The role of the adequate control of the mixed liquor suspended solids (MLSS) in the overall performance of oxidation ditches is discussed. A dynamic model, developed principally for the control of MLSS in the short term incorporating diurnal variations, is described. The predicted model states are MLSS, return sludge suspended solids, dissolved oxygen, ammonia, sludge blanket height, effluent suspended solids and effluent BOD and the forcing functions are influent flow, suspended solids and ammonia. The simplified nature of the model makes it suitable for real time control purposes with the input variables being measurable on a continuous basis. The initial calibration and validation used data collected from a full-scale plant and gave satisfactory results for short-term predictions with good indications that the model is also valid for longer-term applications.

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