Glossary 

Draft – Jan 14, 2010 

Aerobic biological reactor – A tank in which material is converted from an unwanted 

form into a desired form by microorganisms in the presence of free oxygen.

Air heater ash – The ash taken from hoppers below the air heater. 

Air heater cleaning wash water – Any water or liquid cleaning solution used for or 

generated from cleaning the air heater. 

Anaerobic biological reactor – A tank in which material is converted from an unwanted 

form into a desired form by microorganisms not in the presence of free oxygen.

Background concentration – The concentration of a substance in an environmental 

media (air, water, or soil) that occurs naturally or is not the result of human activities.

Base load unit – A unit normally operated to produce electricity at an essentially 

constant rate and runs continuously. 

Best Management Practice (BMP) – Pollution prevention practices that help to avoid 

contact between pollutants and water media that may include good housekeeping 

measures, good management techniques, product modifications, operational changes, 

materials substitution, materials and water conservation, and other measures. 

Boiler blowdown – The minimum amount of liquid removed from the boiler/steam 

generator for the purpose of preventing buildup of materials that exceed limits established 

by best engineering practices. 

Boiler fireside cleaning wash water – Any water or liquid cleaning solution used for or 

generated from cleaning the boiler fireside.

Boiler tube cleaning wash water – Any water or liquid cleaning solution used for or 

generated from cleaning the boiler tubes.

Bottom ash – The heavier ash particles that are not carried out of the boiler with the flue 

gas, and which settle in the furnace or are dislodged from furnace walls. Includes boiler 

slag collected in wet-bottom furnaces. Economizer ash is included when it is collected 

with bottom ash. 

Bottom ash sluice – Process wastewater generated from a wet bottom ash handling 

system that is formed by combining bottom ash with the bottom ash transport water. 

Bottom ash sluice is typically transferred to a pond/impoundment or a dewatering bin. 
Carbon capture system – An air pollution control system intended to reduce emissions 

of carbon dioxide. Includes both post-combustion and pre-combustion carbon 

capture/reduction technologies. 

Carbon capture wastewater – Any process wastewater generated from the carbon 

capture system.

Chemical precipitation/flocculation – Process involving the addition of chemicals to 

alter the physical state of dissolved and suspended solids and facilitate their removal by 

sedimentation.

Clarification – A sedimentation process to remove solid particles from a liquid stream 

by gravitational force. 

Clean Water Act (CWA) – Federal legislation enacted by Congress to “restore and 

maintain the chemical, physical, and biological integrity of the Nation’s waters” (Federal 

Water Pollution Control Act of 1972, as amended, 33 U.S.C. 1251 et seq.). 

Coal pile runoff – The runoff from or through any coal storage pile. 

Coal washing – Coal washing, also known as coal cleaning, entails separating out 

foreign material from coal in a liquid medium and may also include processes to remove 

ash, sulfur and moisture. The liquid medium may be combined with finely ground 

heavier minerals, such as magnetite, in a dense medium fluid, better to effect separation 

of unwanted rock and mineral matter from coal particles. 

Cogeneration plant – A plant that produces electricity and another form of useful 

thermal energy (such as heat or steam), used for industrial, commercial, heating, or 

cooling purposes. 

Combustion turbine cleaning wash water – Any water or liquid cleaning solution used 

for or generated from cleaning a combustion turbine, including the air compressor section 

of the turbine.

Cost of service – A ratemaking concept used for the design and development of rate 

schedules to ensure that the filed rate schedules recover only the cost of providing the 

electric service at issue. This concept attempts to correlate the utility's costs and revenue 

with the service provided to each of the various customer classes. 

Cycling unit – A unit used in a constant and specific cycle. For example, a unit that runs 

daily, but reduces capacity or shuts off at night would be a cycling unit. 

Deep (or shallow) well injection – Disposal of fluids underground through any bored, 

drilled, or driven shaft or a dug hole, improved sinkhole, or a subsurface fluid distribution 

system where the depth is greater than the largest surface dimension. 
Design storm – The storm for which a drainage system or hydraulic structure such as an 

impoundment is designed. 

Detection limit – The minimum result which can be reliably discriminated from a blank 

(for example, with a 99% confidence level). 

