DISSOLVED OXYGEN IN WATER

IMPORTANCE OF DISSOLVED OXYGEN

Aquatic animals need a minimum concentration of dissolved oxygen in water to survive

Dissolved-oxygen (DO) content indicates water quality

At 20°C oxygen content in water is about 9 ppm (parts per million) Oxygen is non-polar so it has low solubility Solubility of gas decreases with higher temps

WATER QUALITY

Factors affecting water quality: Presence of disease causing pathogens/

microorganisms Oxygen demanding wastes: organic substances (plants,

animal/ human/ industrial waste)

Water Quality DO Level

Good Quality (20°C, sea level 8-9 ppm O2

Moderate Pollution 4.5 ppm O2

Highly Polluted <4.5 ppm O2

BIOCHEMICAL OXYGEN DEMAND

Ppm BOD Quality of Water

<1 Almost Pure

5 Doubtful Purity

20 Unacceptable Purity

BIOCHEMICAL OXYGEN DEMAND

Biochemical (Biological) Oxygen Demand (BOD): measure of oxygen consumed by biodegradable organic wastes and ammonia in a given time period/ amount of water (normally 5 days, 20° C)

Ppm BOD Quality of Water

<1 Almost Pure

5 Doubtful Purity

20 Unacceptable Purity

AEROBIC DECOMPOSITION

Aerobic Decomposition: the breakdown and digestion of organic mater in water in the presence of oxygen and a bacteria-rich environment

Oxidation processRemoves oxygen from water so dissolved

oxygen (DO) decreases and can kill aquatic life dependent on oxygen

AEROBIC CONVERSIONS

Complex organic material

simple organic material + CO2 + H2O

nitrogen nitrates (NO3-)

sulfur sulfates (SO42-)

phosphorus phosphates (PO43-)

ANAEROBIC DECOMPOSTITION

Anaerobic decomposition: microorganisms not dependent on oxygen help decompose organic matter without oxygen present

ANAEROBIC CONVERSIONS

Typical Products in Presence of Water:Nitrogen and Hydrogen

ammonia and amines (strong fishy smell)

Carbon and Hydrogen

methane (biogas/ marsh gas)

Organic Sulfur

hydrogen sulfide, H2S (rotten egg smell)

Phosphorus

phosphine, PH3

EUTROPHICATIONEutrophication: the process by which

water becomes lifeless Plant nutrients (nitrates- waste and fertilizers,

phosphates- detergents and fertilizers) lead to excessive growth of plant life (algal blooms)

Dead plants fall to the bottom of water Decay in the presence of bacteria (depletes

oxygen concentration) Fish die due to lack of oxygen Toxic substances are released into the water by

anaerobic processes Water becomes lifeless

THERMAL POLLUTION

Thermal Pollution: Origins: water used to cool/ condense steam in

industrial processes is dumped into bodies of water which then become warmer

Effects: reduced levels of oxygen (gas concentration decreases as temperature increases)

Metabolic rates of aquatic animals increase and require more oxygen to survive