Microbial treatments of waste water can be done in membrane reactors. A membrane installed outside the reactor is used to separate bacteria from the treated effluent. A new membrane reactor concept is presented. The separation membrane is introduced in the reactor and not outside as in a normal one. The membrane plays the role of a separator of two streams and is used at the same time as the immobilizing support for the bacteria. The reactor keeps the bacteria active via a specific nutrient stream that is provided on one side of the membrane. The bacteria grow in and on the membrane where they form an active biofilm. The bacteria can treat the effluent on one side and can be kept active via the nutrient stream at the other side without contamination of the effluent by the nutrient. In this work, the performance of the BICMER (Bacteria Immobilized Composite MEmbrane Reactor) is demonstrated via treatments of effluents containing heavy metals or organic xenobiotics. For heavy metal removal Alcaligenes eutrophus CH34 bacteria were used. These bacteria induce a metal bioprecipitation process that results in the formation of crystalline metal carbonates, which are recovered on a separate column in the reactor. In this way metals can be recovered without disturbing the biofilm on the membrane. Metals such as Cd, Zn, Cu, Pb and Y can be reduced to less than 50 ppb. The metals Co, Ni, Pd and Ge are reduced to below 100 ppb. For organic xenobiotics Alcaligenes eutrophus AE1308 bacteria or other strains (depending on the xenobiotic to be degraded) were used, This strain degrades the xenobiotic 3-chlorobenzoate (Cba) and 2,4-dichlorophenoxyacetic acid to CO2, H2O and chloride). Concentrations of 3 mM Cba could be reduced to less than 0.1 mM. For other toxic organic compounds, different biodegrading strains need to be used.