EN21 - Total water discharge by quality and destination

Releases to water, which are carefully monitored in compliance with water quality regulations, represent less than one percent of the releases reported by AEP. These include the chemicals in coal combustion by-products, chlorine, and ammonia.  Detailed information on discharges to water can be found at AEP’s Toxic Release Inventory/Right-to-Know (TRI/RTK) website.

AEP System Releases for 2005 to Water (197,154 pounds)

Discharges
American Electric Power generating facilities discharge approximately 17.8 billion cubic meters of process wastewater per year to surrounding surface waters (table below).  There is a slight discrepancy between the amount of water discharged from AEP facilities and the amount of water that is withdrawn at these same facilities (see EN8).  Total withdrawals equal 14.6 billion cubic meters per year, which is 3.2 billion cubic meters per year less than what is discharged.  The difference is likely due to the way in which these figures were determined.  The withdrawal data are based on 2005 cooling water withdrawals and does not account for any rainwater that may accumulate in facility treatment ponds or lakes.  The discharge data are based on average discharge values, which are determined over a period of months and include contributions from rainfall events, which are not reflected in the withdraw data.  In addition, AEP does not directly meter intake water flows, therefore, these values are based on estimates.

The majority of AEP wastewater releases are from steam electric facilities and 92% of these releases are non-contact cooling water, which is used to cool steam in condensers (table below).  Once-through cooling systems withdraw water from a nearby water body, pass it through a condenser, and discharge it back into the body of water.  Chlorine or other biocides may be added to the water to control biofouling.  In closed cooling systems, water that has passed through a condenser is sent to a cooling tower to lower the temperature.  As water evaporates, the latent heat required to evaporate the water is transferred from the cooling water to the air, cooling the water (U.S. EPA 2006).  Because some of the water evaporates, fresh make-up water is added to the system.  In addition, a small amount of water must be periodically discharged to control the buildup of solids.  This water is referred to as “cooling tower blowdown” (U.S. EPA 2006). 

The next largest wastewater releases are fly ash and bottom ash transport water; however, these effluents combined only make up a tenth of a percent of all AEP discharges (table below).  The burning of coal or oil in steam electric boilers produces a noncombustible residue known as ash.  Heavier particles that collect at the bottom of the boiler are known as bottom ash.  Finer particles that are light enough to be transferred in the flue gas are known as fly ash.  Fly ash and bottom ash can be transported by wet handling systems that produce slurries of ash, referred to as sluices, that are typically transferred to wet surface impoundments.  The ash settles in the impoundments prior to recycling or discharge of the water.  Fly ash and bottom ash sluices typically contain heavy metals and inorganic constituents (U.S. EPA 1982). 

Other waste streams from AEP facilities include metal cleaning wastes, coal pile runoff, boiler blowdown, sump water, turbine seal water, landfill leachate and seepage, and other low volume wastes.  Metal cleaning wastes are those resulting from the cleaning of any metal process equipment.  Chemicals are often used to remove scale and corrosion from boiler tubes.  The major constituents of cleaning wastes are iron, copper, nickel, and zinc.  Alkaline reagents are also used to clean air preheaters and to neutralize acidity.  These alkaline washes can consist of soda ash, caustic soda, phosphates, and detergent.

Coal pile runoff consists of rainwater that has accumulated on and near coal storage piles.  Coal pile runoff is typically acidic and may contain high concentrations of copper, iron, aluminum, nickel, and other constituents present in coal (U.S. EPA 1982).  Boiler blowdown is that water which is periodically discharged from boilers to control the build-up of solids.  There are many sources of impurities in boiler blowdown, including intake water, internal corrosion of the boiler and chemicals added to the boiler system (U.S. EPA 2006).  Examples of impurities include soluble inorganic salts, calcium, magnesium, iron, copper, chromium, phenol, phosphate, and other chemical species.   Other low volume wastes include laboratory and sampling streams, floor drains, cooling tower basin cleaning wastes, and recirculating service water systems (U.S. EPA 2006). 

All AEP facilities that discharge such effluents have National Pollutant Discharge Elimination System (NPDES) permits that have been issued by the appropriate state agencies.  These permits govern the discharge of the treated wastewaters and ensure compliance with all applicable water quality standards.  The Clean Water Act requires facilities that discharge process wastewaters into receiving waters to control these discharges according to effluent guidelines and water quality-based effluent limits specified in NPDES permits.  The Steam Electric Guidelines specify limits for pH, PCBs, TSS, oil and grease, free available chlorine, total residual chlorine, chromium, copper, iron, and zinc.  These limits are based on the available and economically achievable technologies that can be implemented at steam electric facilities.  Monitoring is conducted at each AEP facility to ensure that the discharges comply with these limits.

Total water discharges from AEP generating facilities

Treatment
The majority of water used at AEP generating facilities is used for cooling purposes, either in once-through or recirculating closed systems.  Cooling towers are most frequently used to cool the water in closed systems; however, in both once-through and closed systems various methods are used to remove biocides and residual oxidants.  Typically, biocides are used in low-level applications to treat the biofouling that occurs in the cooling systems.  Natural decay may be utilized to remove biocides or dehalogenation systems may be used to comply with NPDES permit limits.  In these systems, a reducing agent is added to consume the residual oxidizing biocide.  Sulfur dioxide and sodium bisulfite are the most commonly used dehalogenation chemicals.  Bentonite clay can be added to absorb excess non-oxidizing biocides, which are not removed by sulfur dioxide. 

Bottom ash and fly ash ponds are used to treat ash sluice water and are primarily settling basins that allow ash constituents and suspended solids to settle out before the transport water reaches the discharge point or is recycled.  Some iron co-precipitation also occurs in these ponds, aiding with the removal of pollutants, such as arsenic.  The control of pond pH also helps to precipitate out metals, such as copper.