The consequences of a heavy and varying load on a BNR-WWTP (biological nitrogen removing plant) have been investigated in full scale. The investigations took place at a 57.000 PE BNR plant which is operated according to the BIODENITRO principle. The plant had been heavily overloaded with BOD and N, 2–3 times the designed capacity. During the overload events, the BOD was efficiently removed to below the effluent limit. The N-removal, however, was reduced to below 50% during these events. The nitrification may be inhibited considerably after peak loadings due to a drop in pH caused by nitrification without denitrification. Full denitrification was obtained during peak loadings in spite of aeration of the total process volume. During high and varying loads, the most efficient utilisation of the process volume can be obtained by the following: 1) The plant design should allow of a change in aeration volume from 20% to 100% of the total process volume. Changing the aeration volume is much more effective than changing the oxygen set point. 2) On-line control of the aeration time by NH4, NO3 and pH on-line meters. The historic loadings must be included in the control strategy to avoid alkalinity depletion during heavy load variations.

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