Dairy-waste activated sludge was examined for bacterial composition and response to different conditions. Strains isolated were classified mainly into three groups: predominantly coryneform bacteria (largely Arthrobacter), some Achromobacteraceae and a small groups of Pseudomonadaceae. Experiments with representative strains of the three groups confirmed the ultimate predominant position of Arthrobacter.Two thirds of the bacteria gave a neutral reaction on Hugh and Leifson media, and more than half were not proteolytic. The activated sludge showed a ready accumulation of polysaccharides under certain nutritional conditions.Dissimilation of the carbohydrate of the substrate started immediately after adding substrate to an aerated activated sludge suspension. This was demonstrated by a drop in pH, through accumulation of acid intermediates, and a sharp decrease of dissolved O 2 . Dissolved O 2 was estimated by a stirrer-cleaned oxygen electrode. When dissolved O 2 was still available, acetic acid accumulated; when O 2 was exhausted, lactic acid formed.Dissimilation of the proteins usually started after the carbohydrate was exhausted and lasted longer. This meant that there may be an important discrepancy between the original and the actual C/N relation of the substrate.Experiments with Sphaerotilus natans and Arthrobacter strains, and published data yielded an explanation of the mechanism of bulking of activated sludge.
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