Drinking water is becoming scarce in many coastal regions. Seawater desalination can help, but conventional processes consume huge amounts of energy. Siemens engineers have now developed an electric desalination technique that cuts energy consumption in half.
A new biological water purification facility developed by Siemens Water Technologies generates enough methane gas to power its own operations. It also produces much less sludge than conventional systems. The pilot facility for this process, which is located at a site run by Singapore’s Public Utilities Board, has been operating in an energy- neutral manner since June 2010.
The new facility’s predecessor used an aerobic (ventilated) process in which bacteria broke down impurities in water by digesting them and converting them into new bacterial substances. This produced bacteria flakes filled with impurities — forming sludge that is then separated and either deposited in landfills or burned. “This wastes energy, because the organic impurities contain ten times more energy than we need to do the cleaning itself. All we have to do is use it,” says Dr. Rüdiger Knauf, Director of Development at Siemens Water Technologies. However, sludge concentrations in municipal sewage systems are too low to produce methane economically, so Siemens development engineers use a trick. They charge the bacteria flakes with the organic impurities for only a short time under ventilation. As a result, there is very little bacterial reproduction. After most of the water is separated, the bacteria ferment the impurities into methane in an anaerobic process step. After two aerobic steps and one anaerobic step, the sludge has been broken down so that the least possible amount of sludge remains and the largest possible amount of methane is available for energy generation in gas turbines or combined heat and power plants.
The pilot facility now in operation cleans around half a cubic meter of wastewater per day. A conventional water treatment plant requires a little less than 0.25 kilowatt-hours of energy to do this, so the pilot unit needs to generate roughly that amount of energy in the form of methane. Plans call for construction to begin in May 2011 on a pilot facility in Singapore that will be a thousand times larger than the current unit and will be able to clean wastewater for around 2,000 residents. By comparison, a typical urban water treatment plant accommodates water from 10,000 to 100,000 residents.
Market launch is scheduled for 2012. Existing water treatment plants could be retrofitted for the new system, which makes Knauf confident that “the process will be a viable future water treatment alternative as energy prices rise and landfill capacity in many countries declines.”
When it rains in Singapore, it pours. It is therefore difficult to imagine that this tropical nation suffers from a lack of water. However, Singapore measures only 40 kilometers across at its widest point, which means its land mass isn’t big enough to supply all of its five million inhabitants with drinking water obtained from either rain or groundwater. Singapore’s government has therefore come up with ideas for solving the problem. It has transformed large stretches of land into reservoirs, has begun importing some of its drinking water from Malaysia, and now operates several wastewater recycling facilities (see Pictures of the Future, Spring 2010, Green Test Bed).
In addition, Singapore’s government views seawater desalination as an essential part of its water management system. The problem is that the two common desalination processes — distillation and reverse osmosis — require a lot of energy. The first uses the most energy — approximately ten kilowatt hours per cubic meter (kWh/m3) of purified water, while the second is more economical, as it requires around four kWh/m3, most of which is used to operate high-pressure pumps that push water through extremely fine membrane filters.
Engineers at Siemens Water Technologies have therefore been searching for an even more efficient technique. Back in 2008 they set a new energy savings world record in a lab. This led to Siemens winning the “Singapore Challenge” competition initiated by the country’s government, which called for seawater desalination at a maximum energy consumption rate of 1.5 kWh/m3. Since then, Siemens has been developing a commercial version of its process, and in December 2010, using a large pilot facility built with the help of the Singapore Public Utilities Board, the company demonstrated that its process uses only half the energy required with reverse osmosis. “The new method marks a revolution in seawater desalination,” says Dr. Rüdiger Knauf, Director of Development at Water Technologies. “The pilot facility shows that our technology not only functions in the laboratory but also has a daily capacity of 50 cubic meters of water.”
Combining Two Processes. The trick behind desalination à la Siemens lies in the combination of two techniques. First, salt is removed from seawater using an electrodialysis unit (ED) built to handle high concentrations of salt. After that, water undergoes continual electrodeionization — or CEDI — which removes smaller amounts of salt. This approach ensures that both methods are carried out under optimal conditions. In addition, Siemens’ CEDI technique benefits from the company’s experience as the market leader in the production of highly pure water for pharmaceutical applications.
The details are as follows: The salt content of seawater is approximately 3.5 percent — but drinking water can contain a maximum of only one seventieth of that amount. To achieve this tremendous reduction in salt content the ED and CEDI processes use powerful electric fields. Sodium chloride (salt) in seawater consists of charged ions, so the electrodialysis process channels water between two electric poles through an area containing more than 700 semipermeable membrane pairs. The latter ensure a high desalination capacity. The membranes alternate between those that allow only positive and those that allow only negative ions to pass through. The ions follow the pull of the electric field through one membrane and are then stopped by the next one.
Water with a low salt content, which is known as diluate, thus collects in the compartment between each membrane pair. Salt collects in the compartments on either side, and the concentrate that forms is expelled from the system as wastewater. Newly developed membranes now make it possible to use electrodialysis for high salt concentrations such as those found in seawater. “This technology can be combined with advanced electrodeionization technology to develop a marketable product in the medium term,” says Knauf.”
After flowing through three electrodialysis modules, the salt content of the diluate falls to less than one percent. At this point, desalination with electrodialysis is no longer efficient, so the next stage is a continuous electrodeionization process in which an ion exchange resin located between the membranes significantly increases process efficiency. The resin does this by absorbing ions from the salt and transporting them to the membranes. At the same time, this resin also regenerates itself by absorbing the positive and negative ions that are formed by the partial disassociation of the water in the strong electric field.
Lower Noise and Vibrations. The key benefit of this technique is that it does not require either a high level of vaporization energy or high pressure for the filtering process. Instead, only the relatively low electrical resistance of the membranes has to be overcome. Other advantages over the most common technique previously used — reverse osmosis — include the fact that the new method is safer to operate thanks to the elimination of high-pressure pumps. The procedure also generates less noise and fewer vibrations, is less susceptible to corrosion because it uses plastic pipes, and requires only minimal water treatment before and afterwards. In addition, the mineral content required for drinking water can be set by varying the strength of the electric field.
Other Siemens specialists are also involved in this innovative development. Experts from Siemens Corporate Technology (CT) in Singapore, for example, are studying the properties of the membranes in order to optimize the new materials and production technologies employed. CT expert Dr. Andreas Hauser is also contributing his knowledge of system simulations. Over the next three years, Hauser will work with RWTH Aachen University to create an electrodialysis simulation model in a project funded by the German Ministry of Education and Research.
The goal is to depict processes at the molecular level using extremely powerful computers so as to gain a more precise understanding of how ions are transported through the membranes and what form the water flow dynamics take in the electric field. “The results will flow into Siemens’ product development activities,” says Hauser. Researchers will then be able to further optimize the desalination process. “I’m hoping we’ll end up with software that can calculate an optimal facility design for each individual customer,” says Knauf.
Plans call for demonstration units to be set up at customer locations in Singapore, the U.S., and the Caribbean by mid-2012. These units will show that the new and economical desalination technique will work not only in Singapore but also at any other location, despite sharp regional differences in seawater salt content. “We expect global water consumption to rise by 40 percent over the next 15 years, which will make sustainable water supplies extremely important,” Knauf explains. “Because of its high energy efficiency and low CO2 balance, electrochemical seawater desalination can play a major role in regions suffering from freshwater shortages.”