MEMORANDUM

TO: Steam Electric Rulemaking Record

FROM: Elizabeth Gentile, ERG

DATE: September 30, 2015

SUBJECT: Bottom Ash Complete Recycle – Bottom Ash Hopper Overflow and Seal 

Trough Flushing Water

ERG is providing technical support to the U.S. Environmental Protection Agency’s Office of 

Water, Engineering and Analysis Division for development of effluent limitations guidelines and 

standards (ELGs) for the Steam Electric Power Generating Industry. 

EPA calculated plant-specific incremental compliance cost estimates associated with zero 

discharge of bottom ash transport water. EPA estimated zero incremental compliance costs for 

plants currently operating bottom ash complete recycle systems. Plants operating bottom ash 

complete recycle systems recycle all bottom ash transport water back to the bottom ash sluicing 

system or to another plant process that does not discharge. 

Typically, bottom ash falls from the bottom of the boiler into a bottom ash hopper that is filled 

with water to cool the bottom ash (i.e., quench water). Additional water is continuously added to 

the bottom as hopper so that the quench water temperature remains at approximately 140 ̊F, 

which is required to cool the ash. Some quench water is lost due to evaporation and some is 

transported with the bottom ash after it is pulled from the hopper. Because water is continuously 

added, there is also some overflow that is typically collected in sumps and or tanks next to the 

boiler. Because water from the bottom ash hopper overflow does not transport bottom ash, it is 

not considered transport water. 

Additionally, plants also collect seal trough flushing water, typically in a sump or tank next to 

the boiler. This water is required to seal the bottom of the boiler with the bottom ash hopper, 

which is not physically joined to allow for heat expansion. A continuous water supply is required 

for the seal trough. Because the seal trough flushing water does not transport bottom ash, it is not 

considered transport water. 

However, transport water includes bottom ash hopper overflow and seal trough water when 

bottom ash transport water is used to cool bottom ash in the hopper or flush the seal trough. The 

incremental compliance cost estimates for plants currently discharging bottom ash transport 

water include costs associated with the piping and pumps required to recirculate the bottom ash 

hopper overflow and seal trough flush water in a closed-loop bottom ash handling system for 

plants that currently have systems in place for collecting bottom ash hopper overflow and seal 

trough flushing water. It is common practice in the industry to collect this water in a sump or 

tank next to the boiler. 
Memorandum

September 30, 2015

Page 2

EPA estimated that there would be no incremental compliance costs due to the zero discharge 

requirement for bottom ash transport water, for those plants identified as currently operating 

bottom ash complete recycle systems, including those that may use recycled bottom ash transport 

water as bottom ash hopper overflow and seal trough flush water. Plants that use bottom ash 

transport water ash bottom ash hopper overflow and seal trough flush water cannot discharge that 

wastewater based on the zero discharge requirements for bottom ash transport water, the 

definition of transport water, and the anti-circumvention provision. 

At proposal, EPA defined transport water as follows (78 FR 34533):

• (p) The term transport water means any process wastewater that is used to convey fly 

ash or bottom ash from the ash collection equipment and has direct contact with the 

ash.

EPA also proposed an anti-circumvention provision (78 FR 34466) intended to prevent steam 

electric power plants from circumventing the effluent limits and standards by moving wastewater

produced by a process operation for which there is a zero discharge effluent limit/standard (such 

as bottom ash transport water) to another process operation for discharge under less stringent 

requirements than intended by the steam electric ELGs. In other words, bottom ash transport 

water that is reused to cool bottom ash in the hopper or flush the seal trough must meet the zero 

discharge requirements from those processes.

EPA received three company-specific public comments on the topic. These comments expressed 

concern over the limitations these provisions might create in reusing the bottom ash transport 

water. Although only one of these commenters stated specific changes that would need to be 

made to their operations to comply with the rule, EPA included full bottom ash conversion 

compliance costs estimates for the plants at these companies discharging bottom ash transport 

water.

1 EPA’s full conversion costs include the piping and pumps required to completely recycle 

the bottom ash hopper overflow and seal trough wastewater. 

EPA received two additional public comments from other power plant companies that were 

general and did not provide plant-specific information to assess if any change to compliance cost 

estimates were warranted. 

 1 EPA estimated full conversion costs associated with these plants prior to taking into account the implementation of 

the Disposal of Coal Combustion Residuals from Electric Utilities (CCR) regulation and the Clean Power Plan 

(CPP). EPA’s final compliance cost estimates presented in Section 9 of the TDD take into account both the CCR 

rule and CPP.