Bulking of activated sludge is usually caused by an excessive growth of filamentous bacteria. These bacteria belong to the normal sludge population. They have to compete with the flocforming bacteria for the nutrients supplied with the influent. However, the size of the filamentous population will strongly increase if the filamentous species can consume a large portion of the available substrate. Bulking sludge occurs if the Sludge Volume Index exceeds the value of 150 ml/g. Bulking will result in a deterioration of the settling and the dewatering characteristics of activated sludge. Since 1975, many successful bulking sludge control methods and strategies have been developed  

Examples of sludges with good settling properties

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Examples of bulking sludge

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