It has been shown that by combining the patented process for biological phosphorus removal with post-denitrification without additional carbon dosing in membrane bioreactors (MBRs) good nutrient eliminations can be achieved. Especially, reported denitrification rates are higher than expected ones. The reason for this is not understood yet but a correlation between acetate dosing in batch trials and DNR has been reported, The present study investigated the influence of COD sludge loading and acetate inflow concentration on denitrification rates in continuous MBRs. A laboratory scale MBR operated with synthetic wastewater was switched from a multi to a mono substrate (acetate) wastewater and compared to three other MBR connected to separate sewer systems. Better eliminations for COD, TN and TP for the MBR operated with acetate were expected. However, elimination efficiencies were on a good level for all plants and configurations (eliminations: 94-97% COD. 86-94% TN, 92-99% TP) and no significant increase for mono substrate could be found. Average denitrification rate and TN elimination was even a bit lower with mono substrate. For increasing COD sludge loads increasing denitrification rates could be found. However, the variation within the rates has proved that many different influences have to be considered. (C) 2009 Elsevier B.V. All rights reserved.