In Focus: Bioprocessing Editorial
If ancient domestic fermentation processes such as cheese-making and brewing represent the earliest examples of biotechnology, then the containers in which such processes were performed must be the earliest examples of what we now call bioreactors. Although bioreactors have been used continuously since prehistory for making food and beverage products, it has only been in the last 150 years that they have been used as the basic technological component of a wide, and increasing, range of other industrial processes. These include the production of organic acids and solvents; low volume high-added value products such as antibiotics and vitamins; vaccines, novel biopharmaceuticals and other products of the new biotechnology; and high volume low-added value products such as single-cell protein and biofuels. In addition to their importance to product manufacture, bioreactors are also key in the treatment and remediation of municipal and industrial waste streams. Without the bioreactor, much of what we take for granted in modern life would disappear, or lie around in unsavoury heaps.
Chemical engineers have evolved a wide range of bioreactor designs to meet the particular needs of these many and varied biological processes. Continuing effort goes into modelling, improving and optimising reactor design and performance based on the use of sound scientific, technical and mathematical tools, reflecting the growing industrial importance of the bioprocess industries. JCTB supports the activities of chemical engineers and biotechnologists by publishing new and exciting developments in the fields of bioreactor design and operation. In this issue we have brought four such papers 'In Focus', to demonstrate the relevance of this field to the Journal and vice versa. In their different ways, these four papers highlight the effort that is currently being made to understand the underlying scientific bases of bioreactor performance.
The use of response surface methodology to provide a rapid statistical basis for performance optimisation is increasing; indeed, JCTB has published a number of papers in the field. The first of the In Focus papers, by Quaratino et al., reports the use of this approach to optimise the growth medium for production of manganesedependent peroxidase by the white-rot fungus Panus tigrinus. The authors achieved a quick screening of a large experimental domain to determine the influence of each variable using shake flask cultures; the optimised medium formulation was then applied, successfully, as the basis for process scale-up into both stirred tank and airlift bioreactor systems, demonstrating the validity and value of this approach.
The second paper, by Okoth et al., uses mathematical simulation of a membrane bioreactor to model the performance of an industrial degreasing unit. Mathematical modelling is a favourite pastime of many chemical engineers, but the value of such work can only be in the application of the model to real industrial processes. In this study, real data obtained from a galvanisation plant were used to confirm the ability of the model to simulate the dynamic behaviour of a membrane reactor for the biodegradation of an oil-in-water emulsion.
Mathematical analysis also forms the basis of the third paper, by Aguilar-Lรณpez et al. Despite the increasing implementation of online measurements of essential process variables, there is still a need to make estimations of states and uncertainties that cannot yet be directly measured. Here again, the authors' work is based on actual performance data obtained from a sludge-activated wastewater treatment at a petrochemical plant. They report the development of a model of plant performance which they validate against actual data, providing a basis to estimate total plant performance on the means of limited direct measurements.
Any assumption that microbes naturally grow in free suspension would be false; in fact, it is probable that most microbes grow naturally in complex colonies, called biofilms, on the surfaces of particles. This natural association is exploited in the treatment of municipal sewage used as a feedstock for microbial populations growing in biofilms on the surface of particles in filter beds. Also, there are many reported examples of bioreactors in which microbes are immobilised in or on some form of insoluble matrix. In the final In Focus paper, Gikas and Livingston describe the study of the growth and biomass loss pattern for the marine bacterium Beneckea natriegens immobilised on diatomaceous earth particles in an airlift biofilm reactor. The growth of the immobilised biomass was not uniform; specifically, cells growing nearer the external layer exhibited the highest rate of growth and of biomass loss from the biofilm.
The studies of groups such as these represented here provide a deeper scientific comprehension of the factors that affect bioreactor performance, which should inform both reactor design and operation. Bioreactors will undoubtedly continue as the basis of the expanding bio-industries in both developed and developing countries.