The examples and perspective in this article deal primarily with the United States and do not represent a worldwide view of the subject. (June 2019) (Learn how and when to remove this template message)
An ash pond is an engineered structure used at fossil fuel power stations for the disposal of two types of coal combustion products: bottom ash and fly ash. The pond, also called a surface impoundment, uses gravity to settle out large particulates (measured as total suspended solids) from power plant wastewater. This technology does not treat dissolved pollutants.
In the United States, federal design standards for ash ponds were strengthened in 2015, although various provisions of the new regulations are on hold as of 2019, pending ongoing litigation.
Ash ponds are generally formed using a ring embankment to enclose the disposal site. The embankments are designed using similar design parameters as embankment dams, including zoned construction with clay cores. The design process is primarily focused on handling seepage and ensuring slope stability.
Failure of a pond's earthen embankment can cause ash spills on adjacent land and rivers, with serious environmental damage, as evidenced in the 2008 Kingston Fossil Plant spill in Tennessee and the 2014 Dan River coal ash spill in North Carolina.
The wet disposal of ash into ash ponds is the most common ash disposal method, but other methods include dry disposal in landfills. Dry-handled ash is often recycled into useful building materials. Wet disposal has been preferred due to economic reasons, but increasing environmental concerns regarding leachate from ponds has decreased the popularity of wet disposal. The wet method consists of constructing a large "pond" and filling it with fly ash slurry, allowing the water to drain and evaporate from the fly ash over time.
Leachate from fly ash can contain heavy metals in excess of allowable U.S. standards under the Resource Conservation and Recovery Act (RCRA). The flow of water through the fly ash and into ground water is controlled by using low-permeability clay layers and cutoff trenches/walls. Low-permeability clays have permeability on the order of 10−7 cm/s. Vertical flows through the foundation are controlled by siting fly ash ponds on areas of thick clay or rock layers that provide suitably low permeability through the base of the pond. Areas with high sub-surface permeability can be improved by importing suitable clay. Horizontal flows through the embankment are controlled using clay zones within the embankment. Cut off trenches and cut off walls are used to connect the embankment clay zones and the foundation clay layers. Cut off trenches are trenches that are dug into the selected low-permeability sub-surface layer and backfilled with clay to key the embankment clay zone into the sub-surface. Cut off trenches are generally used when the low permeability foundation layer(s) are near surface. Cut off walls are similar to cut off trenches, but are generally much deeper and narrower, and use either slurry or grout instead of clay.
In the United States, coal ash is a major component of the nation's industrial waste stream. In 2017, 38.2 million short tons (34.7×106 t) of fly ash, and 9.7 million short tons (8.8×106 t) of bottom ash were generated. Coal contains trace levels of arsenic, barium, beryllium, boron, cadmium, chromium, thallium, selenium, molybdenum and mercury, many of which are highly toxic to humans and other life. Coal ash, a product of combustion, concentrates these elements and can contaminate groundwater or surface waters if there are leaks from an ash pond.
Regulation in different countriesEdit
In the United States, due to few federal and state regulations concerning ash ponds, most power plants do not use geomembranes, leachate collection systems, or other flow controls often found in municipal solid waste landfills. In 1980 the U.S. Congress defined coal ash as a "special waste" that would not be regulated under the stringent hazardous waste permitting requirements of RCRA. In 2000, the U.S. Environmental Protection Agency (EPA) stated that coal fly ash did not need to be regulated as a hazardous waste.
Following a 2008 failure that caused the Tennessee Valley Authority’s Kingston Fossil Plant coal fly ash slurry spill, EPA began developing regulations that would apply to all ash ponds in the US.
EPA published a Coal Combustion Residuals (CCR) regulation in 2015. The agency continued to classify coal ash as non-hazardous (thereby avoiding strict permitting requirements under RCRA Subtitle C), but with new restrictions:
- Existing ash ponds that are contaminating groundwater must stop receiving CCR, and close or retrofit with a liner.
- Existing ash ponds and landfills must comply with structural and location restrictions, where applicable, or close.
- A pond no longer receiving CCR is still subject to all regulations unless it is dewatered and covered by 2018.
- New ponds and landfills must include a geomembrane liner over a layer of compacted soil.
In 2016, the United States Court of Appeals for the District of Columbia Circuit vacated the "early closure" provisions in the regulation at 40 CFR 257.100. EPA then extended the compliance date for inactive ponds that attempted to utilize the early closure provisions. In 2018, at the request of industry, EPA extended the compliance date for unlined ash ponds from 2019 to 2020, and provided more flexibility to state agencies in determining compliance with standards. The 2018 regulation was challenged in litigation and remanded by the court to EPA for further revision. The court ruled that EPA failed to adequately address the problems with unlined ponds, many of which continue to leak into groundwater. In 2019, the court agreed to a voluntary remand while allowing the 2020 compliance deadline for unlined ponds to stay in effect, pending further rulemaking.
EPA published a proposed rule on August 14, 2019 that would use location-based criteria, rather than a numerical threshold (i.e. impoundment or landfill size) that would require an operator to demonstrate minimal environmental impact so that a site could remain in operation. On December 2, 2019 EPA published another proposed rule that would establish an August 31, 2020 deadline for facilities to stop placing ash in unlined impoundments. The proposal would also provide additional time for some facilities—up to eight years—to find alternatives for managing ash wastes before closing surface impoundments.
Ash ponds are not allowed in the Netherlands, as they are a type of landfill. Instead, all coal ash is recycled in the Netherlands.
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- EPA. "Hazardous and Solid Waste Management System; Disposal of Coal Combustion Residuals From Electric Utilities." 80 FR 21301, 2015-04-17.
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- EPA. "Hazardous and Solid Waste Management System: Disposal of Coal Combustion Residuals From Electric Utilities; A Holistic Approach to Closure Part A: Deadline To Initiate Closure." Proposed rule. Federal Register, 84 FR 65941. 2019-12-02.
- Smith-Schoenwalder, Cecelia (2019-11-04). "EPA Moves to Rollback Coal Power Plant Waste Rules". U.S. News.
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- Coal Ash - U.S. Environmental Protection Agency