Minimizing biological sludge generation in a sidestream enhanced biological phosphorus removal (S2EBPR) system: full-scale evaluation and modeling insights†

This study evaluated sewage sludge production in real-scale and modeled sidestream enhanced biological phosphorus removal (S2EBPR) systems under various mixing, organic feeding, and solids retention time (SRT) conditions. The S2EBPR process involves diverting a portion of recycle activated sludge (RAS) to an anaerobic sidestream reactor (SSR) to stimulate biological phosphorus uptake. A portion of the full-scale wastewater treatment plant (WWTP) was modified to carry out a demonstration study. This study comprised two different biological systems operating in parallel: the S2EBPR system and a conventional activated sludge (CAS) system. Data collected during an 11-month experimental campaign were used to estimate and compare sludge production in terms of observed sludge yield (Y_obs) between the two biological processes. In the S2EBPR system, Y_obs was 47% lower compared to the reference CAS when complete mixing was provided to the SSR and no external carbon was added. When mixing was provided only at the inlet and outlet of the SSR a 23% reduction was observed. The addition of external carbon did not yield significant benefits in terms of reducing Y_obs. Modeling the S2EBPR performances at higher SRT values (20–50 d) indicated a potential further reduction in sludge production, ranging from 12% to 24%, by maintaining the SRT between 30 and 50 days. Sludge settleability was not significantly affected by SSR addition and nor by different operating conditions tested.