Practical application and calibration of apparent half-saturation coefficients for aerobic granular sludge modelling: Limitations and value

Aerobic granular sludge (AGS) models for process optimization and control require balancing model complexity and accuracy. This study explores the strengths and limitations of using 0D-biofilm models based on apparent-half saturation coefficients (K S,app ) and evaluates a recently proposed procedure, based on 1D-biofilm model outcomes ( 1D-biofilm-to-K S,app procedure), for calibrating K S,app usig routine monitoring data commonly available from full-scale facilities. Two case-studies were investigated, differing in reactor size and process conditions, both characterized by operation as sequencing batch reactors. The implications of using such estimates in 0D-biofilm AGS models were assessed, also considering the possibility of incorporating vertical gradients in the reactor model during the non-mixed anaerobic feeding phase. When implemented in a completely mixed batch reactor model, significant relative errors (30–82 %) were observed in predicting steady-state effluent concentrations (ammonium, phosphorous and biodegradable COD), with negligible improvements when vertical concentration gradients were included. Based upon sensitivity analysis outcomes, direct calibration of K S,app was carried out by fitting predicted vs. average measured effluent concentrations. The application of these new K S,app values under dynamic conditions proved that 0D-biofilm models calibrated using routine monitoring data may not reliably capture the typical variability of AGS processes (Theil's Inequality Coefficient > 0.36 for ammonium and phosphorous). • Apparent half-saturation coefficients were estimated for two AGS case studies. • Critical aspects in the procedure adopted for K S,app estimation were identified. • K S,app from the procedure adopted caused errors >30 % for steady state simulations. • Plug-flow hydraulics inclusion in 0D-biofilm models poorly improved model accuracy. • 0D-biofilm models based on K S,app were scarcely appropriate for dynamic simulation.