Fostering the performance and stability of the high-rate activated sludge process: Biomass dynamics, oxygen consumption and clarifier operation

The optimization of performance and long-term stability of a demonstration-scale High-Rate Activated Sludge (HRAS) pilot plant for treating urban wastewater treatment (35 m 3 d −1 ) was evaluated over 497 days. Maintaining the mixed liquor suspended solids (MLSS) concentration between 1800 and 2100 mg L −1 through waste sludge adjustment stabilizes the operation, preventing biomass washout and sludge bulking. Despite significant fluctuations in influent chemical oxygen demand (COD), ranging from 340 to 1580 mg L −1 , the system consistently achieved an organic removal ratio of 58 %, even under elevated loading conditions, with a low oxidation rate of only 6.9 % ± 3.6 %. Oxygen management was crucial for a high system's performance. The oxygen uptake rate (OUR) ranged from 31 to 54 mg O2 L −1 h −1 , and the specific oxygen consumption (SOC) averaged 0.9 kg O2 kg CODrem −1 . The study highlighted the potential of using soluble COD SOC as an effective parameter for optimizing oxygen supply. The study also evaluated solids removal efficiency using clarifiers of different diameters. The influent suspended solids (SS) removal efficiency increased from 71 % in a 1.0 m diameter clarifier to 85 % in a 1.4 m. The HRAS efficiently acted as a filter for the SS influent's peak loads, buffering the load to the CAS process. A detailed sludge stratification analysis revealed a balanced biomass distribution between the reactor and clarifier, with 50 % of the total biomass retained in the clarifier. Solids flux analysis confirmed that the system was not limited by solids loading but rather by hydraulic loading, with potential improvements achievable through optimization of the overflow rate (OFR). • The organic removal rate averaged 58 % under high organic loading conditions. • Oxygen Uptake Rate ranged between 31–54 mg O2 L −1 h −1 . • Soluble COD SOC identified as key for optimizing oxygen supply. • A proper design of a clarifier is crucial for a stable operation. • Solids flux analysis showed hydraulics, not solids loading, limited performance.