Desalination of Sea Water
Decanters Considerably Reduce Transport and Landfill Costs
The market for processing drinking water from salt water will virtually double
According to Global Water Intelligence, the market for processing drinking water from salt water will virtually double by the year 2016. The market research company is assuming up to 12 million cubic meters of sea water which will then be desalinated every day. This development is being driven mainly by the arid regions of the world, climate change as well as increasing demand in agriculture, municipalities and industry. Agriculture in particular will probably significantly boost demand for process water.
Desalination of sea water is applied mainly in countries in the Gulf region of the Middle East, traditionally in the Caribbean and also in the USA, Australia and Spain; it is also increasingly to be found in large conurbations, e.g. city states such as Singapore or Hong Kong. For instance, Singapore aims to recover up to 30 percent of its water requirement from desalination by the year 2061, when the water supply agreements with neighboring Malaysia are due to expire. According to the study of Global Water Intelligence, the technology of reverse osmosis is the preferred solution, a considerable distance ahead of heat treatment.
Decanters from GEA have already been used for more than ten years in this water desalination process; they are used in the essential mechanical pre-treatment of the sea water for concentrating sediment or filtration back-wash water. The most recent applications are two salt water processing plants in Israel and China.
Reverse osmosis is the more common method
In general, two processes can be used for desalinating sea water: reverse osmosis with semi-permeable membranes or heat treatment. However, heat-treatment installations which evaporate the sea water, thus removing salt from the water which then recondenses as drinking water, are very energy-intensive. They are mainly to be found in applications in regions with their own resources from oil or gas production. Reverse osmosis is much the more common process.
Concentration of the backwash water
In general, the reverse osmosis installations are set up down-stream of a flotation unit or an ultrafiltration unit for removing the insoluble substances in the sea water. The sea water is taken from the sea with suction facilities which are installed off the coast. The reverse osmosis installations have to be flushed at certain intervals. This backflush water or the sedimented sludge from the flotation unit are reliably separated by decanters into solids and reusable water. In both systems, the solids are concentrated by the decanter, thus resulting in a much lower transport and disposal volume. In the case of backflush sludges from filters, the solids concentration in the feed is between one and two percent; sludges from sedimentation have higher dry matter contents of between three and five percent, depending on the organic content of the sea water. The decanters are able to concentrate the solids up to ten times, to a dry matter of approximately 20 to 25 percent. This represents a huge reduction in terms of volume.
The right choice of materials is crucial
Decanters operate extremely reliably, have a very long life and their defined service costs can be calculated in the long term. However, the correct design of materials is essential in the process of desalinating sea water. With its extremely high content of chloride ions (around 15,000 ppm), sea water is extremely aggressive. Depending on the specific characteristics in terms of pH value, temperature and chloride concentration, GEA chooses the right material for the parts of the decanters which come into contact with product in order to permanently avoid corrosion. In general, duplex or super duplex steels are used. With 25 percent chromium, four percent molybdenum and seven percent nickel, super duplex is extremely resistant to chlorine corrosion.
Many years of experience
GEA is able to ideally configure the machines. The first application was seen in 2003, at a desalination plant in Cartagena, Spain, followed by many further installations, for instance in Perth, Australia, or in China. The most recent installation is the desalination plant Ashdod in Israel on the Mediterranean, where two GEA waterMaster CF 4000 of the latest ecoforce decanter generation are used.
Recovering drinking water from salt water becomes more important
