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Water Pollution-Oxygen Consuming Wastes

Water Pollution-Oxygen Consuming Wastes. General types of water pollutants. Disease-causing agents (Pathogens) Oxygen-consuming agents Plant nutrients Toxic substances Heavy metals Pesticides Dissolved solids Acids Suspended solids and sediments Oil

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Water Pollution-Oxygen Consuming Wastes

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  1. Water Pollution-Oxygen Consuming Wastes

  2. General types of water pollutants • Disease-causing agents (Pathogens) • Oxygen-consuming agents • Plant nutrients • Toxic substances • Heavy metals • Pesticides • Dissolved solids • Acids • Suspended solids and sediments • Oil • Radioactive substances (Radionuclides) • Heat (thermal pollution)

  3. Oxygen and aquatic life • Animals and plants living in an aquatic habitat depend on oxygen dissolved in the water for their survival. • The availability of O2 in water sets the boundary between aerobic and anaerobic life. This has implications on: • Water quality • Health of ecosystem • Oxygen in water comes from dissolution of atmospheric O2.

  4. Dissolved oxygen The amount of dissolved oxygen (DO) depends on the temperature and the altitude of the water. Relates to atmospheric pressure

  5. Biological oxygen demand • Energy is acquired by oxidation of organic material. In this process, oxygen is required. CH2O + O2 = CO2 + H2O 12 mgC 32mg 1mgC 2.7mg O2 • At 20oC, DO=9 mg/l, therefore, only ~3.4mg of CH2O can be oxidized by the O2 in a liter of water. • When the concentration of organic material is high, water can be easily depleted of DO. • Biological oxygen demand (BOD) is the amount of O2 (in milligram) required by microorganisms to carry out the oxidation of organic carbon in one liter of water. • BOD5: the oxygen consumed by microorganisms in five days.

  6. BOD: example problem What is the BOD of water in which 10 mg of sugar is dissolved in a liter? How does this compare with the O2 solubility at 20oC?

  7. Oxygen-consuming wastes • Organic waste materials released into the water can rapidly deplete dissolved oxygen. • When water is overloaded with organic materials, oxygen-consuming (aerobic) bacteria proliferate. • As a result, DO is consumed more rapidly than it can be replaced from the atmosphere. • When DO<5ppm, fish start to die. • If DO drops further, invertebrates and aerobic bacteria will be unable to survive. • In the absence of DO, anaerobic bacteria take over to decompose organic material  The water begins to smell unpleasant.

  8. DO variation with the distance from organic waste charge point

  9. Sources for organic wastes • Human and animal sewage • Industrial waste from paper mills, tanneries, and food-processing plants

  10. Typical BODs for wastes from various processes

  11. Decomposition products of organic compounds

  12. Three definitions of oxidation and reduction

  13. Natural sequence of redox reactions in aqueous environment

  14. Redox couples in natural environments • O2/H2O • NO3-/N2 • MnO2/Mn2+ • Fe(OH)3/Fe2+ • SO42-/HS- • CO2/CH4 Decreasing redox potential

  15. Study questions • What determines the maximum amount of oxygen that can be dissolved in water? • What consumes oxygen in water? • What happens after oxygen is exhausted in a water body? • List the major redox pairs in the natural environments in the order of decreasing redox potential.

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