1. Dissolved Oxygen In Water
Selin Palabıyıkoğlu

2. The importance of dissolved oxygen
It is one of the most important parameters in aquatic systems. For animals living in water, the water should contain a minimum concentration of dissolved oxygen. Fish require the highest levels and bacteria requires the least.
The dissolved oxygen content of a body of water is one of the most important indicators of its quality.

3. The process
The oxygen dissolves by diffusion from the surrounding air; aeration of water that has tumbled over falls and rapids; and as a waste product of photosynthesis. An  simplified formula is given below: 
Photosynthesis (in the presence of light and chlorophyll):
6 CO2 +6 H20  C6H12O6 +6 O2
Fish and aquatic animals cannot split oxygen from water or other oxygen-containing compounds. Only green plants and some bacteria can do that through photosynthesis and similar processes. Virtually all the oxygen we breath is manufactured by green plants. A total of three-fourths of the earth’s oxygen supply is produced by phytoplankton in the oceans. 

4. Dissolved Oxygen Levels
The solubility of oxygen in water is very low: at 20°C, it is about 2.8 x mol dm -3. This is equivalent to 9 mg of oxygen per dm -3 of water and is best described as 9 ppm (parts per million). As the temperature rises, the solubility of any gas decreases, in contrast to that of most solids.

5. Dissolved Oxygen Levels
The quality of water, at any temperature, can be determined by the amount of oxygen present.
At 20°C, dissolved oxygen content of 8 to 9 ppm O2 at sea level is considered to be water of good quality.
As the dissolved oxygen level reaches 4.5 ppm oxygen, it is considered moderately polluted and below that concentration, it is highly polluted.
Dissolved Oxygen Levels

6. Dissolved Oxygen Levels
How much dissolved oxygen an aquatic organism needs depends upon its species, its physical state, water temperature, pollutants present, and more. Consequently, it’s impossible to accurately predict minimum dissolved oxygen levels for specific fish and aquatic animals. For example, at 5 oC , trout use about milligrams of oxygen per hour; at 25 oC , they may need five or six times that amount. Fish are cold-blooded animals. They use more oxygen at higher temperatures because their metabolic rate increase.

7. The quality of water depends on several factors including oxygen demanding wastes and disease causing pathogens or microorganisms that can affect health. Organic substances such as plants, animal and human waste, waste from industrial processes are the main oxygen demanding wastes.

8. BOD, the Biochemical Oxygen Demand, is a measure of the amount of oxygen consumed by the biodegradable organic wastes and ammonia in a given amount of water over a time period; normally 5 days at 20°C. The greater the amount of oxygen demanding wastes, the higher is the BOD.
ppm BOD
Quality of water
<1
Almost pure water 5
Doubtful Purity 20
Unacceptable Purity

9. Measurement of BOD 2 Mn+2(aq) + 4OH-(aq) +O2(aq)  2MnO2(s)+2H2O(l)
Potassium idodide is then added which is oxidised by the manganese(IV) oxide in acidic solution to form iodine:
MnO2(s) + 2I- (aq) + 4H+(aq)Mn+2 (aq) + I2(aq) +2H2O(l)
The iodine released is then titrated with standard sodium thiosulfate solution:
I2(aq) + S2O3-2(aq)  S4O6-2(aq) + 2I- (aq)
By knowing the number of moles of iodine produced, the amount of oxygen present in the sample of the water can be calculated and hence its concentration

10. Aerobic Decomposition
Aerobic respiration of organic matter in water in the presence of oxygen is a natural biological process in which bacteria that thrive in oxygen-rich environments, break down and digest the organic matter. It is actually an oxidation process, where the complex organic matter is broken into simple organic matter. ( CO2 and water)
Organic nitrogen  nitrates ( NO3- )
Organic sulfur  sulfates ( SO4-2 )
Organic phosphorus  phosphates (PO4-3 )
It removes oxygen from water and causes a decrease in dissolved oxygen.
The dissolved oxygen may decrease even to zero if too much organic matter is present, killing aquatic life that depends on oxygen.

11. Anaerobic Decomposition
Anaerobic decomposition of organic matter in water takes place in the absence of oxygen by microorganisms that do not require oxygen. The products are in reduced form and often foul smelling and toxic.
Typical anaerobic decomposition reaction products of organic matter in the presence of water include:
Ammonia and amines from nitrogen and hydrogen
Methane from carbon and hydrogen
Hydrogen sulfide from organic sulfur
Phosphine from phosphorus

12. ELEMENT
AEROBIC DECAY PRODUCT
ANAEROBIC DECAY PRODUCT
CARBON
CO2
CH4 (marsh gas)
NITROGEN
NO3-
NH3 and amines
HYDROGEN
H2O
CH4, NH3, H2S, H2O
SULFUR
SO4-2
H2S (‘rotten eggs’ gas)
PHOSPHORUS
PO4-3
PH3 (phospine)

13. Eutrophication
Eutrophication means an increase in chemical nutrients -- typically compounds containing nitrogen or phosphorus -- in an ecosystem. It may occur on land or in water. The term is however often used to mean the resultant increase in the ecosystem's primary productivity (excessive plant growth and decay), and further effects including lack of oxygen and severe reductions in water quality, fish, and other animal populations. It usually occurs in lakes or slow-moving streams and they receive excess nutrients that stimulate excessive plant growth . Those nutrients can come from many sources, such as fertilizers applied to agricultural fields, golf courses, and suburban lawns; deposition of nitrogen from the atmosphere; erosion of soil containing nutrients; and sewage treatment plant discharges.

14. Temperature Effect
Power plants and many industrial processes use huge quantities of fresh water to cool and condense steam. The cooling water subsequently becomes warm and if this is dumped into rivers or lakes, it leads to thermal pollution.

15.        

The dissolved oxygen concentration decreased with increasing temperature. If water is heated sufficiently, the dissolved oxygen concentration could decrease to a point that made it unlivable for fish and aquatic plants.

16. Temperature (0C)
Solubility of O2 in fresh water (ppm)
Solubility of O2 in sea water (ppm)
14.71
11.71 10
11.42
9.28 20
9.14
7.50 30
6.21

17. Temperature Effect
A moderate increase in temperature generally speeds up the rate at which biochemical reactions occur, thus, not only the dissolved oxygen level decreased as temperature increases, but also the metabolic rates of aquatic animals increase and require more oxygen. As a result, thermal pollution has adverse effects on both dissolved oxygen and the rate of consumption of oxygen in bodies of water.