WATER QUALITY - Water Quality Engineering in Natural Systems - page 10

2.2.5 Thermal Pollution qualities of surface waters. Discharges of oxidizable

organic substances into water bodies result in the con-
sumption of oxygen and the depression of DO levels. If
DO levels fall too low, the effects on fish can range from
a reduction in reproductive capacity to suffocation and
death. larvae and juvenile fish are especially sensitive
and require higher levels of DO than those required by
more mature fish. Oxygen depletion at the lower depths
of lakes and reservoirs create reducing conditions in
which iron and manganese can be solubilized, and taste
and odor problems may also increase because of the
release of anoxic and/or anaerobic decay products, such
as hydrogen sulfide. Nutrient enrichment in surface
waters is often signaled by excessive oxygen production,
leading to supersaturation of oxygen in some cases, and
to hypoxia or anoxia in deep waters where excessive
plant production is consumed.
Saturation levels of DO decrease with increasing
temperature, as shown in Table 2.2 for a standard atmo-
spheric pressure of 101 kPa.
One of the most commonly used empirical equations
for estimating the saturation concentration of dissolved
oxygen, DO sat , is:

Thermal pollution is typically associated with the dis-
charge of relatively large volumes of heated water into
cooler receiving waters. The impacts of thermal pollu-
tion include reduced oxygen levels and alteration of the
natural ecology in the receiving water. The primary
source of thermal pollution is waste heat from nuclear
and fossil-fuel electric power plants, although discharges
from domestic wastewater treatment plants into cold-
water streams can also elevate receiving water tempera-
tures to unacceptable levels (Cochran and logue, 2011).
Commonly, problematic heated discharges are about
10°C above the natural temperature of the receiving
water. Typically, about half of the fuel energy used by a
power plant is dissipated as waste heat to waterways,
usually to an adjacent water body. Many fish species
(e.g., salmon) are extremely sensitive to temperature
and cannot adjust readily to warmer waters. Conversely,
some fish species thrive in warmer waters near power
plants and can be severely harmed by a sudden drop in
temperature that usually occurs when a plant shuts
down for scheduled maintenance or an unscheduled
outage. Increased water temperatures increase the res-
piration rate of aquatic life, approximately doubling
their respiration rate for each 10°C rise in temperature,
and decrease the saturation concentration of oxygen in
the water, hence increasing the stress on aquatic life.
Most modern power plants are required to install
cooling towers that release waste heat to the atmo-
sphere rather than to water bodies.
In cases of coastal coastal power stations that dis-
charge heated water to temperate seas, heated dis-
charges are generally of little consequence, but in
tropical seas, where summer temperatures are already
near the thermal death point of many organisms, the
increase in temperature can cause substantial loss of life.

1 575701 10 . ×

5

6 642308 10 . ×

7

ln DO sat = − 139 34411 . + −

T T

2

a a

10 11

1 243800 . × 10 8 621949 10 . ×

+ −

T

3

T

4

a a

(2.1)

where T a is the absolute temperature (K) of the water.
Equation (2.1) is commonly referred to as the Benson-
Krause equation. A more compact alternative equation
that is sometimes used is given by

468
31 .

DO sat =

T

(2.2)

+

2.3 CHEMICAL MEASURES

where T is the water temperature in °C. Equation (2.2)
is accurate to within 0.03 mg/l, as compared with Equa-

Several chemical compounds or combinations of com-
pounds are considered to be toxic to human and aquatic
life and have the potential to occur in the water environ-
ment at harmful levels. In some cases, it is not the pres-
ence of a toxic substance that is of concern, but the lack
of a substance that is essential for the well-being of the
aquatic ecosystem. Dissolved oxygen falls into this latter
category.

TABLE 2.2. Saturation Dissolved Oxygen in Water

Temperature (°C) Dissolved Oxygen (mg/l)

0 14.6

5 12.8

10 11.3

15 10.1

2.3.1 Dissolved Oxygen

20 9.1

25 8.2

Dissolved oxygen (DO) is one of the most important
water-quality parameters affecting the health of aquatic
ecosystems, fish mortality, odors, and other aesthetic

30 7.5

35 6.9