A case study of the conversion to grey water to a flush water source in a student residence hall of the Turkish megacity Istanbul is presented. Grey water from bathtubs/showers and washbasins is treated and reused for flushing toilets. Raw and membrane bio-reactor (MBR) treated grey water quality was monitored throughout one academic year using parameters to characterize organic matter (COD/BOD), suspended solids (TSS/VSS), nutrients (N/P) and microbiological indicators (total/fecal coliform, Escherichia coli, Enterococcus). For raw grey water, all mean concentrations are below the characteristic concentrations for weak conventional domestic wastewater in terms of physicochemical parameters. However, total coliform concentrations were 7 logs, which is compatible with weak/intermediate conventional domestic wastewater and the mean concentration of fecal coliforms was 6 logs, corresponding to intermediate/strong conventional domestic wastewater. Results of this work revealed that organic matter and microbiological indicators were the important pollutants for this specific grey water. After treatment in an MBR system, grey water quality complied with standards given for reuse as flush water. The student residence hall was a successful example of conversion to grey water as a flush water source. As such, 25% of the wastewater could be revaluated and 25% of the regular daily water supply could be saved.