Oxic settling anaerobic (OSA) process for excess sludge reduction: 16 months of management of a pilot plant fed with real wastewater

An OSA (oxic settling anaerobic) pilot plant, preliminarly ran as a conventional activated sludge (CAS) system, was managed under several operating conditions for 16 months. The aim was to evaluate the reduction of excess sludge production due to the recirculation of activated sludge trough the sludge holding tank (SHT). The sludge retention time varied from 37 to 406 days according to the excess sludge wastage. The oxidation/reduction potential (ORP) inside the sludge holding tank was of – 248 ± 133 mV. The observed sludge yield, obtained by feeding the system with real wastewater, varied from 0.112 to 0.465 gTSS/gCOD, with a reduction of the excess sludge production of 49.6 ± 20.7% compared to the CAS system. The recirculation of the sludge through the SHT favoured the COD and ammonia nitrogen solubilization connected to the sludge decay. The release of soluble phosphorus due to the phosphorus accumulating organisms metabolism was also noticed.