Sequential anaerobic-aerobic digestion has been demonstrated as a promising alternative for enhanced sludge stabilization. In this paper, a feasibility study of the sequential digestion applied to real waste activated sludge (WAS) and mixed sludge is presented. Process performance is evaluated in terms of total solid (TS) and volatile solid (VS) removal, biogas production, and dewaterability trend in the anaerobic and double-stage digested sludge. In the proposed digestion lay out, the aerobic stage was operated with intermittent aeration to reduce the nitrogen load recycled to the wastewater treatment plant (WWTP). Experimental results showed a very good performance of the sequential digestion process for both waste and mixed sludge, even if, given its better digestibility, higher efficiencies are observed for mixed sludge. VS removal efficiencies in the anaerobic stage were 48 and 50% for waste and mixed sludge, respectively, while a significant additional improvement of the VS removal of 25% for WAS and 45% for mixed sludge has been obtained in the aerobic stage. The post-aerobic stage, operated with intermittent aeration, was also efficient in nitrogen removal, providing a significant decrease of the nitrogen content in the supernatant: nitrification efficiencies of 90 and 97% and denitrification efficiencies of 62 and 70% have been obtained for secondary and mixed sludges, respectively. A positive effect due to the aerobic stage was also observed on the sludge dewaterability in both cases. Biogas production, expressed as Nm(3)/(kgVSdestroyed), was 0.54 for waste and 0.82 for mixed sludge and is in the range of values reported in the literature in spite of the low anaerobic sludge retention time of 15 days.

Combined anaerobic-aerobic sludge digestion has been demonstrated as a valid technological solution to improve the sludge stabilization process, in terms of solid reduction and sludge dewaterability properties. In this study, we investigated the fate of all nitrogen species (ammonia, nitrites, nitrates) in the sequential digestion process when the post-aerobic step is operated with intermittent aeration. Nitrogen course has been followed in the two digesters operated in semi-continuous mode and in batch nitrification-denitrification kinetic tests. Two-stage digestion was applied to real waste activated sludge. High volatile solid removal efficiencies (47 and 26 % in the anaerobic and aerobic phases, respectively) confirmed the potential of this technology. Moreover, the post-aerobic stage allowed to successfully achieving nitrogen removal through the simultaneous nitrification-denitrification process. Nitrification and denitrification efficiencies were 95 and 70 %, respectively. Batch tests for nitrification and denitrification were also carried out to investigate the process kinetics. A process model has been formulated and calibrated with a first set of experimental kinetic data. Evaluated kinetic parameters were employed in the validation phase successfully performed (correlation coefficients R 2 [ 0.98) with different series of experimental data.

The requirement for enhanced stabilization processes to obtain a more stable, pathogen-free sludge for agricultural use is an increasing challenge to comply with in the waste hierarchy. With this in mind, the Routes European project ('Novel processing routes for effective sewage sludge management') is addressed to assess innovative solutions with the aim of maximizing sludge quality and biological stability. In order to increase anaerobic stabilization performances, the sequential anerobic/aerobic process and the thermophilic digestion process, with or without integration of the thermal hydrolysis pre-treatment, were investigated as regards the effect on sludge stabilization, dewaterability and digestion performances. Thermal pre-treatment improved anaerobic digestion in terms of volatile solids reduction and biogas production, but digestate dewaterability worsened. Fluorescence in situ hybridization (FISH) quantification showed an increase of methanogens consistent with the increase of biogas produced. The aerobic post-treatment after mesophilic digestion had a beneficial effect on dewaterability and stability of the digested sludge even if was with a reduction of the potential energy recovery.

The original publication of this article contains a mistake. The correct title is: "Sequential anaerobic/aerobic digestion for enhanced sludge stabilization: comparison of the process performance for mixed and waste sludge", which replaces the previous one : "Sequential anaerobic/anaerobic digestion for enhanced sludge stabilization: comparison of the process perfor-mance for mixed and waste sludge".The online version of the original article can be found at http:// dx