A full scale experiment with an activated sludge process in a pharmaceutical manufacturing plant showed that the operation of two settling tanks in series, with the first tank full of sludge, resulted in selection of sludge particles which had a lower than average filament content for retention in the process. Series operation of the two clarifiers was very successful in controlling sludge bulking. The functioning of the settling tanks as selectors of sludge particles which settle and thicken well has not been considered to any great extent. Two laboratory-scale activated sludge plants fed with an artificial wastewater were operated to explore this selection function further. The SVI values were measured and assessed. At the start of operation, the SVI was about 60–80 ml/g in both of the parallel and the series plants. After eight days of the operation, the SVI started to increase rapidly in the parallel plant, and in twenty days of operation, the averaged values of SVI were 163 ml/g and 116 ml/g for the parallel and the series plants, respectively. On day 38, MLSS from two aeration tanks were interchanged. In a few days after that interchange, the SVI values in the parallel, which was originally in the series mode, rapidly increased from 130 ml/g to more than 350 ml/g(SV30 = 930 ml/g). This experiment clearly supports the hypothesis of a microorganism selection function of the settling tank and also corresponds to the results obtained from the field study.
Self-Optimizing Control of a Continuous-Flow Pharmaceutical Manufacturing Plant