Slugs help to clear bacteria-blocked filters
15 Jan 2000
Bioreactors are the focus of intense interest in several process spheres at the moment, particularly areas like speciality chemicals and pharmaceuticals. Using modified bacteria to produce low-volume, high-value chemicals has many advantages, such as the high selectivity of the process. However, the process itself is difficult, because the living cells have to be separated from the product.
Membrane bioreactors get around this problem by allowing the products to diffuse through a membrane. Traditionally, in continuous fermentation processes, a crossflow filter is used; but when the concentration of the feedstocks is increased to boost the rate or yield, the filter blocks very quickly, which can almost stop the flow through the membrane.
A team of researchers from the Department de Genie Biochemique et Alimentaire at L'Institut National des Sciences Appliquees in Toulouse have now found that, surprisingly, the answer might lie in moving away from a `steady state' system. They found that disrupting the flow by injecting air into the system can increase flow through the membrane threefold; moreover, they claim, it reduces energy consumption by 30-50 per cent.
Presenting their results at a recent conference on bioreactor fluid dynamics, the team said that slugs of air trapped in the stream that flows across the membrane `destabilise' the conditions immediately adjacent to the membrane. The local pressure, velocity and shear stresses all fluctuate, and the combination of these factors prevents the membrane from becoming blocked. `Air injection is a simple, economic and successful technique,' they comment. `This process could easily be applied to any kind of cell culture.' If the process requires oxygen, then air can be used as the injected gas-phase; if the process is anaerobic, effluent gas from the reaction could be recycled into the flow.
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