The consequences of a heavy and varying load on a BNR-WWTP (biological
nitrogen removing plant) have been investigated in full scale. The in
vestigations took place at a 57.000 PE BNR plant which is operated acc
ording to the BIODENITRO principle. The plant had been heavily overloa
ded with BOD and N, 2-3 times the designed capacity. During the overlo
ad 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. F
ull denitrification was obtained during peak loadings in spite of aera
tion of the total process volume. During high and varying loads, the m
ast efficient utilisation of the process volume can be obtained by the
following: 1) The plant design should allow of a change in aeration v
olume from 20% to 100% of the total process volume. Changing the aerat
ion 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 met
ers. The historic loadings must be included in the control strategy to
avoid alkalinity depletion during heavy load variations. (C) 1998 Pub
lished by Elsevier Science Ltd. All rights reserved.