The aim of LIFE NIMBUS project is to demonstrate the power-to-gas technology based on biological processes to produce biomethane by biological methanation and bio-H2 by bioelectrochemical system. From a technical point of view, the main objective is to produce biomethane that accomplishes the quality to be used as a biofuel and, at the same time, store renewable energy that would be lost otherwise as gas. In order to tackle this objective, a biological methanation demonstration plant will be designed, built and operated in the Baix Llobregat WWTP in Barcelona. The biomethane produced by this demonstration plant will be used to fuel one public bus. Additionally, there is the aim to increase the efficiency of the energy storage and so an innovative mixture of H2 and bio-H2 sources will be evaluated. The project will demonstrate the technical and economic feasibility of a power-to-gas solution based on biological processes, contributing to increase the renewable energy production and therefore reducing carbon footprint, which is one of the proposed solution environmental benefits. LIFE NIMBUS will also include a business and a replication plan to ensure the implementation of the proposed power-to-gas solution in other sites, such as Sète WWTP in France.