Discharge – The conveyance of process wastewater to: (1) surface waters; or (2) a 

publicly owned, privately owned, federally owned, combined, or other treatment works. 

Dry bottom ash handling system – A system that does not use water to convey bottom 

ash away from the boiler. It includes systems that collect and convey the ash without any 

use of water, as well as systems in which bottom ash is quenched in a water bath and then 

mechanically or pneumatically conveyed away from the boiler. 

Dry FGD system – Dry FGD system, also referred to as semi-dry FGD system, captures 

sulfur dioxide from flue gas by a spray dryer absorption process that produces calcium 

sulfite with low moisture content. 

Dry fly ash handling system – A system that does not use water to convey fly ash as a 

dry material away from particulate collection equipment. 

DUNS Number – Unique nine-digit numeric sequence (“Data Universal Numbering 

System”) assigned to a corporate entity by Dun and Bradstreet. 

Economizer ash – The ash taken from hoppers below the economizer. 

Evaporation – The process by which water or other liquid becomes a gas. 

FGD scrubber absorber – As depicted in Figure B-1, the FGD scrubber absorber is the 

module where contact between flue gas and sorbent occurs, which results in the capture 

of sulfur dioxide from the flue gas. 

FGD scrubber purge – Process wastewater that exits an FGD scrubber system 

(typically from a solids separation process) and that is transferred to a wastewater 

treatment system or discharged. Note: The scrubber purge stream may be the same as 

the FGD slurry blowdown stream if the plant does not operate a solids separation system 

prior to wastewater treatment. 

FGD scrubber system – As shown in Figure B-1, a system that captures sulfur dioxide 

from flue gas. An FGD scrubber system may be wet or dry. For wet FGD systems, the 

solids separation and solids dewatering processes are part of this system. 

FGD slurry blowdown – Slurry water that exits an FGD scrubber absorber to control 

the solids/chlorides levels in the FGD scrubber absorber. FGD slurry blowdown is 

typically transferred to a solids separation process. 
FGD slurry preparation process water –Water used for the preparation of FGD slurry 

water (e.g., water that is added to ball mills for limestone slurry preparation). 

FGD slurry water – All water that enters into, is used within, or recycles through the 

FGD scrubber absorber. FGD slurry water is replenished by make-up water and the 

solids level is controlled by FGD slurry blowdown. 

FGD solids – Any solid material generated by the FGD scrubber system. This may also 

be called FGD sludge (e.g., calcium sulfite and calcium sulfate). 

FGD solids separation – The process that separates FGD slurry blowdown into two 

separate streams: the solids-rich stream (i.e. underflow) that contains FGD solids and the 

solids-lean stream (i.e. overflow) that contains water and fines. 

FGD solids separation recycle – The FGD wastewater that is returned to the FGD 

scrubber absorber following the FGD solids separation process.

FGD wastewater – Process wastewater generated specifically from the FGD scrubber 

system.

Filter – An apparatus using woven, granular, or other material to remove solid particles 

from wastewater or water. 

Filter backwash – Any water generated from reversing the direction of flow through a 

filter for the purpose of washing and/or eliminating solids from the filter.

Filter press – An apparatus used in solids dewatering that forces sludge through a filter

to remove water and form a cake. 

Floor drain wastewater – Liquid collected in any of the floor drains at the plant.

Flue gas mercury control system – An air pollution control system installed or operated 

for the purpose of removing mercury from flue gas. In this questionnaire, do not include 

FGD or SCR/SNCR systems as flue gas mercury control systems.

Flue gas mercury control system wastewater – Any process wastewater generated 

from the flue gas mercury control system. 

Fly ash – The ash that is carried out of the furnace by the gas stream and collected by 

mechanical precipitators, electrostatic precipitators, and/or fabric filters. Economizer ash

is included when it is collected with fly ash. 

Fly ash sluice – Process wastewater generated from a wet fly ash handling system that is 

formed by combining fly ash with the fly ash transport water. The fly ash sluice water is 

typically transferred to an ash pond/impoundment.
Forced generator outage – The hours for any failure, misoperation, or malfunction 

that results in the immediate automatic or manual removal of a generating unit from 

connection to the transmission system, or that prevents such connection from being 

accomplished when desired. 

