Construction or modernization of a wastewater treatment plant (WWTP) with membrane bioreactor (MBR) technology requires selection of optimal configuration and design. The objective of this paper is to evaluate a hybrid MBR, i.e., a combination of a conventional activated sludge (CAS) process and an MBR, in comparison to a stand-alone MBR. This paper evaluates two different hybrid MBR configurations and a stand-alone MBR. The impact of these MBR configurations on operation, performance, energy consumption and economy is evaluated. A hybrid MBR operated in series provides certain operational flexibility, ensures more stable conditions for the activated sludge leading to a more stable MBR operation at energy and cost efficient conditions. Nevertheless, determination of the optimal plant configuration depends on the particular local situation.