Water & Enviro Week 4

Dissolved oxygen

Concentration of molecular oxygen dissolved in water

Because O2 content of water is so low, balance between…

Aquatic life and dissolved oxygen is very delicate

How does O2 become dissolved in water

Diffuses through air-water interface

O2 saturation in water is dependent on

1. Temperature

2. Pressure

3. Salt concentration

True or false: Fish need minimum amount of O2 to survive

True - need it for their blood

Name another way O2 gets into water

Photosynthesis - aquatic vegetation, algae convert solar energy to chemical energy

Respiration / Decomposition

Production of energy by utilisation of O2 in breakdown organic molecules releasing carbon dioxide & water as by-product

How does photosynthesis compare to respiration & decomposition with regards to when it occurs

Photosynthesis - Daylight only, Respiration & Decomposition - 24 hours

How is Dissolved oxygen measured

In mg/l using Winkler titration or do meter

Carbohydrates

Carbon, hydrogen & oxygen - typical sugars (glucose, starch, cellulose)

Nitrogenous compounds

Carbon, hydrogen, oxygen, nitrogen & occasionally sulphur - typically proteins

True or false: Oxygen is used to breakdown/oxidise organic matter by aeroboic micro organisms

True

Oxygen demand

Amount of O2 required to degrade organic matter present in water

Theoretical oxygen demands

Calculated amount of O2 which would be required to fully oxidise an organic compound

Oxidation process consumes oxygen, reducing

Dissolved oxygen concentration in water

How is it impractical to calculate theoretical oxygen demand

1. Expensive to identify all organic compounds in water

2. Microbes may not be capable of degrading all organic matter present

Biochemical oxygen demand

Measures actual amount of O2 required by aerobic microbes in oxidation of organic matter

Biochemical Oxygen Demand test steps

1. Water sample incubated in darkness - prevent photosynthesis

2. Samples diluted to prevent microbes depleting O2

3. Samples may have to be seeded with bacteria if not enough are present to degrade matter

4. 5 days incubuated at 20 degrees

True of false: Biochemical oxygen demand is always greater than theoretical oxygen demand

False

How long does ultimate biochemical oxygen demand take

More than 20 days

Chemical oxygen demand

Measure of total O2 required to oxidise all organic material in water

How is chemical oxygen demand carried out

Water mixed with potassium dichromate in acid with catalyst and boiled for 2 hours - difference of potassium dichromate measured at start and end

Advantage and disadvantage of chemical oxygen demand

Adv - measurements made quickly, Disadv - No distinction with biologically available matter

Nitrogen in water is from

Decomposition of organic & inorganic matter, excretion, atmosphere industry and agriculture

Excess nitrogen can

Clog water intakes, use up dissolved oxygen using decomposition, block light

Nitrification

Further oxidising by nitrifying bacteria

Concentration of nitrogen forms can

Indicate water quality

Phosphorus

Nutrient required by all organisms for basic processes of life

Phosphorous concentration in polluted and unpolluted waters

Polluted - high, unpolluted - low

Eutrophication

Lack of phosphorous/nitrogen prevents growth - algae and plants die consuming O2 - dissolved oxygen drops and fish die