Treatment is currently necessary for municipal and industrial wastewaters, given the decli- ning quantity and quality of available water. An anaerobic biological reactor is a suitable option for wastewater treatment because it generates biogas which contains methane, a potential source of energy. The objective of this research was to treat wastewa- ter from a chocolate industry with an applied organic loading rate (OLRappl) of 4.5 kg of chemical oxygen demand (COD)/m3/d and a hydraulic retention time (HRT) of 10.3 h to obtain both, the removal efficiency, and the mass balance in terms of COD. Even though there were variations in the OLRappl and the TRH, the average removal efficiency of COD was 91%. The cal- culated mass balance in terms of COD was close to 100%.