Exergetic Life Cycle Assessment (ELCA) is applied to a wastewater treatment plant (WWTP) by activated sludge in order to demonstrate the environmental and exergetic effect of biogas recovery within the whole system boundaries. Two methods of recovery are compared: a simple burner (Burner scenario) and a combined heat and power engine (CHP scenario). The results showed that biogas recovery reduces the depletion of natural resources and decreases ecological impacts. Eutrophication, toxicity and ecotoxicity impacts are not affected by biogas recovery in the Burner scenario and the change is not significant in the CHP scenario. Irreversibility is added to the system by biogas recovery. This is because of the nature of the burnt mixture, which is different from conventional fossil fuel and because of the addition of units such as CHP and heat exchangers. The CHP fed with the biogas cannot fulfill the total demand in thermal energy for heating and drying in comparison to the burner. Natural gas should be burnt additionally and a heat exchanger introduced. The irreversibility of the CHP scenario is larger than the Burner's one, but the former allows reducing electricity consumption by 22.7%. Moreover, exhaust gas has a positive effect from the exergy point of view. It contains an excess potential work which could be recovered in another function. Based on ozone layer depletion, photochemical oxidation, acidification and marine aquatic ecotoxicity impacts, biogas recovery with a CHP scenario is better than with a Burner scenario. But based on abiotic resources depletion, global warming potential, irreversibility and excess potential work the second strategy, with a Burner scenario, is the best one. In process optimization, the focus should not be only on potential impacts reduction. The decrease of the irreversibility produced by new functions introduced in the system should also be taken into account. The other subsystems within the system boundaries should be looked at especially those related to electricity production. The ELCA is a useful tool for system ecodesign and for finding the best way to promote renewable fuels versus fossil ones.