Ash pond
Encyclopedia
Ash ponds are engineered structures for the disposal of fly ash
. The wet disposal of fly ash into ash ponds is the most common fly ash disposal method, but other methods include dry disposal in landfill
s. 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. Ash ponds are generally formed using a ring embankment
to enclose the disposal site. The embankments are designed using similar design parameters as embankment dam
s including zoned construction with clay
cores. The design process is primarily focused on handling seepage and ensuring slope stability.
Leachate from fly ash can contain heavy metals in excess of allowable RCRA standard. 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/sec. 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.
Due to few federal and state regulations concerning ash ponds, most do not use geomembranes, leachate
collection systems, or other flow controls often found in municipal solid waste
(MSW) landfills. Following a failure that caused the Tennessee Valley Authority’s Kingston Fossil Plant coal fly ash slurry spill
, the Environmental Protection Agency began overseeing the regulation of all ash ponds in order to establish national fly ash pond standards.
Fly ash
Fly ash is one of the residues generated in combustion, and comprises the fine particles that rise with the flue gases. Ash which does not rise is termed bottom ash. In an industrial context, fly ash usually refers to ash produced during combustion of coal...
. The wet disposal of fly ash into ash ponds is the most common fly ash disposal method, but other methods include dry disposal in landfill
Landfill
A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...
s. Wet disposal has been preferred due to economic reasons, but increasing environmental concerns regarding leachate
Leachate
Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed....
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. Ash ponds are generally formed using a ring embankment
Levee
A levee, levée, dike , embankment, floodbank or stopbank is an elongated naturally occurring ridge or artificially constructed fill or wall, which regulates water levels...
to enclose the disposal site. The embankments are designed using similar design parameters as embankment dam
Embankment dam
An embankment dam is a massive artificial water barrier. It is typically created by the emplacement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay and/or rock. It has a semi-permanent waterproof natural covering for its surface, and a dense, waterproof...
s including zoned construction with clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
cores. The design process is primarily focused on handling seepage and ensuring slope stability.
Leachate from fly ash can contain heavy metals in excess of allowable RCRA standard. 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/sec. 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
Slurry
A slurry is, in general, a thick suspension of solids in a liquid.-Examples of slurries:Examples of slurries include:* Lahars* A mixture of water and cement to form concrete* A mixture of water, gelling agent, and oxidizers used as an explosive...
or grout
Grout
Grout is a construction material used to embed rebars in masonry walls, connect sections of pre-cast concrete, fill voids, and seal joints . Grout is generally composed of a mixture of water, cement, sand, often color tint, and sometimes fine gravel...
instead of clay.
Due to few federal and state regulations concerning ash ponds, most do not use geomembranes, leachate
Leachate
Leachate is any liquid that, in passing through matter, extracts solutes, suspended solids or any other component of the material through which it has passed....
collection systems, or other flow controls often found in municipal solid waste
Municipal solid waste
Municipal solid waste , commonly known as trash or garbage , refuse or rubbish is a waste type consisting of everyday items we consume and discard. It predominantly includes food wastes, yard wastes, containers and product packaging, and other miscellaneous inorganic wastes from residential,...
(MSW) landfills. Following a failure that caused the Tennessee Valley Authority’s Kingston Fossil Plant coal fly ash slurry spill
Kingston Fossil Plant coal fly ash slurry spill
The TVA Kingston Fossil Plant coal fly ash slurry spill occurred just before 1 a.m. on Monday December 22, 2008, when an ash dike ruptured at an solid waste containment area at the Tennessee Valley Authority's Kingston Fossil Plant in Roane County, Tennessee, USA. of coal fly ash slurry was...
, the Environmental Protection Agency began overseeing the regulation of all ash ponds in order to establish national fly ash pond standards.