Design guidelines for a reverse osmosis desalination plant

Abstract

There are two basic needs globally and that is the control and supply of reliable electricity and clean water. However, one of the biggest challenges the world is facing today is the lack of fresh water resources. Lower rainfall, together with population and industry growth, are only a few factors contributing to the fast increasing strain on existing water supplies around the world. This fast increasing need therefore necessitates the investigation into finding alternative sources. One such option is that of desalination. In the last 50 years desalination technologies have been applied to produce high quality fresh water from brackish and seawater resources. In the 1980's a breakthrough was made with the introduction of the membrane desalination technology, known as the reverse osmosis (RO) process. Today newly developed technologies are improving the competitiveness of the reverse osmosis process against the traditional distillation processes. There are a number of options to increase the efficiency of a reverse osmosis plant and one option is to use warm industrial waste water as the feed water to the desalination plant. It is known that the viscosity of water is inversely proportional to its temperature. Therefore, if the feed water temperature of a reverse osmosis plant is increased the membranes will become more permeable. This will result in a higher production volume or in a lower energy demand. South Africa is on the edge of building the first fourth generation nuclear power plant, called the Pebble Bed Modular Reactor (PBMR) at Koeberg. The PBMR will produce a cooling water outlet temperature of 40°C which can be used as feed water to a reverse osmosis plant. In this study design guidelines of a reverse osmosis plant are given in nine steps. These steps were then used during a basic component design of a reverse osmosis plant coupled to the waste water stream of a PBMR nuclear power plant. Furthermore design software programs were used to simulate the coupling scheme in order to validate the outcome of the design guidelines. The results of the two design approaches compared well to one another. It furthermore showed that by using the waste water from the PBMR nuclear power plant the efficiency of the RO plant is increased and the operating cost is decreased. Fresh water can be produced at a cost of R 5.64/m3 with a specific electricity consumption of 2.53 kWh/m3.