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Section: Application Domains

Keywords : biology, environment, process engineering, optimal control, impulse control.

Control of sequencing batch reactors

From an engineering point of view, biological reactors are classified according to the way they are fed. When treating industrial as well as urban waste-waters, batch processes present a number of advantages with respect to continuous ones. In particular, the reaction rates are usually faster and the separation step, during which the biomass is separated from the effluent to be finally rejected into the environment, is much easier to control than during continuous operation. A batch processoperates in a sequential mode (this is why they are called Sequencing Batch Reactors or SBR): the water to be treated is first introduced into a closed tank. Then, the reaction takes place (the biomass degrades the substrates), the biomass settles and the supernatant (clean water) is finally discharged from the process before another cycle begins.

A classical objective for improving the functioning of these processes is the minimal time fedbatch strategyfor a SBR treating both the organic carbon and nitrogen. When only one biological reaction in involved, and furthermore its growth law is monotonic, the optimal solution is well-known : it consists in filling the tank as fast as possible and waiting. For more complex cases (i.e. non-monotonic growths or several species in competition on the same substrate to be degraded), the optimal solution is most of time far less simple, because of the presence of singular arcs. In these cases, a true feedback is required to achieved the optimal trajectory, but the problem of determining optimal syntheses is still widely opened.


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