Gross generation – Amount of power produced by an electric power plant, measured at 

the terminals of the plant (i.e., prior to the point at which the power leaves the station and 

is available to the system). This amount includes electric power generated at a power 

plant that is used to operate equipment at the plant.

Gypsum pile – A temporary storage pile on site containing gypsum. 

Gypsum pile runoff – The runoff from or through any gypsum pile.

Gypsum stacking – For plants that sluice gypsum to a pond/impoundment, the process 

used to dig out the gypsum from the pond/impoundment and stack it along the sides of 

the pond/impoundment or in separate piles for dewatering. 

Gypsum wash water – Process wastewater generated during the solids dewatering

operation of gypsum or gypsum solids. 

IGCC generating unit – An integrated gasification combined cycle generating unit. 

Immediate parent firm – The first entity in the facility’s ownership structure 

responsible for facility’s expenses associated with steam electric generating units. Note 

that for the purpose of Part I of the questionnaire, if a plant that has multiple owners, 

detailed financial and economic data are requested of the immediate parent firm that 

holds the largest equity share in the plant. 

Independent power producer – A corporation, person, agency, authority, or other legal 

entity or instrumentality that owns or operates facilities for the generation of electricity 

for use primarily by the public, and that is not a utility. 

Intake water – Water from public utilities, streams, rivers, lakes, or underground 

aquifers that is used to supply or feed process unit operations or treatment processes. 

Intermediate unit – A unit that not used in a constant and specific cycle. The unit is 

instead used more sporadically on an as needed basis when energy requirements are less 

than peak load but more than base load. 

Ion exchange – Reversible exchange of ions adsorbed on a mineral or synthetic polymer 

surface with ions in solution in contact with the surface.

Landfill – A disposal facility or part of a facility where solid waste, sludges, or other 

process residuals are placed in or on any natural or manmade formation in the earth for 

disposal and which is not a storage pile, a land treatment facility, a surface impoundment, 
an underground injection well, a salt dome or salt bed formation, an underground mine, a 

cave, or a corrective action management unit. 

Leachate - Liquid, including any suspended or dissolved constituents in the liquid that 

has percolated through or drained from waste or other materials emplaced in a landfill, or 

that pass through the containment structure (e.g., bottom, dikes, berms) of a surface 

impoundment. Leachate also includes the terms seepage, leak, and leakage, which are 

generally used in reference to leachate from an impoundment.

 

Leachate collection system - A system that gathers leachate and conveys it to a 

collection area for treatment, discharge, or other use.

 

Leak detection system - A system whose primary purpose is to monitor performance of 

the containment structure of a pond/impoundment or landfill by collecting fluid which 

flows through the liner. 

Liner – A continuous layer of natural or man-made materials, beneath or on the sides of 

a pond/impoundment, landfill, or landfill cell, which restricts the downward or lateral 

escape of the wastes placed therein or leachate.

Mill reject sluice – Water stream that is generated by combining mill rejects with water 

to aid in transport and/or disposal. 

Mill rejects – Material such as stone, slate and iron pyrite that is rejected by coal 

pulverizers because it could not be ground. 

Nameplate capacity –The full-load continuous rating of a generator, prime mover, or 

other electric power production equipment under specific conditions as designated by the 

manufacturer. Installed generator nameplate rating is usually indicated on a nameplate 

physically attached to the generator. 

Natural wetlands – An area created naturally, not man-made, that is saturated by surface 

or ground water with vegetation adapted for life under those soil conditions, as swamps, 

bogs, fens, marshes, and estuaries.

Non-contact cooling water – Water used for cooling which does not come into direct 

contact with any raw material, product, byproduct, or waste.

Non-pond wastewater treatment unit – A wastewater treatment unit that is not a 

pond/impoundment. Non-pond wastewater treatment units include, but are not limited to: 

chemical precipitation/flocculation, pH adjustment, clarification, anaerobic/aerobic 

biological reactor, thickeners, and filters. 

Nonutility – A corporation, person, agency, authority, or other legal entity or 

instrumentality that owns or operates facilities for electric generation and is not an 

electric utility. These entities are not owned by a governmental unit or the consumers that 
the entity serves and do not operate within the traditional cost-of-service price regulation. 

