Approaching the removal of selenium in coal ash ponds with a chemical solution can save money and time investing in extra treatment equipment to address this contaminant. Standard wastewater treatment that removes both arsenic and selenium can provide the solution.
Regulations Require Selenium Treatment
Coal combustion residuals (CCRs), the byproducts of the combustion of coal at power plants, include contaminants like selenium, mercury, cadmium, and arsenic associated with various serious health effects. Coal-fired power plant wastewater can be high in these contaminants.
EPA regulations, including the Disposal of Coal Combustion Residuals from Electric Utilities (CCR rule) and the revised effluent limitation guidelines (ELGs), set federal limits on the levels of toxic metals in wastewater that can be discharged from power plants. The regulations discourage additional CCR waste storage in surface ponds and encourage coal facilities to dewater and close the ponds when possible.
The concern with CCR storage in ash ponds is that contaminants may leak into groundwater, blow as dust into the air, or be released into surface water due to catastrophic failure of coal ash impoundments. Both growing regulations and environmental concerns make it important to choose a treatment option for coal ash ponds that is efficient and sustainable.
Removing Selenium from Coal Ash Ponds with a Mobile Solution
Selenium treatment solutions often rely on equipment that is not generally included in a standard wastewater process train but requires additional, specialized handling. However, selenium can be removed from wastewater by placing mobile equipment at the ash pond temporarily to provide a cost-effective treatment solution that uses conventional equipment.
Most wastewater treatment processes do not already include the equipment required for selenium treatment, and adding the extra pieces creates additional complexity. Specialized treatment processes could include:
- Biological treatment specifically designed to target selenium and post filtration
- Reverse osmosis or ion exchange, both with subsequent brine treatment
Simple Treatment for Arsenic and Selenium Together at Coal Ash Ponds in the Eastern USA
WesTech has a successful history of addressing customer needs for coal ash pond wastewater treatment with mobile, temporary equipment. One example of an effective use of a mobile treatment solution was at an Eastern U.S. power plant with multiple coal ash ponds. The plant required the mobile system to treat the water for discharge from a number of ponds so they could be closed. Once the first pond was dewatered, the plan was to shut down the temporary system and relocate it to the next pond.
|Parameter||Design Influent||Effluent Limits||Results|
|TSS||<10,000 ppm||100 ppm||<10 ppm|
|Total Arsenic||<1,000 ppb||340 ppb||<10 ppb|
|Total Selenium||<200 ppb||56 ppb||<5 ppb|
The decision was made to use a chemical approach to treatment, and to accommodate this, a WesTech mobile equipment solution was installed – a RapiSand™ ballasted flocculation unit, a pressure filter, and a chem center – and it was expected that its compact design would require less energy and resources than other treatments.
The selenium removal at the first pond was tested using added caustic, coagulant, oxidant, and polymer, and the results were on target. To assure that the selenium treatment was successful, the setup also included a biological selenium treatment system.
Surprisingly, this additional system was never started because effluent results without the biological treatment system were sufficient. Developing this simple yet effective solution at this site was critical to keeping costs down.
WesTech’s selenium treatment solution uses a system that targets arsenic and follows a conventional treatment structure. This chemical treatment of arsenic in high levels requires coagulation and oxidation, filtration, sedimentation, and filtration, the processes found in one form or another in most wastewater treatment systems.
The results of the chemical selenium removal at the Eastern U.S. ash pond are shown in the graph above and were attainable due to the speciation of the selenium in the ash pond. Selenium in water is generally found in two main forms:
- Reduced selenium, selenite, Se4+
- Oxidized selenium, selenate, Se6+
Since a majority of selenium at the ash pond that was being treated was selenite, excellent selenium treatment was accomplished through the arsenic removal process. It is also worth noting that small doses of oxidant were not enough to oxidize the selenium but remained strong enough to keep the arsenic oxidized, which is the removable form of arsenic.
Approaching selenium removal through the arsenic treatment process using a conventional water treatment system is a cost-effective, practical way to address selenium contamination in ash ponds. Since the chemical process used for treating arsenic can also be effective at treating selenium, the selenium can be reduced with a minimum amount of added equipment.
Upcoming regulations will require a solution to selenium contamination in coal ash ponds. Even those states without regulations now will likely see selenium restrictions in the future.