A BULKING-SLUDGE WITH HIGH STORAGE RESPONSE SELECTED UNDER INTERMITTENT FEEDING

In various activated sludge systems, the biomass grows under transient (unbalanced) conditions and the storage response (formation of intern al polymers as the fastest adaptation to the changing environment) bec omes important. The aim of the present work was to select mixed cultur es with high storage response by cultivation on an acetate-limited med ium in a completely mixed reactor operated with intermittent feed (CST R-i) and to characterise their storage response by using batch experim ents under different experimental conditions. Even though kinetic sele ction based on storage response should bring about predominance of flo c-formers, in this work a bulking sludge was established steadily for long periods and starting from different inocula. Batch experiments sh owed that the mixed culture dominated by the filamentous bacteria pres ented immediate response to the substrate spike with both substrate re moval and storage of it as PHB at high rates, in the range 740 divided by 920 and 510 divided by 610 COD mg/COD g h, respectively. According to the COD balance, storage of poly-3-hydroxybutyrate (PHB) was pract ically the only significant metabolism Moreover, these filaments were able to store PHB about up to their dry weight and were highly resista nt to long starvation (24 h). Both high storage response and resistanc e to starvation can contribute to competitive advantage of these filam ents in a large range of operating conditions. Nevertheless, inactivit y under anoxic conditions makes easy to control bulking by metabolic s election. Although better insight into implications for full-scale pla nts must be gained, these results confirm that population dynamics und er transient conditions is mainly due to storage phenomena and indicat e that a competitive advantage of floc-formers against filaments, unde r such a selective pressure, has not to be considered as an absolute r ule. (C) 1998 Elsevier Science Ltd. All rights reserved.