Aerobic storage by activated sludge on real wastewater

Activated sludge processes are often operated under dynamic conditions, whe re the microbial response can include, besides of growth, several COD remov al mechanisms, and particularly the storage in form of polymers. While abun dant evidence of aerobic storage under dynamic conditions with synthetic su bstrates can be found (Majone et al., Water Sci. Technol. 39(1) (1999) 61), there is still little knowledge about COD removal mechanisms with real act ivated 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 t ests were performed on the same sludge by using acetate, filtered wastewate r and raw wastewater as carbon source along with determination of acetate u ptake and storage polymer formation. Comparison of results obtained has sho wn that poly-3-hydroxybutyrate (PHB) storage gives always the main contribu tion to acetate removal and that in the case of wastewater PHB is also form ed 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 comp ounds or other non-storage phenomena, In the low-rate SBCOD phase of respir ogram PHB is clearly utilised in tests with acetate as internal reserve mat erial once the acetate is depleted. In tests with filtered and raw wastewat er the PHB concentration decreases much slower. probably because more PHB i s formed due to the availability of external SBCOD (soluble and not). Moreo ver. reported OUR in the SBCOD-phase from filtered or raw wastewater are qu ite 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 canno t be easily separated by the comparison of tests on filtered and raw wastew ater, because both substrates are simultaneously present also in tests with the filtered wastewater. As a side consequence, the chemical-physical meth od for evaluation of true soluble and biodegradable COD tends to overestima te the respirometry-based RBCOD, at least for the wastewater under observat ion. Even though modelling by ASM3 (Gujer et al., Water Set. Technol. 39(1) (1999) 183) makes it possible to well describe the whole experimental beha viour, it requires that much more storage compounds are formed than the exp erimentally observed PHB. These compounds have still to be identified and q uantified in order to confirm the conceptual structure of ASM3. (C) 2001 El sevier Science Ltd. All rights reserved.