Aerobic storage by activated sludge on real wastewater

Activated sludge processes are often operated under dynamic conditions, where the microbial response can include, besides of growth, several COD removal mechanisms, and particularly the storage in form of polymers. While abundant evidence of aerobic storage under dynamic conditions with synthetic substrates can be found (Majone et al., Water Sci. Technol. 39(1) (1999) 61), there is still little knowledge about COD removal mechanisms with real activated sludge and wastewater. The aim of the present paper is therefore to give a direct evidence of storage phenomena occurring when a real sludge is mixed with influent wastewater and of their influence onto OUR profiles in typical respirometric batch tests. For this purpose, respirometric batch tests were performed on the same sludge by using acetate, filtered wastewater and raw wastewater as carbon source along with determination of acetate uptake and storage polymer formation. Comparison of results obtained has shown that poly-3-hydroxybutyrate (PHB) storage gives always the main contribution to acetate removal and that in the case of wastewater PHB is also formed from other substrates. PHB formation clearly occurs during the high-rate RBCOD-phase, however for wastewater it accounts for only a fraction (18–22%) of overall RBCOD removal, so calling for other unidentified storage compounds or other non-storage phenomena. In the low-rate SBCOD phase of respirogram PHB is clearly utilised in tests with acetate as internal reserve material once the acetate is depleted. In tests with filtered and raw wastewater the PHB concentration decreases much slower, probably because more PHB is formed due to the availability of external SBCOD (soluble and not). Moreover, reported OUR in the SBCOD-phase from filtered or raw wastewater are quite higher than those reported in batch tests with acetate, so confirming a main contribution of external SBCOD. However, the respective contributions for utilisation of previously stored compounds and of external SBCOD cannot be easily separated by the comparison of tests on filtered and raw wastewater, because both substrates are simultaneously present also in tests with the filtered wastewater. As a side consequence, the chemical–physical method for evaluation of true soluble and biodegradable COD tends to overestimate the respirometry-based RBCOD, at least for the wastewater under observation. Even though modelling by ASM3 (Gujer et al., Water Sci. Technol. 39(1) (1999) 183) makes it pos