Bulking sludge prevention by an aerobic selector

Activated sludge bulking caused by the filamentous bacteria 021N, was repeatedly detected in the mixed liquor of a beet sugar mill treatment plant, equipped with an aerated selector. The organic pollution of the waste water consisted of about 70% easily degradable dissolved substrate (sugar, fatty acids). Only in cases when the elimination of the readily biodegradable substrate from the liquid phase in the selector was incomplete a rapid increase of filamentous bacteria could be detected consistently. The readily biodegradable substrate is predominantly removed in the selector by uptake and storage by the biomass. The oxygen demand for the storage in the selector depends on the kind of substrate. To obtain storage capacity in the selector, the sludge must have the opportunity to regenerate the capacity for substrate storage in the aeration tank. In the case of overloading and/or oxygen and/or nutrient deficiency the storage capacity can not be regenerated and the aerobic selector loses its effectiveness. From the findings about the factors influencing the elimination of the readily degradable substrate in the selector, a simple calculation method for dimensioning of aerobic selectors as well as a simulation model have been developed. In two plants, (60,000 m(3)/d, 40 t COD/d) that were built according to these findings it could be verified that the growth of 021N can be avoided effectively by using aerobic selectors, dimensioned with the developed calculation method. The results could be proved by successful operation of the treatment plants during the last two years. In a paper mill plant the SVI reaches values of 300 to 600 ml/g caused by the filamentous bacteria Type 0041 and Type 1701. Some days after installing an "adequate aerobic selector system" the growth of filamentous bacteria could be suppressed and the SVI reached values of 60 to 90 ml/g. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.