Water conservation is an increasingly important requirement in the economic and environmentally sound operation of process plant over a range of industrial sectors. ESDU 00020 and computer program AQUAINT are part of ESDU's Guide to Industrial Water Conservation. Their purpose is to familiarise process engineers with the latest techniques for water conservation and allow exploration and evaluation of conservation schemes though water re-use. ESDU 00020 describes the economic and environmental importance of water conservation and summarises the various conservation means. Techniques are then provided for the estimation of minimum required quantities of freshwater (freshwater-use targets) and the design of water re-use networks. The methods can be applied to single or pseudo-single contaminant systems with contaminants such as biological or chemical oxygen demand (BOD or COD)@ suspended or total solids or single species such as chloride ion concentration. Multi-contaminant systems will be covered in later parts of the Guide. A methodology is described for identifying the minimum amount of freshwater required by a system whilst highlighting critical operations within the system. This provides a means of identifying maximum possible water savings and provides guidance on effective data collection by focusing on critical operations. Techniques for designing water networks that recognise the importance of plant economics and operability and achieve minimum water-use targets are presented. Water regeneration steps can also be incorporated into network design and suitable specifications of regeneration systems can be explored. AQUAINT incorporates all the procedures described in ESDU 00020. It allows rapid minimum freshwater-use targeting using appropriate field data and provides network design information. The use of AQUAINT is illustrated by a number of worked examples.