problem
The Mead Corporation has considered closing unlined wastewater treatment
impoundments, including a 2.8 hectars strong waste pond at the Mahrt,
Alabama mill. The Mahrt strong waste pond contains a mixture of
spent liquor, fiber and some ash. In addition to receiving the occasional
liquor spill material; the pond receives some storm water. The pond
is frequently covered with some depth of water, and any exposed
solids are typically weak and unstable. The pond would be closed
under guidelines promulgated by the Water Division of the Alabama
Department of Environmental Management (ADEM).
One requirement stipulates that any cap should maintain a minimum
slope of 5 percent. For the 2.8 hectars impoundment, this would
require significant earth fill volumes. However, the guidelines
allow some variance in the stipulated closure approach provided
the closure method provides equal or better protection of the groundwater.
Mead’s proposed concept involved the use of ash as a construction
material in covering the solids in place. The landfill operator
would use ash as a fill to construct the cover incrementally, without
double handling of material. This would save significant construction
costs and preserve valuable landfill airspace.
solution
A demonstration project for the closure concept was performed using
ash and a multi-layer geogrid for bridging the soft solids. The
closure strategy was successfully demonstrated and a closure concept
developed. A closure design concept was developed for the unlined
pond to achieve:
- Facilitate preparation of future phase closure plan for the
existing impoundment to comply with/ADEM’s guidelines for
industrial wastewater impoundments;
- Stabilize the existing ground surface of the impoundment with
a multi-layer geogrid to prepare for the additional fill material
for closure; and
- Utilize on site ash as a fill material to facilitate closure
of the existing impoundment.
conclusions
The use of a multi-layer geogrid to stabilize the existing ground
surface of the impoundment to prepare for the additional fill material
for closure was considered a success. With the geogrid reinforcement
at the base of the ash layer, the geogrid was able to withstand
heavy equipment and loads, which is critical for the short term
and long term closure activities of the impoundment. At an economic
standpoint, the amount of airspace saved for the mill’s landfill
by using the industrial wastewater impoundment for ash fill was
estimated to be around 100.000 m3. The mill produces around 135.000
m3 per year, which is about an 8-month savings of valuable landfill
airspace.
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