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 BOD = B iochemical O xygen D emand can be used as an indicator of the amount of organic waste (=oxygen-demanding waste) in water impure water has more oxygen-demanding waste and therefore a higher BOD than pure water BOD = amount of oxygen used (in ppm) needed by aerobic bacteria to decompose organic matter in a fixed volume of water at 293K over a set period of time, usually five days

Biochemical Oxygen Demand The greater the quantity of degradable organic waste, the higher the BOD BOD vs. Dissolved Oxygen (DO) content of the water Rivers: oxygen level is regenerated Lakes: limited oxygen level

Biochemical Oxygen Demand

Dissolved oxygen in water solubility of oxygen at 1 atm and 293K is 9 ppm solubility 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 in Water Vary with season and time of the day Lowest during summer (because of high temperature) Highest during winter (because of low temperature) Because of Global Warming there is increase in temperature, so fish are looking for cool place which has high level of DO

DO in Water Rapidly moving water usually contain a large amount of DO and stagnant water tends to contain less Stagnant water contains rotting organic matter, which is deterimental for aquatic life Decrease in DO leve  Polution

Dissolved oxygen in water organic waste = faeces, dead organisms, urea, untreated sewage, waste from food production organic waste is first broken down by aerobic respiration by bacteria (for energy) dissolved oxygen used up in this oxidation products of aerobic respiration: water, carbon dioxide, inorganic ions like nitrates, nitrites, phosphates

Dissolved oxygen: measurement water sample is collected and added to oxygen saturated water ([O 2 (aq)] is known) water is left for 5 days new [O 2 (aq)] is measured using an oxygen electrode or titration the BOD = [O 2 (aq)] - [O 2 (aq)] pure water has BOD of less than 1 ppm

Measuring Dissolved Oxygen Winkler method: Sample of water is collected, ensuring that no space is left in the sample bottle The amount of oxygen in the water sample is fixed by using manganese sulfate and an alkaline–iodide reagent. 2Mn 2+ (aq) + 4OH - (aq) + O 2 (aq)  2 MnO 2 (s) + 2H 2 O (l) Formation of a brownish floc of manganese(II) oxide indicates the presence of oxygen in the sample.

Measuring Dissolved Oxygen Winkler method: Concentrated sulfuric acid is added, removing this floc and fixing the amount of oxygen in the sample. MnO 2 (aq) + 2I - (aq) + 4H + (aq)  Mn 2+ (aq) + I 2 (aq) + 2H 2 O(l) At this stage the darker brown the solution is, the higher the concentration of I 2 (aq) and therefore the higher the amount of DO. Titration is performed, using a standardized solution of sodium thiosulfate and starch as an indicator 2S 2 O 3 2- (aq) + I 2 (aq)  S 4 O 6 2- (s) + 2I - (aq) The greater the volume of sodium thiosulfate used the greater the level of DO in the sample.

Aerobic versus Anaerobic Decomposition Aerobic decomposition Decomposition of organic matter uses oxygen in the process and, as a result, oxides or oxyanions are produced. Elements are oxidized or lose electrons

Aerobic versus Anaerobic Decomposition Anaerobic decomposition. Not enough oxygen present or when decomposition is done by organisms that do not require oxygen Reduction or gain in electrons Associated with a foul smell

Eutrophication large amounts of organic matter produce large amounts of inorganic ions such as nitrates (fertilizers) and phosphates (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 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

Eutrophication

Eutrophication: causes untreated sewage 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

Eutrophication A process in which water bodies (rivers, lakes,..) receive excessive amount of nutrients which causes excessive plant growth e.g. algae which in turn reduces the amount of dissolved oxygen and prevents sunlight from entering the water

Eutrophication Animation:

Thermal Pollution Higher water temperatures: Level of DO in a water sample is dependent on the water temperature, and that the solubility of gases decreases with increased temperature. Decrease amount of dissolved oxygen in water with rising temperature When water temperature increase, Increase rate of metabolism of aquatic life requiring them to need more food and oxygen Spawning, fertilization and hatching of eggs is very sensitive to temperature