THERMAL DISCHARGE FIELD STUDIES
Margaret Crum, Environmental Control Engineer
Aluminum Company of America
Pittsburgh, Pennsylvania 15129
INTRODUCTION
In June and August of 1975, data were collected in the field to determine the characteristics of thermal discharges from two power plants located along the Ohio River.   The
data obtained were utilized as part of a successful effort to obtain variances from the
steam-electric power generating effluent guidelines as allowed by Section 316(a) of the
Federal Water Pollution Control Act.   Section 316(a) allows less stringent effluent limitations to be imposed on the thermal component of a discharge, provided that the owner
or operator of the source can demonstrate that the effluent limitations are more stringent
than necessary to protect aquatic life.   Field studies documenting the extent of the thermal plume are a necessary part of the demonstration.
This study is of interest since it was one of the few thermal discharge studies on the
Ohio River in which an outside consultant was not utilized.   The study consisted of characterizations of thermal plume configurations under two different flow regimes and  a
literature search of thermal tolerances of fish species commonly occurring in the Ohio
River.   The techniques used for this study could be utilized by other power companies
to perform 316(a) demonstrations at low cost.   The data obtained give accurate indications
of maximum temperatures in the plume, plume size and dispersion rates.   Such data also
can be used to validate the thermal components of a water quality model.
BACKGROUND
Steam-electric power generating plants are the largest users of cooling water in the
United States.   The term steam-electric power plants includes both nuclear and fossil-fueled
plants, both of which operate on similar principles.   Water is converted into high-pressure,
high-temperature steam in a boiler by the heat produced by combustion of fossil fuels or
atomic reactions.   The steam is used to drive a turbine.   Mechanical energy from the turbine is converted to electrical energy by a generator.   Exhaust steam from the turbine is
condensed and returned to the boiler.   Most of the heat which was not converted to electrical energy is rejected as waste heat to either the stack or the condenser cooling water.
This can represent a substantial amount of heat since the most efficient fossil-fueled power
plants operate at about 40% efficiency; the most efficient nuclear plants operate at about
33% efficiency.   Large volumes of cooling water are provided either as once-through cooling water from a large water body or recirculating water from a large evaporative cooling
system such as a cooling tower or lake.   Nonevaporative cooling systems are not commonly
used.
Both once-through and closed-cycle cooling water systems can have measurable environmental impacts.
Once-through cooling systems are simple in principle and in operation.   Water is screened
at the intake by traveling screens, pumped through the condensers and discharged. Continuous and/or shock chlorination is commonly used to control growth of slime and organisms, such as the asiatic clam which can foul cooling water tunnels.   Environmental effects
can include the impingement of fish on the intake screens, mechanical and thermal damage
to small organisms entrained in the cooling water, damage to aquatic organisms by chlorine, and damage to organisms in the receiving stream caused by the thermal discharge.
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