Waste activated sludge minimization and carbon footprint reduction to foster transition to circular economy in the water sector

The urgent need to address environmental challenges such as increasing sewage sludge production and greenhouse gas emissions in wastewater management has spurred a global transition towards a circular economy approach. This transition often involves transforming traditional wastewater treatment plants into water resource recovery facilities, essential for sustainable water resource management. In this view, different schemes were evaluated in a pilot scale and full-scale WWTP, to find a trade-off between sludge production, effluent quality, N2O emissions and carbon footprint assessment. This thesis investigates two case studies: UNIPA (pilot scale) and Corleone (full scale) wastewater treatment plants, focusing on the critical role of plant configuration and optimization in mitigating sewage sludge production and reducing the carbon footprint. In both cases, innovative techniques such as oxic-settling-anaerobic (OSA) process and the intermittent aeration (IA) strategy were implemented. Results of both cases demonstrated that OSA process exemplifies tangible reductions in sewage sludge production and carbon footprint, aligning with circular economy objectives and European Commission recommendations. Furthermore, advancements in phosphate removal highlight improved nutrient recovery capabilities under the OSA process. Moreover, results also showed that the combination of these two strategies is very effective in terms of carbon footprint reduction, greenhouse gas emissions and sludge production. In UNIPA case study, with the aim to provide a general comparison between the several layouts investigated, a semiquantitative method was adopted based on the average values achieved under each configuration for six different parameters chosen as representatives: Yobs (for sludge production), emission factor (for N2O emissions), total nitrogen and phosphorus removal efficiency (to evaluate the effluent quality), membrane fouling rate and carbon footprint. In Additionally, in UNIPA case study, further strategies were evaluated in view of finding a trade-off between sludge production, carbon footprint, nutrients removal and greenhouse gases (GHG) emissions: membrane bioreactor (MBR) and Integrated Fixed Film Activated Sludge (IFAS), and the combination of them. Moreover, the influence of different parameters (organic load rate, sludge retention time, hydraulic retention time of the anaerobic side stream reactor, intermittent aeration times, location and percentage of the sludge recirculation line) were monitored during UNIPA experimental campaign. The results obtained at UNIPA highlighted the significant influence exerted by plant configuration and operational parameters in reducing the amount of produced sludge. In general, Results showed that MBR schemes demonstrated good results in terms of excess sewage sludge production in all cases. Moreover, when combined with IA strategy, this configuration demonstrated very good performances in terms of emission factor and carbon footprint. Regarding the IFAS strategy, very good performances were verified in terms of sewage sludge production and emission factor, compared to the other schemes studied. Regarding Corleone WWTP, the implementation of the OSA configuration reduced wasted sludge production significantly (in 27%), improved phosphate removal, but a worsening in the TN performances. On the other hand, the intermittent aeration strategy demonstrated very effectiveness in terms of reduction of the WWTP carbon footprint (mainly due to the reduction of energy consumption) and in terms of TN and NH4 removal efficiency, even if combined with OSA system. Therefore, a proper trade-off between sludge reduction and process performance must be assessed. This study presents the evaluation of waste activated sludge reduction from urban wastewater treatment with the simultaneous control of N₂O emissions and nutrient removal efficiencies. It uniquely combines pilot and full-scale experiments within a circular economy framework. Additionally, this study systematically tests advanced technologies (OSA and IA) across different scales, providing valuable insights on their practical applicability and environmental benefits.