This paper quantifies the influence of operating conditions on the efficiency of domestic wastewater treatment and of membrane bioreactors (MBR) performance with a statistical method (multivariate linear regression model). The impact of the following seven parameters-(1) transmembrane pressure (TMP), (2) mixed liquor suspended solids (MLSS), (3) dissolved oxygen concentration (DO), (4) hydraulic retention time (HRT), (5) sludge retention time (SRT), (6) temperature (T) and (7) pH on the accumulated filtrate volume (V) and water quality (COD, NH3-N) -were investigated. The results showed that MLSS and temperature had 34% and about 23% respectively the majority of the influence on the resulting filtrate volume among the seven influencing parameters. The mathematic expression obtained by the multivariate linear regression model between the accumulated filtrate volume and operating conditions was V = 249.64TMP-0.056MLSS+ 31.69DO + 55.06pH-5.25T-8.82HRT + 16.24. The COD of the filtrate primarily depended on the process temperature, which carried about 64% of the weight among all the seven investigated parameters. The ammonia nitrogen concentration (NH3-N), of the effluent was mainly a function of MLSS (about 40% influence) and pH (about 27% influence). No effect of the TMP on the filtrate quality was found. In addition, the mathematic expressions between COD, NH3-N and operating conditions were COD = 0.0073A4LSS-5.64DO + 2.86T + 0.099SRT + 2.48HRT-88.82 and NH3-N = 0.017MLSS-12.99DO-28.86pH + 0.90T + 0.13SRT + 201.30. These mathematic expressions could possibly be used to optimize and predict the efficiency of domestic wastewater treatment in bioreactor parameters.