This part of the study presents the theory, design and appropriate models of a new distillation system with natural vacuum technique. Natural vacuum distillation (NVD) system is considered as boolean AND (opposite U) to obtain 10.33 m of water elevation and create natural vacuum. Theoretical models of this system are investigated and appropriate models are selected for practice applications. In the scope of this study, the theory and design criteria of NVD system which include balance design and selection of distillation method, natural vacuum formation, balancing and feeding processes are introduced and discussed in detail. Additionally, P-v and T-s diagrams including the process flow of each model are obtained by depending on thermodynamic behaviours of the system. Consequently, the operation height between the tanks and columns should be stable and conserved as 10.33 in of water in order to create the natural vacuum and keep the natural balance of the system for a long time. Hence, the water is transported throughout the system due to the pressure differences between evaporation and distillation parts. The pressure losses that are caused by the water-level differences between tanks and columns should be taken into account and absolutely reduced. Moreover, the system should be definitely operated in the natural vacuum to efficiently conduct the production of distilled water from wastewater with the high temperature. Copyright (C) 2004 John Wiley Sons, Ltd.