Ash Basin Cleanup, Lining & Service

SITUATION
Charah was hired to excavate, reconstruct and improve standards of an operating ash basin while ensuring that plant operations were not impacted. Specifically, that included a complete excavation of the basin and construction of a liner system that is in compliance with engineering standards and local, state and pending U.S. Environmental Protection Agency (EPA) regulations.

The utility’s earthen-dammed ash pond was put into service prior to the June 2010 EPA proposed management standards for Coal Combustion Residuals (CCRs). The pond, approximately 60 acres in area and up to 100 feet in depth, had reached 80% of its capacity. As part of a previous project, the pond was divided into two cells − a lower polishing cell and an upper active cell, used to impound ash. With the changing regulatory environment and a growing need to address and minimize ash impoundment storage, the utility turned to Charah for a long-term containment solution that would extend the pond’s service life. Charah looked at a number of possibilities, including conventional dredging, excavating “fingers” into the basin, and construction of an auxiliary impoundment. Each of those options proved cost-prohibitive or impractical from a permitting and engineering perspective. An innovative solution was required.

INNOVATIVE SOLUTIONS
EnviroDitch for Sluice Control and Ash Harvesting
The innovative EnviroDitch is a patent-pending rim ditch system designed by Charah to control sluice water and comply with existing EPA regulations and minimize reliance on ash impoundments for final ash disposition. The EnviroDitch diverts water into a newly constructed high density polyethylene (HDPE) lined basin, leaving the existing ponds dry for excavation. The EnviroDitch operates similarly to a traditional rim ditch, but uses innovative techniques to maximize effectiveness, including a concrete-lined channel with a geomembrane liner and flow controls for increased settling of solids, thus reducing Total Suspended Solids (TSS).

Stabilizing the Ash Dike
Charah installed 84 well points along the dike to control the internal water surface, stabilizing the ash dike and creating a drier, safer work site. During dewatering and cleaning operations, work on the upper cell was limited to 48 feet from the top elevation of the dike.

Total Suspended Solids (TSS) Controls
To maintain operations in the lower cell, Charah installed state-of-the-art TSS controls to prevent negative impact to the lower pond or to the final discharge to the primary outfall.

Flow Control Structures
Flow control structures allow for year-round ash removal, and enhance settling of TSS.

RESULTS

  • The 60-acre pond was successfully cleaned and restored at depths up to 80 feet, unprecedented in the industry.
  • Charah provided the utility with the flexibility to use the cleaned pond for one of the following:
  • A lined and active pond with a 15-year life-span.
  • A lined dry stack or landfill with a 20-year life-span.
  • Effluent discharges met regulatory standards.
  • The project did not interrupt utility operations.