Thermal Pollution

By Rute Neves and Silvia Lourenço

We thank Paulo Fonseca for his cooperation.

Environmental engineers and chemical engineers take a narrow view of thermal pollution, unfortunately. Their jobs are to remove heat from waste streams so that discharge regulations are satisfied. The regulation may be stated as the volumes and temperatures that are permissable for discharge or as the thermal rise that is that is tolerable for the receiving water. But first we need to answer an important question: What is Thermal Pollution?

In a broader sense, and with the concern about global warming, engineers do nothing to mitigate the effects on earth because they take air or cool water from the environment to exchange heat with their waste streams.

Almost half of all water withdrawn in the United States each year is for cooling electric power plants. The cheapest and easiest method is to withdraw water from a nearby body of surface water, pass it through the plant and return the heated water to the same body of water.

Warmer temperatures lower dissolved oxygen content by decreasing the solubility of oxygen in water. Warmer water also causes aquatic organisms to increase their respiration rates and consume oxygen faster, and it increases their susceptibility to disease, parasites, and toxic chemicals. Discharge of heated water into shallow water near the shore of a lake also may disrupt spawning and kill young fish.

Fish and other organisms adapted to a particular temperature range can also be killed from thermal shock

While some scientists call the addition of excess heat to aquatic systems thermal pollution, others talk about using heated water for beneficial purposes, calling it thermal enrichment. They point out that heated water results in longer commercial fishing seasons and reduction of winter ice cover in cold areas.

SOLUTIONS: For Reducing Thermal Water Pollution

Waste Heat - A pollutant as dangerous to waters as more tangible of forms of waste Human activity can change normal temperature:

1. By altering environment of watercourse: Road building , logging,

poundments, diverting flows for irrigation.

2. Adding or removing heat.

On national scale, industrial cooling waters is a first-order source of heat. Electro power generation uses 80% of cooling waters. Best single index of thermal pollution lies in projecting future electric power generation.

Past experience has indicated that thermal pollution has not multiplied as fast as power generation because of improvements in thermal plant efficiency and developement of hydropower.

Nulear plants - waste even higher proportion of heat than fossil-fuel plants.

Heat rejection is expected to increase nine fold by the year 2000.

Problem is one of managing tremendous amounts of waste heat in a manner that will maintain or enhance, physical, chemical and biological nature of our water resources.

Use of cooling water by U.S. industry, 1964 (click to see table)

Approximately one-half of all water used in U.S. is used for cooling by power and manufacturing industries.

During past half-century power generation has virtually doubled every 10 years.This trend is expected to continue or perhaps increase.

Power generation has been either for hydropower or steam-electric. Latter requires cooling water.

Remaining sites for hydropower are limited.

Thermal plants continue to gain a larger portion of market for power.

Plant Evaluation

National averages of steam-electric plant statistics (click to see table)

Effects of Heat


last update 5 - December - 1996