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 internal polymers as the fastest adaptation to the changing environment) becomes important. The aim of the present work was to select mixed cultures with high storage response by cultivation on an acetate-limited medium in a completely mixed reactor operated with intermittent feed (CSTR-i) and to characterise their storage response by using batch experiments under different experimental conditions. Even though kinetic selection based on storage response should bring about predominance of floc-formers, in this work a bulking sludge was established steadily for long periods and starting from different inocula. Batch experiments showed that the mixed culture dominated by the filamentous bacteria presented immediate response to the substrate spike with both substrate removal and storage of it as PHB at high rates, in the range 740÷920 and 510÷610 COD mg/COD g h, respectively. According to the COD balance, storage of poly-3-hydroxybutyrate (PHB) was practically the only significant metabolism. Moreover, these filaments were able to store PHB about up to their dry weight and were highly resistant to long starvation (24 h). Both high storage response and resistance to starvation can contribute to competitive advantage of these filaments in a large range of operating conditions. Nevertheless, inactivity under anoxic conditions makes easy to control bulking by metabolic selection. Although better insight into implications for full-scale plants must be gained, these results confirm that population dynamics under transient conditions is mainly due to storage phenomena and indicate that a competitive advantage of floc-formers against filaments, under such a selective pressure, has not to be considered as an absolute rule.