12
Dissolved Oxygen Theory
37
partial pressure of oxygen in the water will be the same as it is in air. Therefore,
all you need to know is the temperature of the air in which the probe is placed.
By consulting solubility tables for oxygen at the particular barometric pressure
and salinity of the water being measured, the corresponding concentration
(mg/L or ppm) can be found for air saturated water at the air calibration
temperature, and the meter can be set accordingly. Because most meters are
temperature compensated, they will still give correct readings in mg/L even
though the actual water temperature may be different to the air calibration
temperature. Note: The closer the air calibration temperature is to
the water temperature, the more accurate the calibration.
Applications
Oxygen is essential for fish, invertebrate, plant, and aerobic bacteria respiration.
DO levels below 3 ppm are stressful to most aquatic organisms. Levels below 2
or 1 ppm will not support fish. Fish growth and activity usually require 5 to 6
ppm of DO, an important consideration for Aqua-culture industry.
Low DO indicates a demand on the oxygen of the system. Natural organic
material such as leaves accumulate in the stream and create an oxygen
demand as it is decomposed. Organic materials from human activities also
create an oxygen demand in the system. Micro-organisms consume oxygen as
they decompose sewage, urban and agricultural run-off, and discharge from
food-processing plants, meat-packing plants and diaries. There is an optimum
DO level for this process and if DO level falls too low, the micro-organisms die
and the decomposition ceases. If DO level is too high, more power is used
than necessary for aeration and the process becomes costly.
In boiler water application, presence of oxygen in the water will increase
corrosion and helps build up boiler scale that inhibits heat transfer. In such
instance it is critical to keep DO concentration to a minimum.
Some pollutants such as acid mine drainage produce direct chemical demands
on oxygen in the water. DO is consumed in the oxidation-reduction reactions of
introduced chemical compounds such as nitrate (NO
3
1-
) and ammonia (NH
4
1+
),
sulfate (SO
4
2-
), and sulfite (SO
3
2-
) and ferrous (Fe
2
+
) and ferric (Fe
3
+
) ions.
These are important consideration for water and wastewater treatment industry.
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