Step feeding flow configuration is an attractive process alternative for large treatment systems designed for nitrogen removal, eliminating the need for internal recycling and optimizing organic carbon utilization for denitrification. Volume ratios of anoxic and aerobic volumes and wastewater fraction to be diverted from the inlet of the system are important parameters to be considered in the design of the step feeding process. Wastewater characterization and especially C/N ratio significantly affect the design. The effect of wastewater characterization is illustrated for the Riva Plant in Istanbul, with a capacity of 4 million population equivalent, designed as a two-stage step feeding system for biological nutrient removal. It is shown that the proposed design cannot meet the effluent standard of 10 mgl−1 total N for the relevant sewage characterization assessed on the basis of an extensive experimental study, totally different from the one adopted for design.

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