Anaerobic waste water purification.

Process for purifying waste water, comprising at least a step of anaerobic biological purification, performing anaerobic biological purification in continuous operation in at least one bioreactor complete mixing via biomass, wherein the biomass of psychrophilic microorganisms is selected, concentrating biomass by microfiltration and reconduciendose to anaerobic biological purification, and wherein the biomass in the bioreactor has a concentration of 25 g / l.