The design and optimisation of MBR units require knowledge of the biokinetics, fouling potential and mixing. Although the mixing within an MBR system is of critical importance to the performance, MBRs are mainly designed on the basis of biokinetics and fouling potential of the treatment system while assuming the hydrodynamic characteristics. One method to characterise the mixing is the residence time distribution (RTD). In this work, tracer studies using lithium chloride were performed to acquire RTD profiles of two full-scale MBR systems with different membrane configurations (flat sheet and hollow fibre). Analysis of the RTD profiles indicated that that both MBRs, including their respective filtration tanks, are very close to completely mixed. The mixing energy per volume of permeate used by the hollow fibre membrane vessel was lower than that of flat sheet module MBR; both in terms of whole MBR energy usage and membrane blower only energy usage. Hence, it is possible to conclude that the flat sheet MBR, per square metre of membrane, in this case, requires more energy to achieve a similar degree of mixing. (c) 2010 Elsevier B.V. All rights reserved.