Bench-scale systems, using conventional and compact hybrid activated sludge configurations, were set up to evaluate the systems' nitrification–denitrification performance, operating sludge age/MLSS concentration and sludge settleability at a Hong Kong municipal STW. Configurations tested were the continuous clarifier modified Ludzack Ettinger (MLE) and the sequencing batch reactor (SBR) with and without hybrid suspended biofilm carriers. Results demonstrated that the hybrid SBR and MLE systems consistently achieved close to complete nitrification (effluent NH4-N = 2.4 and 6.9 mg/L) and 75% and 67% removal of nitrogen (N) (effluent NO3-N < 10 mg/L) with an overall hydraulic retention time of only 7.5 hours, operating sludge age as short as 5.2 days, and mixed liquor suspended solids concentration of ∼1,300 mg/L with a sludge volume index of 109 and 229 mL/g, respectively. The most sensitive and slowest growing nitrifiers attached to the hybrid biofilm carriers. This allowed the hybrid processes to be operated at a sludge age shorter than the critical nitrifying sludge age while still retaining near complete nitrification. In contrast, to achieve complete nitrification, the conventional MLE system needed to be operated at 1.5 to 2.5 times the critical sludge age. These results indicate that the hybrid MLE configuration is a suitable process for use in upgrading existing conventional works for N removal and for increasing hydraulic capacity of existing N removal works, without major civil works modifications, in Hong Kong. For new works, consideration might be given to the use of the hybrid SBR, which shows a more stable N removal performance than the MLE process due to its inherent in-basin equalization capacity and better reaction conditions for nitrification in terms of higher initial NH4-N level. It was also observed that the conventional SBR produced better nitrification performance than the hybrid MLE process tested. Design parameters and operating conditions of the hybrid systems should be subjected to further full-scale trial for higher hydraulic capacity and N removal performance.

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