The nitrogen removal efficiency of the step-feed anoxic-oxic activated sludge process, which has two anoxic tanks and two oxic tanks, was theoretically discussed on the basis of the stoichiometry of denitrification and nitrification reactions. As the first step, effluent NH4-N and NO3-N concentrations were formulated with four parameters; 1) a, equivalent ratio of alkalinity to ammonia in influent, 2) b, that of substrate to ammonia, 3) r, step ratio of influent to the second anoxic tank and 4) R, return (+ recycle) sludge ratio. This calculus was done for the possible sixteen (=24) cases which show different reaction patterns in four tanks, and 12 cases out of 16 were found to be available. The effects of step ratio, r were examined in its range of 0 - 1 at a fixed R value, and it was found that the increase of r alters the outcome in a different way depending on the ranges of a and b. Consequently, zoning of a-b coordinates was successfully made, and the optimal r value for maximum total nitrogen removal was obtained in each zone. In addition, the optimal volume allocation of the four tanks was discussed and the ratios were formulated for each zone.

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