E5 Dissolved oxygen in water

For animals to survive in aquatic systems, water must contained a minimum concentration of dissolved oxgyen as the organisms require oxygen for aerobic respiration. Oxygen is a diatomic non-polar molecule that is only slightly soluble in water, as water is polar. At 20°C, the solubity of oxygen is ≈0.00028cm3, which is 9mg of water per million mg of water (9ppm). Solubility decreases as temperature rises.

Quality of water is dependent on the dissolved-oxygen (DO) content of a body of water. Water of good quality has a DO content of 8-9ppm, while moderately polluted water is about 4.5ppm, and anything below that concentration is highly polluted. Quality of water depends on factors like oxygen demanding wastes, like organic substances (plants, animals, human waste) and waste from industrial processes (meat packing and food processing plants and paper mills), and diseases causing microorganisms (pathogens).

E.5.1 Outline biochemical oxygen demand (BOD) as a measure of oxygen demanding wastes.

BOD is the measure of the amount of oxygen consumed by biodegradable organic wastes and ammonia in a given amount of water over a time period, normally 5 days, at 20°C. A sample is diluted in oxygen saturated water and enclosed without any air in a BOD sample bottle for a 5 day incubation period, after which the decrease in dissolved oxygen is measured using an oxygen electrode. Almost pure water has a ppm BOD of less than 1, while water of doubtful purity has a ppm BOD of 5, and water of unacceptable purity a ppm BOD of 20 (e.g. untreated sewage has a BOD of 100-400ppm, while food processing plants can be up to 10000ppm).

E.5.2 Distinguish between aerobic and anaerobic decomposition of organic material in water.


Aerobic decomposition
-Oxidation process where complex organic matter is broken down into simple organic material, carbon dioxides and water. Simple organic material can be further converted to nitrates, sulphates, and phosphates.

-DO decreased as oxygen is used up, and if too much organic matter is present, DO may decrease to zero and kill aquatic life that depends on oxygen.

Anaerobic decomposition
-Typical products include ammonia and amines from nitrogen and hydrogen (strong fishy smell), methane/biogas/marsh gas from carbon and hydrogen, hydrogen sulphide (rotten egg smell) from organic sulphur, and phosphine (PH3) phosphorus.
-Oxygen is not used up.

E.5.3 Describe the process of eutrophication and its effect.

Plant nutrients are often soluble and leach into water bodies. This leads to excessive growth of aquatic plant life, often in the form of algal blooms.

Effects:

-water smells and tastes bad, becomes lifeless.
-red tides: marine plankton produce chemical toxins.

-Dead plants are decomposed anaerobically, depleting oxygen

-Fish die from asphyxiation (lack of oxygen)

-Anaerobic processes produce toxic substances like phosphine and hydrogen sulfide.

E.5.4 Discuss the source and effects of thermal pollution of water.

Thermal pollution occurs when water that is heated in power plants/industrial processes is dumped into streams, rivers or lakes. Two major effects would be on dissolved oxygen and the metabolic rates of aquatic life.

OXYGEN:

Concentration of oxygen decreases as temperature increases. Furthermore, warm water is less dense than cool water and stays near the top. This water is unable to absorb as much oxygen from the atmosphere; at 0°C, DO is 15ppm, at 20°C, DO is 9ppm, and at 40°C, DO is 6.5. This is for pure water (polluted water will have even lower DO contents).

AQUATIC LIFE:

Increased temperatures increase the rate of biochemical processes, and hence the metabolic rates of aquatic animals increase and, in so doing, require more oxygen. The rate of consumption of oxygen increases.