Presentation on theme: "E 5. Dissolved oxygen Outline biochemical oxygen demand (BOD) as a measure of oxygen- demanding wastes in water. Distinguish between aerobic and anaerobic."— Presentation transcript:
E 5. Dissolved oxygen Outline biochemical oxygen demand (BOD) as a measure of oxygen- demanding wastes in water. Distinguish between aerobic and anaerobic decomposition of organic material in water. Describe the process of eutrophication and its effects. Describe the source and effects of thermal pollution in water.
Dissolved oxygen in water solubility of oxygen at 1 atm and 293K is 9 ppm solubility of gases decreases as temperature rises needed for aerobic respiration of aquatic plants and animals, e.g. fish need 3 ppm or 3 mg dm -3 (kg of water) large biodiversity needs 6 ppm
Dissolved oxygen Aerobic respiration lowers the amount of dissolved oxygen in water. The more aerobic respiration, the lower the amount of dissolved oxygen. At times organic waste and other nutrients end up in the water of a river or lake.
Dissolved oxygen in water Organic waste = faeces, dead organisms, urea, untreated sewage, waste from food production. Organic waste in water is first broken down by aerobic respiration by bacteria (for energy). Dissolved oxygen in the water is lowered. Products of aerobic respiration: water, carbon dioxide, inorganic ions like nitrates, nitrites, phosphates.
Dissolved oxygen in water BOD or B iochemical O xygen D emand. BOD can be used as an indicator of the amount of organic waste (=oxygen-demanding waste) and other nutrients in a sample of water. BOD = measured by determining the amount of oxygen in a sample of water (in ppm) at the start and determining it again after 5 days. The difference is the BOD or amount of oxygen used up by aerobic bacteria to decompose the organic matter in the water sample a fixed over 5 days at 293K and in the dark.
Dissolved oxygen in water The greater the BOD, the less dissolved oxygen there is. The greater the BOD, the more organic waste there was in the water sample.
Dissolved oxygen: measurement Water sample is collected and added to oxygen saturated water ([O 2 (aq)] is known). Water is left for 5 days at 293K and in the dark. New [O 2 (aq)] is measured using an oxygen electrode or titration. BOD = [O 2 (aq)] - [O 2 (aq)] Pure water has BOD of less than 1 ppm.
Measuring dissolved oxygen Winkler method: 2Mn 2+ (aq) + 4OH - (aq) + O 2 (aq) 2 MnO 2 (s) + 2H 2 O (l) MnO 2 (aq) + 2I - (aq) + 4H + (aq) Mn 2+ (aq) + I 2 (aq) + 2H 2 O(l) 2S 2 O 3 2- (aq) + I 2 (aq) S 4 O 6 2- (s) + 2I - (aq)
Eutrophication Aerobic respiration of large amounts of organic matter in water produce large amounts of inorganic ions such as nitrates and phosphates; nitrates and phosphates are also added as a result of use of fertilizers and detergents in water. These ions are nutrients to algae. An algal bloom (or ‘red tide’) occurs. This bloom removes a lot of dissolved oxygen from the water to below a level many aerobic organisms cannot survive including aerobic bacteria – they die. More organic waste is added reducing oxygen further. Anaerobic bacteria take over decomposition of organic waste. Products of anaerobic respiration: methane, ammonia, amines, hydrogen sulphide – foul-smelling gases.
Eutrophication: causes Untreated sewage (has a lot of organic waste). Run-off animal waste from farms. Nitrates from fertilizers. Phosphates from detergents. Eutrophication is apparent as increased turbidity in the northern part of the Caspian Sea, imaged from orbit.turbidityCaspian Sea http://en.wikipedia.org/wiki/Eutrophication
Eutrophication A process in which water bodies (rivers, lakes,..) receive excessive amount of nutrients (organic waste, nitrates, phosphates) which causes excessive plant growth e.g. algae which in turn reduces the amount of dissolved oxygen and prevents sunlight from entering the water http://www.umanitoba.ca/institutes/fisheries/eutro.html
Dissolved oxygen: Dissolved oxygen: temperature Higher water temperatures: decrease amount of dissolved oxygen in water increase rate of metabolism of aquatic life requiring them to need more oxygen