Abstract Nowadays, energy consumption is one of the major concerns of wastewater treatment plants (WWTPs). Time ago, anaerobic digestion was usually implemented for sewage sludge stabilization but energy recovery optimization has recently gained importance. The energy balance of five WWTPs located in Catalonia revealed that depending on the configuration of the plant and its operation, between 39% and 76% of the total electric energy consumed in the WWTP could be supplied by the biogas produced. In the second part of this work, a carbon, nitrogen and sulphur flux analysis was carried out, together with an energy content evaluation for each stream in the WWTP. Results showed that 37% of the carbon found in the raw wastewater was removed during the active sludge process and 24% was transformed into biogas. The remaining carbon was found in the anaerobic dewatered sludge (22%) and in the treated water (19%). As a result, 34% of the initial energy was recovered in the form of biogas.