Reactors and biosensors for improved microbial degradation of acetonitrile
Abstract: Biodegradation is becoming an attractive mode of operation for eliminating toxic chemicals released from industrial processes or already present in the environment. One needs, however, not only to identify organisms capable of degrading such chemicals, but also to address the issue of whether these chemicals, after being degraded, will be toxic to the degrading organisms if present in high concentrations. The present thesis concerns the isolation of a microbial culture capable of degrading acetonitrile. Various aspects of designing a process for degrading acetonitrile are also considered. To obtain an efficient and robust process, immobilised cells were used, making the system less sensitive to sudden spikes in the concentration of the toxic compound. Letting the cells grow in an adhesive mode on suspended plastic carriers was found to be an efficient arrangement. Adhesive growth was used since it is a low-cost method of obtaining an immobilised preparation, and since the use of plastic carriers enabled stirred tank technology to be employed, eliminating unfavourable concentration gradients that often appear with use of packed bed reactors. The process was scaled up to a 20 liter reactor, the degradation rates obtained reaching 0.77 g/(l day). Sometimes, one bioreactor alone is not sufficient for achieving the degradation needed. One can then use a sequence of reactors, these differing slightly from one another in the conditions they provide the microorganisms. The study aimed at obtaining complete mineralisation of acetonitrile. This was achieved by using a sequence of five bioreactors. How best to maintain control over the feed of the toxic compound within the reactor was a key issue, since the level of degradation is low when a reactor operates at very low concentrations and at high concentrations inhibition can occur, or worse yet cell death can take place. To solve this dosage problem, a cell-based biosensor was constructed using the same type of cells as those present in the reactor. The cells were immobilised in the proximity of an oxygen electrode, which registered the metabolic response of the cells when exposed to acetonitrile. A potential problem, however, was that acetic acid is formed when acetonitrile is degraded, its likewise being found to be degraded by the immobilised cells in the biosensor, producing a measurable signal. To eliminate these disturbances, a membrane that kept the dissociated acetic acid separate from the layer of immobilised cells was installed in the biosensor. The acetonitrile sensor was used for intermittent assay of the reactor mixture. The signal was also used to control a pump for feeding new acetonitrile into the reactor system. The degradation of acetonitrile shall be regarded as a model system for the degradation of other toxic compounds. Thus, the methods presented in the thesis could have applicability in many different systems in the future.
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