Washington, DC -- In a pilot-scale test supported by the U.S. Department of Energy (DOE) Office of Fossil Energy, Clemson University researchers have shown that manmade or "constructed" wetlands can be used to treat non-traditional water sources which could then be used in power plants or for other purposes. The successful test, which was managed by DOE's National Energy Technology Laboratory (NETL), could help power plants economically meet criteria for water reuse or discharge established by the National Pollution Discharge Elimination System and the Clean Water Act.
Power plants need large quantities of water to operate efficiently. An economical treatment method that allows the use of non-traditional water--water that is not drawn from a traditional freshwater source, such as a lake, river, or municipal water supply--could decrease the impact of electricity generation on the Nation's limited freshwater supplies.
Four kinds of non-traditional water sources were treated during the test: ash basin water, simulated cooling water, flue gas desulfurization water, and produced water (water that is extracted from the ground along with oil and natural gas). All of the water sources contained contaminants such as salts, heavy metals, and hydrocarbons. In addition, all were capable of causing bio-fouling or corrosion because of their pH, ionic strength, or nutrient content. Specific contaminants of concern included the following:
- Ash basin water--arsenic, chromium, copper, mercury, selenium, and zinc.
- Cooling water--bromine, chlorine, copper, lead, peroxides, and zinc.
- Flue gas desulfurization water--arsenic, boron, chlorides, mercury, selenium, and zinc.
- Produced waters--arsenic, benzene, cadmium, chlorides, chromium, copper, lead, mercury, nickel, sulfide, toluene, zinc, and oil and grease.
Recognizing the differences in each water source, unique wetland treatment systems were designed and constructed for each type of water. The artificial wetlands were created in large (70- to 250-gallon) vats containing vegetation that would be found in natural wetlands, such as California bulrush and narrow leaf cattail. Each type of water was gravity fed through its own series of vats, residing for about 24 hours in each.
Before and after treatment, researchers measured pH, temperature, dissolved oxygen, hardness, alkalinity, and conductivity, along with the levels of contaminants. Treatment goals were determined by--
- National Pollution Discharge Elimination System permit limits.
- Water quality criteria established by the U.S. Environmental Protection Agency.
- Irrigation standards established by the U.S. Department of Agriculture.
- Toxicity to the water flea Ceriodaphnia dubia.
- Reuse standards focused on minimizing damage to the power pant by treated waters.
Test results showed that, while limited to chloride concentrations of less than 4,000 milligrams per liter, constructed wetland treatment systems could remediate all four non-traditional water sources for reuse or discharge. Since constructed wetland treatment systems cost 10-50 percent less than conventional treatment systems, they could provide an economical alternative to conventional water-treatment approaches, which are comparatively costly and are often unable to achieve new, rigorous water-quality standards.
The link between water and energy is one of several research areas in NETL's Innovations for Existing Plants program. The program aims to sustain the use of coal in the Nation's energy mix by developing technologies that will enable the current fleet of coal-fired power plants to comply with existing and emerging environmental regulations.