Dissolved Oxygen; What Is Dissolved Oxygen; Why Measure Do; How Is Do Measured - In-situ Aqua TROLL 600 Operator's Manual

Dissolved Oxygen

What is Dissolved Oxygen?

The amount of dissolved oxygen (DO) in both natural water and wastewater is a
function of several parameters. DO is highly dependent on temperature and
atmospheric pressure. An increase in temperature causes a decrease in the amount of
oxygen that can dissolve in water. Conversely, higher atmospheric pressures result in
higher DO values. Salinity is also a factor. Oxygen solubility is greater in freshwater
than in salt water. There are also chemical and biochemical processes that affect DO.
Most of the dissolved oxygen in water comes from the atmosphere, but oxygen from the
photosynthesis of aquatic plants is also a key source. DO levels in lakes and other
surface water will actually follow a cyclic or diurnal pattern over the course of a day,
rising and falling as light intensity changes from dawn to dusk.

Why Measure DO?

Most aquatic life requires an average DO value greater than 5.0 milligrams dissolved
oxygen per liter of water (mg/L) in order to survive. Although the amount of dissolved
oxygen in a body of water fluctuates due to natural processes, large deviations from
normal levels are usually a result of human activity. Changes in DO levels are usually
the result of a buildup in organic waste. Organic waste can enter surface water from
treatment facilities, runoff from agricultural feed lots or domestic areas, and from
industrial discharge. Organic wastes often contain nitrates and phosphates. Nitrates
and phosphates are nutrients for aquatic plants and algae, stimulating overproduction
when present in excessive levels. Accelerated growth of blooms increase the number
of photosynthesizing plants, which temporarily increases the amount of dissolved
oxygen. However submerged aquatic vegetation eventually experience a reduction in
sunlight die to increased coverage on the surface. This decrease in sunlight leads to a
reduction in photosynthesis and eventual death. Bacterial processes take over and
consume even more dissolved oxygen. Fish and other aquatic species die due to lack
of dissolved oxygen. This tragic process is know as eutrophication.
Conditions
100% DO, 0°C, 1 atm, 0 ppm Chlorine*
100% DO, 20°C, 1 atm, 0 ppm Chlorine
100% DO, 0°C, 0.75 atm, 0 ppm Chlorine
100% DO, 20°C, 1 atm, 20 ppm Chlorine
Safe level for most aquatic life

How is DO Measured?

Traditional methods for DO determination include the Winkler titration method and
electrochemical methods, such as polarographic (Clark cell) and galvanic probes. With
1-970-498-1500
Typical DO values
DO Value
14.6 mg/L
9.09 mg/L
6.77 mg/L
7.35 mg/L
> 5.0 mg/L
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