Modeling of an aerobic granular sludge process for ammonia retention: Insights into granule size, dissolved oxygen concentration, and solids retention time

This study investigated the optimization of a continuous aerobic granular sludge (CAGS) process for effective ammonia retention, chemical oxygen demand (COD) removal, and reduction of nitrous oxide (N2O) emissions by a multi-species and multi-substrate one-dimensional granular sludge model. Increasing demand for ammonia recovery from high-strength nitrogenous wastewater requires stable ammonification, ammonia retention, and a low suspended solids concentration in effluent; therefore, applying CAGS is significant. By utilizing the granular sludge model, this research determined optimal operating conditions. A CAGS process without floccular sludge at DO concentration ≤ 1 g O2/m3 and granule size of 200 µm achieved an ammonia retention efficiency (ARE) of 62.8–86.1%. The COD removal efficiencies exceeded 90% in most cases except at a low DO ≤ 0.5 g O2/m3 and a large granular size (≥ 3000 µm). Incorporating floccular sludge in the model increased the chance of harboring ammonia-oxidizing bacteria (AOB), compromising the success of ammonia retention. The impact of the SRT for the floccular sludge on ARE depended on DO concentration and granule size. The simulation elucidated the presence of a boundary granule size drastically changing the phenomena between SRT and ARE at a DO concentration ≤ 2 g O2/m3, showing an increase and decrease in AREs with a longer SRT above and below the boundary granule size, respectively. These findings provide a detailed blueprint for the strategic operation of a CAGS process, balancing granule size, DO, and floccular SRT to achieve efficient ammonia recovery and organic matter removal while minimizing N2O emissions. [Display omitted] •A mathematical model consisting of granular and floccular biomasses was constructed.•Operation conditions of granular sludge for ammonia retention were determined.•Floccular sludge proportion affected ammonia retention in the granular sludge system.•Marginal N2O emissions were achieved during successful ammonia retention.