Oxic-Settling-Anaerobic under intermittent aeration: effect on system performance, sludge minimization and greenhouse gas emissions

The paper presents the results of an experimental study carried out on a pilot scale activated sludge system operated under intermittent aeration and fed with real wastewater at the Water Resource Recovery Facility of Palermo University (Italy). The experimental campaign was divided into two periods: in Period I, the pilot plant was operated as a conventional activated sludge system under intermittent aeration with fixed duration of aerated and non-aerated phases. In Period II, the plant layout was modified according to the Oxic-Settling-Anaerobic (OSA) configuration, by adding one anaerobic reactor in the recycling line from the final settler to the biological reactor. The final aim was to assess the reduction of excess sludge production while monitoring both system performance and N2O emissions. The results highlighted that the implementation of OSA configuration lead to a decrease of the sludge yield from 0.45 to 0.35 mgVSS/mgCOD. The COD efficiency remained quite stable and close to 90%. Conversely, OSA configuration negatively affected the pilot plant nitrification, with a significant decrease of NH4-N removal from 91% to 79%. Such reduction was also confirmed by respirometric analysis which showed a change in kinetic behavior from Period I to Period II.