The supply of energy in sufficient quantities and the access to clean water for every human are among the most significant challenges to address in the decades to come, as these are key elements of well-being and development. The present energy system is clearly reaching its limits in terms of sustainability, and the "business as usual' scenario is no longer acceptable. New approaches have been proposed based on much improved energy efficiency, development of renewable and new energies, and carbon capture and sequestration for fossil resources. The industrial deployment of these alternate scenarios is intrinsically related to that of water at large scale, and its use for human consumption, food production and industrial usage. A typical example is given by the exploitation of Athabasca tar sands by open pit, and the production of synthetic crude. Because the access to freshwater is getting scarce in many countries, the recycling and reuse of grey water (where it is possible), and the desalination of brackish and sea water are the most likely technologies to emerge for the supply of industrial water. Chemical engineering is the enabling discipline that will make the evolution possible.

In this communication, we will first begin with a brief description of our present energy system based on fossil resources, a legacy of the industrial revolution. We will then review the main drivers supporting the energy and water demand, and the constraints they are facing. In a next step, we will explain some of the tasks that chemical engineers will have to address in dealing with the energy-environment nexus in the future. Several examples will be used to illustrate the close interactions between clean energy production and water management.