Pilot plant studies with settled domestic waste water were carried out to study the influence of a contact tank on bulking sludge control. It has been clearly proved that the configuration of the contact tank plays a decisive role in preventing or curing bulking sludge. A rectangular contact tank with a substrate gradient ensured well-settling sludge, while a circular contact tank - completely mixed - was not able to control bulking sludge. Stirring or aerating of the rectangular contact tank did not make a difference in the sludge quality during continuous process operation. Great differences in biosorption and COD-removal were measured between well-settling and poor-settling sludge. A detention time of 30 minutes of the sludge-water mixture in the rectangular tank guaranteed - on the basis of biosorption and COD-removal - a more stable and reliable process for controlling bulking sludge than 10 minutes.

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