Nonutility power producers include qualifying cogenerators, qualifying small power 

producers, and other nonutility generators (including independent power producers). 

Nonutility power producers are without a designated franchised service area and do not 

file forms listed in the Code of Federal Regulations, Title 18, Part 141. 

NOx control system – An air pollution control system that removes NOx from air 

emissions. Types of NOx control systems include, but are not limited to, the following: 

selective catalytic reduction (SCR), selective non-catalytic reduction (SNCR), overfire 

air, and low NOx burners. 

NPDES permit – Permits issued under the National Pollutant Discharge Elimination 

System (NPDES) program authorized by Sections 307, 318, 402, and 405 of the Clean 

Water Act that applies to plants that discharge wastewater directly to United States 

surface waters. 

On site – Property and equipment under the operational control of the plant, including 

landfills, ponds/impoundments, and outfall structures located on non-contiguous 

property. 

Particulate matter control system – An air pollution control system that removes 

particulates from the flue gas. Particulate matter control systems include, but are not 

limited to, the following: electrostatic precipitators (ESP), fabric filters/baghouses, 

mechanical collectors, and venturi scrubbers.

Peaking unit – A unit normally used only during peak-load periods of electricity 

demand. 

pH Adjustment – Decreasing the acidity or alkalinity of a substance by adding alkaline 

or acidic materials, respectively. 

Plant – Includes all contiguous and non-adjoining property and equipment that is under 

operational control of the facility, including non-adjoining landfills, surface 

impoundments, and outfall structures. 

Pollutant – Dredged spoil, solid waste, incinerator residue, filter backwash, sewage, 

garbage, sewage sludge, munitions, chemical wastes, biological materials, radioactive 

materials (except those regulated under the Atomic Energy Act of 1954, as amended (42 

U.S.C. 2011 et seq.)), heat, wrecked or discarded equipment, rock, sand, cellar dirt and 

industrial, municipal, and agricultural waste discharged into water. (See 40 CFR 122.2) 

Pollution prevention – The use of materials, processes, or practices that reduce or 

eliminate the creation of pollutants or wastes. It includes practices that reduce the use of 

hazardous and nonhazardous materials, energy, water, or other resources, as well as those 

practices that protect natural resources through conservation or more efficient use. 
Pollution prevention includes but is not limited to source reduction, in-process 

recycle/reuse, and water conservation practices. 

Pond/impoundment – A natural topographic depression, man-made excavation, or diked 

area formed from earthen materials or man-made materials or a combination of them, 

which is designed to hold an accumulation of liquid process wastes or process wastes 

containing free liquids, and which is not an injection well. Examples of 

ponds/impoundments include holding, storage, settling, and aeration pits, ponds, and 

lagoons. 

Pond/impoundment system – A treatment system consisting of one or more 

ponds/impoundments. 

Pond outlet – The point at which the pond/impoundment releases water to another 

pond/impoundment, surface water, or other process 

Power marketers – Business entities engaged in buying and selling electricity. Power 

marketers do not usually own generating or transmission facilities. Power marketers, as 

opposed to brokers, take ownership of the electricity and are involved in interstate trade. 

These entities file with the Federal Energy Regulatory Commission (FERC) for status as 

a power marketer. 

Primary purpose – Provides the predominant source of revenue for the plant. The 

principal reason for which the plant operates. 

Priority pollutant – Priority pollutants are a set of 126 chemical pollutants listed at 40 

CFR part 423, Appendix A.

Privately Owned Treatment Works (PrOTW) – Any device or system which is (a) 

used to treat wastes from any facility whose operator is not the operator of the treatment 

works and (b) not a “POTW.” 

Process operation – One or more pieces of process equipment used to change the 

physical or chemical characteristics of one or more process streams. Process operations 

include, but are not limited to, boilers, scrubbers, SCR/SNCR systems, air heaters, EMC 

systems, and cooling towers. 

Process wastewater – Any water which comes into direct contact with or results from 

the storage, production, or use of any raw material, intermediate product, finished 

product, byproduct, or waste product. Examples of process wastewater include, but are 

not limited to, wastewater from ash handling, equipment cleaning, air pollution control

devices, rinse water, coal pile runoff or other contaminated stormwater, and cooling 

water are considered process wastewater. Process wastewater can be treated, recycled, 

discharged, or hauled offsite for disposal. Sanitary wastewater, non-contact cooling 

water, potable water, sewage, fire protection, car washes, and uncontaminated 
stormwater are not considered process wastewater for the purpose of this information 

collection request. 

