Grundfos BioBooster (GBB) installed and operated a membrane bioreactor (MBR) test plant in 2012. During the period it became evident that the nitrification rate was lower than expected and a study was carried out to investigate the possible reasons for the observed low-nitrification rate. Tests were conducted at a pilot plant and the effect of shear from the BioBooster membrane system and the pressure reduction component on the nitrification rate was investigated. The possible effect of selection of microbial communities caused by the filtration unit was also investigated. The results revealed an unchanged nitrification rate when exposed to shear from the filtration unit and the pressure reduction component. When testing the effect of selection, the nitrification rate was also unchanged.