MBR, Membrane BioReactor, is a relatively new wastewater treatment technique using membrane filtration to separate particles from the biologically treated water. By means of analyzing analysis results from KARV, the existing wastewater treatment plant in Kalmar, from the recent six years and assuming that PO4-P in the water from the secondary settling tank equalizes with total effluent phosphorus in an MBR, the opportunity of an MBR to reach future effluent requirements could be assessed. The future effluent requirement for phosphorus is expected to be 0.2 mg/L or 0.1 mg/L. Today the phosphorus effluent requirement in Kalmar is 0.3 mg/L total phosphorus as annual average. Results from data analysis presented in figures shows that with the same operation strategy as during the six evaluated years, the future requirements would not be reached. Phosphorus is essential for yield and performance of biomass in the biological treatment step. The starting position for the work is that the way of operation during the six recent years would have given exactly the right amount of P to the bacteria in the biological treatment step. An increased dose of chemicals for phosphorus removal could lead to P-deficiency in the biological treatment step and thus decreased efficiency. This effect could be an essential aspect in the design of a future MBR in Kalmar.