characteristics of both large-scale and scale-down reactors.     

Low-dissolved-oxygen nitrifying systems exploit ammonia-oxidizing bacteria with unusually high yields

In wastewater treatment plants, nitrifying systems are usually operated with elevated levels of aeration to avoid nitrification failures. This approach contributes significantly to operational costs and the carbon footprint of nitrifying wastewater treatment processes. In this study, we tested the effect of aeration rate on nitrification by correlating ammonia oxidation rates with the structure of the ammonia-oxidizing bacterial (AOB) community and AOB abundance in four parallel continuous-flow reactors operated for 43 days. Two of the reactors were supplied with a constant airflow rate of 0.1 liter/min, while in the other two units the airflow rate was fixed at 4 liters/min. Complete nitrification was achieved in all configurations, though the dissolved oxygen (DO) concentration was only 0.5 ± 0.3 mg/liter in the low-aeration units. The data suggest that efficient performance in the low-DO units resulted from elevated AOB levels in the reactors and/or putative development of a mixotrophic AOB community. Denaturing gel electrophoresis and cloning of AOB 16S rRNA gene fragments followed by sequencing revealed that the AOB community in the low-DO systems was a subset of the community in the high-DO systems. However, in both configurations the dominant species belonged to the Nitrosomonas oligotropha lineage. Overall, the results demonstrated that complete nitrification can be achieved at low aeration in lab-scale reactors. If these findings could be extended to full-scale plants, it would be possible to minimize the operational costs and greenhouse gas emissions without risk of nitrification failure.     

Development and structure of drinking water biofilms and techniques for their study

Drinking water systems are known to harbour biofilms, even though these environments are oligotrophic and often contain a disinfectant. Control of these biofilms is important for aesthetic and regulatory reasons. Study of full-scale systems has pointed to several factors controlling biofilm growth, but cause-and-effect relationships can only be established in controlled reactors. Using laboratory and pilot distribution systems, along with a variety of bacterial detection techniques, insights have been gained on the structure and behaviour of biofilms in these environments. Chlorinated biofilms differ in structure from non-chlorinated biofilms, but often the number of cells is similar. The number and level of cellular activity is dependent on the predominant carbon source. There is an interaction between carbon sources, the biofilm and the type of pipe material, which complicates the ability to predict biofilm growth. Humic substances, which are known to sorb to surfaces, appear to be a usable carbon source for biofilms. The finding offers an explanation for many of the puzzling observations in full scale and laboratory studies on oligotrophic biofilm growth. Pathogens can persist in these environments as well. Detection requires methods that do not require culturing.     

Modified reactive SMB for production of high concentrated fructose syrup by isomerization of glucose to fructose

This work presents modifications to the Hashimoto's hybrid simulated moving bed reactor (SMBR) system which was used to produce 55% high fructose syrup (HFS55). The purpose of this study is to develop a new SMBR system to overcome the disadvantages of Hashimoto system (3-zone SMB with seven reactors), i.e., low utility of reactors when feed being a 50/50 blend of glucose and fructose. Two different configurations of modified system were presented in this paper: the first configuration is 4-zone SMB with one reactor, while the other one consists of one additional reactor. Both of these configurations aim at improving the concentration and purity of glucose at the inlet of the reactor, which will lead to both high productivity and high purity of fructose in the product. A state-of-the-art optimization technique, viz., non-dominated sorting genetic algorithm (NSGA) is used in finding the optimal design and operating parameters for the modified reactive SMB and Varicol processes. Compared with the Hashimoto's system, high productivity and purity of fructose can be achieved in these new systems using less number of reactors.     

