Saline strip

Siemens has combined its CEDI process with electrodialysis for a low-power desalination method for drinking water. Berenice Baker reports

Siemens Water Technologies is researching a lower energy method to desalinate sea water using membrane technology originally developed to make ultra-pure water for power plants and pharmaceutical applications.

Current desalination plants use evaporation, which consumes 10kWh per cubit metre, or reverse osmosis, in which water is pushed through a filter at pressure, which uses around 3kWh per cubic metre. Siemens plans to combine its continuous electrode de-ionisation (CEDI) process with electrodialysis (ED) to make drinking water; it claims this would only use 1.5kWh per cubic metre.

Siemens’s research is based at the Singapore government’s Water Hub, an R&D facility that was set up to study solutions to the global scarcity of drinking water. Siemens began a three-year project in 2008 to reduce the power consumption of desalination technologies.

Tim LeTourneau, director of global desalination at Siemens Water Technologies, said: ‘Both ED and CEDI use membrane technology. In a reverse-osmosis system, water passes through a membrane in a sieving action and the contaminants remain on one side and the treated water goes through the membrane.

‘The ED CEDI solution uses an ion-transport mechanism whereby the water doesn’t pass through the membrane, the ions do. In salt water, the sodium and chloride ions will pass through membranes leading to separate channels and ultimately be removed from the flow stream.’

Dr Rüdiger Knauf, vice-president of R&D for Siemens Water Technologies, said: ‘You are pulling the salt through the membrane using an electrical field as the driving force by having an anode and a cathode either side. By using an alternating set of membranes, you are able to separate the salt from water.’

The membrane is made using a process that imbues it with chemical properties, allowing it to be employed in a variety of ways, such as allowing different ions through.

A water desalination plant that uses this method consists of a series of chambers separated from each other by a membrane. Every other chamber has a different kind of membrane; one will have a cation exchange membrane, the next will have an anion exchange membrane. The sodium ions can pass through only one type of membrane, the chloride ions through the other. Because they cannot pass through the other type, they are prevented from mixing again and can be removed from the system.

Siemens had to make a number of changes to its existing CEDI system to make it suitable for desalination. The components of the CEDI wall had to be made much thinner, requiring a considerable amount of engineering to make modules that could accept greater membrane areas in a restricted space.

One of the challenges in adapting the technology was that in the ultra-pure water application, conductivity is extremely low, just microsiemens per cubic centimetre. ‘You are then applying it to seawater, which has 35g of salt in one metre of water, giving it a very high conductivity,’ added Knauf. ‘There are major process changes required to manage the power and current, which leads to the uniqueness of our ED CEDI process.’

The project will culminate in a 50m3 demonstration unit in late 2011. Siemens is currently running small lab units and will make progressive steps in size through alpha then demo units.

At the Singapore Water Hub, Siemens Water Technology employs 40 people working on other types of research, including advanced biological treatment, novel disinfection and oxidation methods and a process for removing trace contaminants such as mercury and arsenic.

‘There is a lot written about global water trends, water scarcity, globalisation, industrialisation and urbanisation,’ said Knauf. ‘Our customers also care about availability, affordability, safety and reliability, which we try to address by providing lifecycle cost-advantages through advanced engineering and advanced process technologies.

‘It is not always a new technology that solves these problems. As in this project, it is very often a solution that comes from existing established technologies that you combine in hybrid processes to provide an integrated solution.’