In a large hall on the outskirts of Vienna, shiny pipes carry treated wastewater through three giant heat pumps, part of Austria's drive to reduce carbon emissions and its dependence on Russian gas, with more and more European cities eyeing this alternative. The plant -- billed as Europe's most powerful one -- is churning out district heat to up to 56,000 Vienna households, with operator Wien Energie planning to double its capacity to 112,000 households by 2027. "It is very clear that we have to restructure our energy system to become independent of fossil fuels or of different individual countries," Linda Kirchberger, a manager at Wien Energie, tells AFP. IMAGES AND SOUNDBITES