Online- and offline- monitoring of stem cell expansion on microcarrier

In the biopharmaceutical industry, adherent growing stem cell cultures gain worldwide importance as cell products. The cultivation process of these cells, such as in stirred tank reactors or in fixed bed reactors, is highly sophisticated. Cultivations need to be monitored and controlled to guarantee product quality and to satisfy GMP requirements. With the process analytical technology (PAT) initiative, requirements regarding process monitoring and control have changed and real-time on-line monitoring tools are recommended. A tool meeting the new requirements may be the dielectric spectroscopy for online viable cell mass determination by measurement of the permittivity. To establish these tools, proper offline methods for data correlation are required. The cell number determination of adherent cells on microcarrier is difficult, as it requires cell detachment from the carrier, which highly increases the statistical error. As an offline method, a fluorescence assay based on SYBR^®GreenI was developed allowing fast and easy total cell concentration determination without the need to detach the cells from the carrier. The assay is suitable for glass carriers used in stirred tank reactor systems or in fixed bed systems, may be suitable for different cell lines and can be applied to high sample numbers easily. The linear dependency of permittivity to cell concentration of suspended stem cells with the dielectric spectroscopy is shown for even very small cell concentrations. With this offline-method, a correlation of the cell concentration grown on carrier to the permittivity data measured by the dielectric spectroscopy was done successfully.     

Performance of staged and non-staged up-flow anaerobic sludge bed (USSB and UASB) reactors treating low strength complex wastewater

The use of anaerobic processes to treat low-strength wastewater has been increasing in recent years due to their favourable performance-costs balance. For optimal results, it is necessary to identify reactor configurations that are best suited for this kind of application. This paper reports on the comparative study carried out with two high-rate anaerobic reactor systems with the objective of evaluating their performances when used for the treatment of low-strength, complex wastewater. One of the systems is the commonly used up-flow anaerobic sludge blanket (UASB) reactor. The other is the up-flow staged sludge bed (USSB) system in which the reactor was divided longitudinally into 3, 5 and 7 compartments by the use of baffles. The reactors (9 l) were fed with a synthetic, soluble and colloidal waste (chemical oxygen demand (COD) < 1000 mg/l) and operated at 28°C and 24 h hydraulic retention time. Intermediate flow hydraulics, between plug-flow and completely-mixed, in the UASB and 7 stages USSB reactors allowed efficient degradation of substrates with minimum effluent concentrations. Low number of compartments in the USSB reactors increased the levels of short-circuiting thus reducing substrate removal efficiencies. All reactors showed high COD removal efficiencies (93–98%) and thus can be regarded as suitable for the treatment of low strength, complex wastewater. Staged anaerobic reactors can be a good alternative for this kind of application provided they are fitted with a large enough (≥7) number of compartments to fully take advantage of their strengths. Scale factors seem to have influenced importantly on the comparison between one and multi staged sludge-bed reactors and, therefore, observations made here could change at larger reactor volumes.     

Enzymatic membrane reactors for biodegradation of recalcitrant compounds. Application to dye decolourisation

Membrane bioreactors are being increasingly used in enzymatic catalysed transformations. However, the application of enzymatic-based treatment systems in the environmental field is rather unusual. The aim of this paper is to overview the application of enzymatic membrane reactors to wastewater treatment, more specifically to dye decolourisation. Firstly, the basic aspects such as different configurations of enzymatic reactors, advantages and disadvantages associated to their utilisation are revised as well as the application of this technology to wastewater treatment. Secondly, dye decolourisation by white-rot fungi and their oxidative enzymes are discussed, presenting an overall view from for in vivo and in vitro systems. Finally, dye decolourisation by manganese peroxidase in an enzymatic membrane reactor in continuous operation is presented.     

High density cell culture by membrane-based cell recycle

Enhancement of productivity of a bioprocess necessitates continuous operation of bioreactors with high biomass concentrations than are possible in conventional batch, fedbatch or continuous modes of culture. Membrane-based cell recycle has been effectively used to maintain high cell concentrations in bioreactors. This review compares membranebased cell recycle operation with other such high density cell culture systems as immobilized cell reactors and reactors with cell recycle by centrifugation or gravity sedimentation. A theoretical of production of primary and secondary metabolites in membrane-based recycle systems is presented. Operation of this type of system is discussed with examples from aerobic and anaerobic fermentations.