Following the creation of the first reverse osmosis (RO) membrane in the 1960s, the technique has been widely used for the purposes of both small scale and municipal seawater desalination. Forward osmosis (FO) is now also emerging as a possible contender, with the potential for much lower energy consumption. In this study, we have developed a thermodynamic benchmark for use in assessing the suitability of a potable water system for purifying small amounts of brackish water in emergency situations. The light, portable and re-usable purification system is driven by FO. A Pouch is filled with draw solution and immersed in brackish water; the pouch incorporates a traditional RO membrane. The 'draw solution' contains digestible salts and/or sugars to provide an osmotic pressure difference across the membrane, thus drawing in purified water across the membrane. Three such draw solutions were produced and tested, allowing the osmotic potential of the solution to be determined over a succession of dilutions. The results Could be fitted with a power law function. In order to take account of the solution non-ideality and the non-linearity Of flux rates, a thermodynamic relationship was used in conjunction with a membrane transport model to develop a benchmark which describes the ideal behaviour of a FO water system. This benchmark, in conjunction with the power law function, showed that such a system could be used in an emergency to provide safe, potable water in a reasonable time interval and without the need for a power source. The study has also suggested the possibility of a Continuous water Purification system based upon this principle, and has drawn attention to the benefits of novel draw Solutes ill such a system.