In this work, a mathematical model representing two series interconnected chemostats where the mortality of the species is taken into consideration, is studied in detail. The study is carried out with different mortalities of the two tanks. The specificity of this study is the intervention of two types of heterogeneities. There is heterogeneity in relation to the distribution of the total volume in both tanks and heterogeneity in relation to the different mortalities of the two tanks. We study the performance of the serial configuration under two different criteria which consists on the substrate concentration leaving the second tank and the biogas flow rate production. A comparison is made with a single chemostat where the mortality rate is considered to be the same in all tanks, i.e. in the single chemostat and in the interconnected tanks. Conditions depending on the mortality rate, on the parameter; defining the distribution of the total volume between the two tanks and on the operating parameters that are the substrate concentration at the entrance of the first tank and the dilution rate, are involved. These conditions allow to have a serial configuration with mortality more efficient than a single chemostat with the same mortality.