In this study two desalination technologies have been compared by means of LCA with the focus on energy supply with a variety of scenarios based on different assumptions. The studied technologies are reverse osmosis and the newly developed technology Memstill where electrical energy demand is reduced by using external thermal energy source and use of chemicals is reduced. The scenarios are chosen to simulate different locations of sea water desalination plants and to demonstrate the impact of energy demand and energy source, chemical use and longer components' life times on the environment. The results show the dominant impact of energy demand on the overall results for all applied environmental impact assessment methods; transports, material and even chemical use are of negligible impact on the total score. The use of waste heat in case of Memstill offers a great opportunity to reduce the negative environmental impacts related to energy use. As the LCA analysis tool the GaBi software has been applied with the Ecoindicator99, CML 2001 and Ecopoints environmental impact assessment methods. © IWA Publishing 2011.