The great diurnal variation in the quality of wastewater of small communities is an obstacle to the efficient removal of high nitrogen with traditional activated sludge processes provided by pre-denitrification. To verify this problem, the authors developed a pilot plant, in which the domestic wastewater of community of 15,000 inhabitants was treated. The results demonstrate that average and peak nitrogen removal efficiencies of over 60% and 70%, respectively, are difficult to obtain because of

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... the strong variations in the BOD 5 /NO 3 -N ratios and the unexpected abnormal accumulation of dissolved oxygen during denitrification when the BOD 5 load is low. These phenomena cause inhibitory effects and BOD 5 deficiency in the denitrification process. The results demonstrate the need for a more complex approach to designing and managing small wastewater treatment plants (WWTPs) provided with denitrification than those usually adopted for medium-and large-size plants.