NOVELTY - The psychrophilic anaerobic reactor (10) comprises a fluid inlet, a mixing chamber (12) supplied by the inlet, a biofilm chamber (16) present in fluid communication with the mixing chamber, a biofilm carrier medium (18) locatable within the biofilm chamber, a separation chamber (28) present in fluid communication with the biofilm chamber, a porous retainer for retaining the carrier medium within the biofilm chamber while permitting the flow of effluent around the carrier medium, and a recirculation chamber disposed between the mixing chamber and the biofilm chamber.    USE - The anaerobic reactor is useful for treating liquid effluent at psychrophilic and/or mesophilic temperatures.    ADVANTAGE - The anaerobic reactor is capable of efficiently and economically treating the liquid effluent with less energy consumption in an environmentally friendly manner.    DETAILED DESCRIPTION - The psychrophilic anaerobic reactor (10) comprises a fluid inlet, a mixing chamber (12) supplied by the inlet, a biofilm chamber (16) present in fluid communication with the mixing chamber, a biofilm carrier medium (18) locatable within the biofilm chamber, a separation chamber (28) present in fluid communication with the biofilm chamber, a porous retainer for retaining the carrier medium within the biofilm chamber while permitting the flow of effluent around the carrier medium, a recirculation chamber disposed between the mixing chamber and the biofilm chamber, a recirculation line permitting the recirculation of effluent and/or biogas from the separation chamber to the recirculation chamber, a unidirectional valve disposed between the mixing chamber and the biofilm chamber and arranged to permit fluid flow from the mixing chamber directly or indirectly into the biofilm chamber, a unidirectional valve disposed between the biofilm chamber and the separation chamber and arranged to permit fluid flow from the biofilm chamber into the separation chamber, a mixing unit disposed within the mixing chamber to supplement the mixing produced by biogas generated within the mixing chamber, a two phase separator in the separation chamber and operable to separate biogas from treated effluent, and a passive vacuum stripper downstream of the separation chamber and operable to strip residual dissolved biogas from the treated effluent. The carrier medium has a specific gravity lower than the effluent to be treated in the reactor. The retainer comprises a cassette shaped and dimension for location within the biofilm chamber and operable to retain the biofilm carrier medium and having a porous base and top in order to permit the flow of fluid through the cassette. The biofilm carrier material is granular in form, and graded in size, where the largest elements are located closest to the biofilm chamber and the smallest elements are located closest to the separation chamber. The mixing unit comprises an array of baffles located within the mixing chamber. The mixing chamber is operable as a fluidized bed. The biofilm carrier medium comprises a pumice based material. An INDEPENDENT CLAIM is included for a method of anaerobic treatment of a liquid effluent at psychrophilic and/or mesophilic temperatures.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a psychrophilic anaerobic reactor.    Anaerobic reactor (10)    Mixing chamber (12)    Biofilm chamber (16)    Biofilm carrier medium (18)    Separation chamber. (28)