Dissolved Oxygen

3/19/2014

Dissolved Oxygen Sag Curve

 

Oxygen Depletion

Microbes and higher plants and animals obtain the energy stored in the chemical bonds of organic matter, C(H₂O), through the process of respiration:

C(H2O) + O2

CO2 + H2O + Energy

Organisms that depend on others to produce that organic matter are called heterotrophs (or other-feeders). Examples of heterotrophs include bacteria, fungi and even people! We tap into the sun’s energy when we eat organic matter (e.g. pizza) and breathe in oxygen and then breathe back out carbon dioxide and water vapor.
If there is too much organic matter in a lake or river, oxygen can be consumed by bacterial respiration faster than it is re-supplied from the atmosphere. This can lead to oxygen depletion and fish kills. In rivers, oxygen depletion takes a special form called the dissolved oxygen sag curve. The oxygen concentration in the river drops rapidly following the discharge of a waste containing organic matter. Later, as the organic matter becomes used up, the oxygen resources of the river are replenished.
Biochemical Oxygen Demand
Biochemical oxygen demand or BOD is a chemical procedure for determining is the amount of dissolved oxygen needed by aerobic biological organisms in a body of water to break down organic material present in a given water sample at certain temperature over a specific time period. It is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20 C and is often used to determine how badly a body of water is polluted by organic waste.
BOD can be used as a gauge of the effectiveness of wastewater treatment plants, as sewage being an organic waste can deplete the oxygen in a body of water.
Oxygen Sag Curve
In a flowing stream, when a pollutant is dumped in the stream, the concentration of the pollutant will decrease as the water flows down stream.  The Oxygen Sag curve depicts graphically how this concentration impacts the dissolved oxygen in the stream and the biological diversity of the organisms in the stream as the water flows further from the source of the pollution.

Using the Oxygen Sag Curve and the information given above, answer the following questions.
1.  In the clean zone, the BOD (biochemical oxygen demand) was low, why?


2.  In the decomposition zone, the level of oxygen fell.  Why?  The BOD started going up.  Why?  The number of fish decreased.  Why?


3.  In the septic zone, describe the level of dissolved oxygen.  What types of organisms found in this zone?  If you were testing the water quality of a nearby stream and found only these types of organism, what conclusion might you draw about the stream?


4.  In the recovery zone, what was happening to the dissolved oxygen and the BOD?  Was this change in the types of organisms found in that part of the stream?  How?


5.  In the final zone – describe the DO, BOD and the types of organism.  Which earlier zone does the final zone most resemble?


6.  What factors might cause the recovery of the stream to be slower or require more distance of flow?

Quiz