Publicly Owned Treatment Works (POTW) – In general terms, any device or system 

owned by a state or municipality that is used to recycle, reclaim, or treat liquid municipal 

sewage and/or liquid industrial wastes. See 40 CFR part 403.3 for an expanded definition 

of this term. 

Qualifying Cogenerator and Qualifying Small Power Producer (QF) – A 

cogeneration or small power production facility, respectively, that meets certain 

ownership, operating, and efficiency criteria established by the Federal Energy 

Regulatory Commission (FERC) pursuant to the Public Utility Regulatory Policies Act 

(PURPA). 

Raw intake water – Intake water prior to any treatment or use. 

Recycle/reuse – To return a stream or a portion of a stream to an earlier step in the 

process/treatment process or to another process at the plant. 

Residue – Amount of a pollutant remaining in the environment after a natural or 

technological process has taken place; e.g., the sludge remaining after initial wastewater 

treatment, or particulates remaining in air after it passes through a scrubbing or other 

process. 

Reverse Osmosis (RO) – A filtration process designed to separate particulate, colloidal, 

and dissolved matter from a liquid using a semi-permeable membrane, where pressure in 

excess of the osmotic pressure is applied to the concentrated side of the membrane. 

RO reject water – Waste water released from the reverse osmosis process. 

Rural Electric Cooperatives – For the purpose of this questionnaire, rural electric 

cooperatives are electric utilities that are legally established to be owned by and operated 

for the benefit of those using its service. This entity will generate, transmit, and/or 

distribute supplies of electric energy to a specified area not being serviced by another 

utility. Such ventures are generally exempt from Federal income tax laws. Most electric 

cooperatives have been initially financed by the Rural Utilities Service (prior Rural 

Electrification Administration), U.S. Department of Agriculture. 

Sanitary wastewater – Wastewater that is generated from restrooms, cafeterias, 

showers, and domestic (versus industrial) activities. 

Scheduled generator outage – The hours during which the generating unit is offline due 

to planned, scheduled repairs, maintenance, or upgrades, such as routine repetitive 

maintenance and repair that have been programmed into the power schedule, 

Semi-dry FGD systems – Refer to dry FGD system. 
SCR catalyst regeneration wastewater – Any water generated from the SCR catalyst 

regeneration process.

SCR catalyst washing wastewater – Any water generated from the SCR catalyst 

washing process.

SCR catalyst regeneration – Process by which catalysts used in the SCR system are 

regenerated after a period of time because the catalysts have become less reactive through 

use. 

SCR catalyst washing – Process by which catalysts used in the SCR system are washed 

to remove fly ash and/or other particulates. 

Settling pond – A pond used to remove solid particles from a liquid stream by 

gravitational force (i.e., sedimentation process). 

Settling tank – A tank that uses a sedimentation process to remove solid particles from a 

liquid stream by gravitational force. 

Sludge – Any solid, semi-solid, or liquid waste generated from a municipal, commercial, 

or industrial wastewater treatment plant, water supply treatment plant, or air pollution 

control facility exclusive of the treated effluent from a wastewater treatment plant. 

Solids dewatering – The process that removes water from the solids-rich stream 

generated in the solids separation process. Typically a vacuum belt filter or a vacuum 

drum filter is used in this process. FGD solids such as gypsum are produced by this 

process.

Steam turbine cleaning wash water –Any water or liquid cleaning solution used for or 

generated from cleaning the steam turbine.

Stormwater runoff – Runoff generated when precipitation from rain and snowmelt 

events flows over land or impervious surfaces and does not percolate into the ground. 

Sulfur dioxide control systems – An air pollution control system that removes sulfur 

dioxide from flue gas. Sulfur dioxide control systems include, but are not limited to: wet 

FGD systems, dry FGD systems, and lime/limestone addition to the boiler. 

