A mathematical optimisation method utilising the BLIND algorithm was used for model calibration and subsequent simulations on an ASM No.1. based platform. The calibration of model parameters was fulfilled with the use of operational data of a large wastewater treatment plant (of 80000 m3·d-1 capacity) owing a high-load activated sludge stage (sludge age of 1.8-2.5 d) with a subsequent nutrient removal biofiltration system. The calibrated model was applied for the optimisation of the multi-point addition of nitrifying biomass (i.e. step-seeding) and air supply in the high load system that has a bioreactor with plug flow characteristics. Results of model simulation showed, that with addition of nitrifyers in three points may enhance nitrification by about 9% comparing to the single point addition at the head of the reactor. Nitrogen removal can be further increased by about 8% with optimising air supply along the reactor length. Results showed, that the step-seeding concept may convey a potential in nitrogen removal enhancement in combined systems with separate nitrification/denitrification units, since optimised addition of the biomass can be easily performed.

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