Operational strategies enhancing sewage sludge minimization in a combined integrated fixed-film activated sludge - oxic settling anaerobic system

Abstract

A lab-scale integrated fixed-film activated sludge (IFAS) reactor was mplemented with the oxic-settling anaerobic (OSA) cycle for reducing sewage sludge production through the addition of an anoxic/anaerobic sludge holding tank (SHT) along the sludge recycle line. The IFAS-OSA system was operated under the different hydraulic retention time (HRT) in the SHT (HRT_SHT) of 12 h and 6 h, at an oxidation-reduction potential (ORP) < -91 mV and solid retention time (SRT) between 39 and 126 d. Furthermore, the effect of temperature increase in the SHT (T_SHT) from ambient (19.8-25.6 °C) to mesophilic (35 °C) conditions was investigated. The system performances were monitored in terms of sludge minimization and dewaterability efficiencies as well as carbon and nutrients reduction. The observed sludge yield (Y_obs) for the IFAS system was 0.37(±0.06) mg VSS/mg COD. After OSA implementation Y_obs decreased by 32% and 46-65% at HRT_SHT of 12 h and 6 h, respectively, indicating that prolonged exposure to anoxic/anaerobic conditions was not beneficial for sludge reduction. The lowest Y_obs of 0.09(±0.05) mg VSS/mg COD (76% lower than that in the IFAS system) was obtained at an HRT_SHT of 6 h and when T_SHT was set at 35 °C. OSA implementation did not affect COD and NH₄⁺ oxidation of the IFAS system (90-96% and 99%, respectively) and improved total nitrogen (TN) reduction (31-53%) due to improved denitrification in the SHT. On the contrary, sludge dewaterability worsened following OSA implementation, which was linked to the increased levels of exopolymeric substances in the suspended biomass.

Keywords: Extracellular polymeric substances; Integrated fixed-film activated sludge; Oxic settling anaerobic; Sewage sludge; Sludge minimization; Temperature.