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Sulfates are a form of salt that cause scaling in equipment, resulting in reduced performance and premature equipment failure. This is a particular concern in feed water for boiler furnaces and condensers in power generation facilities. Sulfates can also be present in blow down wastewater from emissions control systems and ash ponds.
In addition to process water concerns, many jurisdictions are imposing tighter discharge limits for sulfate because of concerns about the impact on the surrounding environment, with evidence that the presence of sulfates can negatively impact crop yields and fertility in livestock. Although not considered harmful to humans except at very high levels, sulfate imparts a bitter taste and odor that makes it undesirable for drinking water.
The growing legislative focus on sulfates will have an impact on virtually all industries -- from power generation to municipal water systems. The power generation industry in particular will be affected significantly by the U.S. Environmental Protection Agency's review of effluent guidelines for the fossil fuel-burning power generation sector.
With the added restrictions, it may very well be that many conventional wastewater treatment systems will begin to fall short of the mark. Some operations have been working with membrane technologies with partial success. But membrane technologies are becoming a less appealing option because of their high energy consumption and the associated waste generated by the process.
A newer, much less energy-intensive alternative that can dramatically decrease energy requirements and increase the amount of water for reuse is ion exchange technology. This process works quite differently from membranes by using chemistry to safely breaking down sulfate molecules, leaving operations with high water recovery rates, as well as a "clean" waste product that can be reused in other applications.
Sulfate reduction technologies
Prevailing technologies to treat sulfate include reverse osmosis (membrane systems), processes based on ettringite formation, biological sulfate reduction, and precipitation with barium.
Typically these technologies are either not able to treat contaminated water to a level which complies with new regulations and standards that are being imposed on industry, or they bring either environmental or economic disadvantages.
For example, membrane systems have been used for treating wastewater streams in a number of industries, including the energy sector. They operate by pushing water through a series of micro filters that capture contaminants in the form of a supersaturated liquid. Once the system collects the residual liquid, the liquid is then heated to evaporate the water content, producing a crystalline byproduct that often requires special handling and disposal.
There are several limitations with this approach. First, water recovery rates can be low because of the complex cocktail of contaminants typically found in industrial wastewater. It is not uncommon to have up to 50 percent of the water end up in the waste product stream. Second, membrane processes can have high energy costs. As with any process that relies on heat, the act of crystallization also consumes high amounts of energy. In addition, the cost of handling the brine byproduct and ongoing maintenance -- including membrane replacement -- can be extremely high.
Other techniques pose their own unique challenges. For example, processes based on ettringite formation have not been commercially proven, with limited effectiveness and high cost. Biological sulfate reduction requires a large input of energy to raise the temperature of feed water, and generates a residual waste that can be toxic. Passive biological systems leave a large environmental footprint, and are suitable for only small flows of water.
Ion exchange
Ion exchange uses an entirely different process, using chemistry to remove sulfate from water. In the first stage, feed water is passed through a series of contactors containing cation exchange resin to remove calcium and magnesium by loading the cations onto the resin, and then through contactors containing anion exchange resin to remove sulfate. In the second stage, the resins are regenerated using the low-cost reagents, sulfuric acid and lime, so that the only products of the process are a solid gypsum product and clean water that can be re-used or safely discharged. There are no residual wastes that require special disposal or ongoing management.
Utilities can realize a number of environmental and economic advantages with ion exchange treatment. It usually requires no pre-treatment and there is no residual brine waste that would otherwise require special disposal. The process also consumes 90 percent less energy than a similar capacity membrane system and has been shown to achieve water recovery rates of up to 95 percent. Depending on the operation and its water treatment needs, ion exchange technology can reduce operating and capital costs for water treatment by at least half.
For power generation facilities, this new water treatment process can improve the rate of water recycle and re-use, and in doing so, facilities can reduce the quantity of water required for make-up feed, and improve the quality of water discharged to the environment. By reducing water consumption, the process can help facilities meet zero-discharge limits. In addition, facilities can prevent scaling and corrosion by treating feed water, reducing their equipment maintenance costs.
With the current government's renewed focus on the environment, the energy sector has an opportunity to break new ground in waste water treatment and reuse. There is no question that all industries need to explore technologies that can effectively handle contaminants such as sulfates, while preserving the integrity of the water supplies for the communities they serve.