Thickener – A sedimentation process to remove solid particles from a liquid stream by 

gravitational force. In contrast to clarification, the primary purpose of the thickener is to 

increase the concentration of suspended solids of the feed stream (i.e., to remove liquids), 

thereby increasing the concentration of solids in sludge. Note that thickening should not 

be confused with solids dewatering; the cake formed from solids dewatering is handled 

as a solid and not a liquid. 
Treated (water or process wastewater) – Water that has been processed by physical, 

chemical, biological, or other means to remove specific constituents of the water stream 

or to alter the physical or chemical state of specific constituents of the water stream. 

Treated intake water – Water that is acquired from a source and treated prior to use by 

physical, chemical, biological, or other means to remove specific constituents of the 

water stream or to alter the physical or chemical state of specific constituents of the water 

stream. 

Ultimate parent firm – The highest level domestic business entity in the facility’s 

ownership structure. A firm that is owned by another U.S. firm is not an ultimate 

domestic parent firm. In contrast, a U.S. firm that is owned by a foreign firm is an 

ultimate domestic parent firm. 

Uncontaminated stormwater – Stormwater runoff that has not come into contact with 

raw materials, byproducts, or waste products from the electricity generation process. 

Utility – Any entity that generates, transmits, or distributes electricity and recovers the 

cost of its generation, transmission or distribution assets and operations, either directly or 

indirectly, through cost-based rates set by a separate regulatory authority (e.g., State 

Public Service Commission), or is owned by a governmental unit or the consumers that 

the entity serves. Examples of these entities include: investor-owned entities, public 

power districts, public utility districts, municipalities, rural electric cooperatives, and 

State and Federal agencies. Electric utilities may have Federal Energy Regulatory 

Commission approval for interconnection agreements and wholesale trade tariffs 

covering either cost-of-service and/or market-based rates under the authority of the 

Federal Power Act. 

Vacuum drum filter – A system that consists of a tank containing a rotating drum 

covered with a cloth filter. A vacuum is used to pull water through the cloth filter to 

dewater the solids. Also referred to as a rotary drum filter.

Vacuum filter belt – An apparatus used in dewatering that uses a vacuum to remove 

water by pulling it through a revolving filter belt.

Variable O&M costs – Operation and maintenance costs that vary directly in proportion 

to the amount of electricity generated by a plant. 

Wastewater treatment – The processing of wastewater by physical, chemical, 

biological, or other means to remove specific pollutants from the wastewater stream or to 

alter the physical or chemical state of specific pollutants in the wastewater stream. 

Treatment is performed to allow for discharge of wastewater or recycle/reuse of 

wastewater. 

Wastewater treatment system – A combination of one or more wastewater treatment 

units, other than ponds/impoundments, designed to achieve wastewater treatment. 
Wastewater treatment unit – A unit operation used to remove pollutants from process 

wastewater. Wastewater treatment units include, but are not limited to: primary settling 

ponds, chemical precipitation, pH adjustment, clarification, biological reactor, thickeners, 

filters, and constructed wetlands. 

Waste coal – Usable material that is a byproduct of previous coal processing operations. 

Waste coal is usually composed of mixed coal, soil, and rock (mine waste). Most waste 

coal is burned as-is in unconventional fluidized-bed combustors. For some uses, waste 

coal may be partially cleaned by removing some extraneous noncombustible 

constituents. Examples of waste coal include fine coal, coal obtained from a refuse bank 

or slurry dam, anthracite culm, bituminous gob, and lignite waste. 

Wet bottom ash handling system – A system in which bottom ash is conveyed away 

from the boiler using water as the transport medium. Wet bottom ash systems typically 

send the ash slurry to dewatering bins or a pond/impoundment.

Wet FGD system – Wet FGD systems capture sulfur dioxide from the flue gas using a 

sorbent that has mixed with water to form a wet slurry, and that generates a water stream 

that exits the FGD scrubber absorber.

Wet fly ash handling system – A system that conveys fly ash away from particulate 

removal equipment using water as the transport medium. Wet fly ash systems typically 

dispose of the ash slurry in a pond/impoundment. 

Zero discharge – Disposal of process and/or non-process wastewaters other than by 

direct discharge to surface water or by indirect discharge to a POTW. Examples include 

evaporation, deep-well injection, hauling, and/or offsite recycle/recovery.