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E.6 Water Treatment

E.6 Water Treatment. E6.1 Primary pollutants. Heavy metals (i.e. mercury, lead, cadmium) Pesticides Dioxins Polychlorinated biphenyls (PCBs) Organic matter Nitrates Phosphates. Sources of pollutants. Mercury – paints, batteries, fungicides/seed dressings, dental almalgams

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E.6 Water Treatment

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  1. E.6 Water Treatment

  2. E6.1 Primary pollutants • Heavy metals (i.e. mercury, lead, cadmium) • Pesticides • Dioxins • Polychlorinated biphenyls (PCBs) • Organic matter • Nitrates • Phosphates

  3. Sources of pollutants • Mercury – paints, batteries, fungicides/seed dressings, dental almalgams • Lead – lead pipes, lead paint or glaze, tetraethyl lead in gasoline • Cadmium – metal plating, rechargeable batteries, pigments, by-product of zinc refining • Nitrates – fertilizers • Phosphates - detergents

  4. Pesticides • Include insecticides, fungicides, herbicides  washed off land into water • DDT was used as a pesticide – very stable and fat soluble; remains in food chain and magnified at higher trophic levels

  5. Dioxins • From incineration of organochloro- compounds • Some weed killers (herbicides) • Two benzene rings connected via 1-2 oxygens • Each ring can have up to 4 Cl atoms • 10 000 times more poisonous than cyanide ion

  6. PCBs • Electrical transformers and capacitors • Plasticizers • Adhesive industries • Polychlorinated biphenyls

  7. 6.2 Waste water treatment Before returning to environment • Remove hazardous materials • Reduce BOD • Kill microorganisms

  8. Primary methods • Not sufficient to improve water quality to safe levels • Can remove about 30-40% of BOD waste • Steps: • Passed through screens and grids to filter large insoluble solids and remove floating objects and grease • Passed into sedimentation tank to settle • Sludge (bottom of tank) is removed • Sedimentation speeded up by adding chemicals to allow suspended particles to form large clumps = flocculation – flocs are formed by adding Al2(SO4)3 and Ca(OH)2 to make a gelatinous precipitate

  9. Secondary Methods • Activated sludge process • Removes about 90% of organic oxygen-demanding wastes and suspended particles • Primary & secondary do not remove inorganic substances

  10. Secondary methods • Waste water mixed with bacteria-laden sludge • Large blowers bubble high-oxygen air through mixture (bacteria oxidize organic matter) • Water with decomposed suspended particles passes through sedimentation tank  large quantity of biologically active sludge collects • Part of active sludge is recycled

  11. Tertiary methods • Precipitation • Ion exchange • Biological methods • Activated carbon bed

  12. Precipitation • Heavy metal ions (Cd, Pb, Hg) removed as sulfide salts • H2S bubbled through • Solids removed by filtration • Cd2+(aq) + H2S(g) CdS(s) + 2H+(aq) • Phosphates removed by addition of calcium or aluminum ions 3Ca2+(aq) + 2PO43-(aq) Ca3(PO4)2(s) Al3+(aq) + PO43-(aq)  AlPO4(s)

  13. Ion exchange • For soluble nitrates and cations • Resins or zeolites exchange nitrate ions with hydroxide ions X-OH- + NO3-(aq) X-NO3- + OH-(aq) (ion exchange) (ion exchange) • Exchange cations with H+ Y-H++ M+(aq) Y-M+ + H+(aq) (ion exchange) (ion exchange) • H+ and OH- ions combine to form H2O • Very expensive for large volumes since resins and zeolites need to be regenerated • Ion exchange can also remove salt from sea water

  14. Biological methods • Anaerobic denitrifying bacteria turn nitrates into N2 • Algae ponds can also remove nitrate ions

  15. Activated carbon bed • Activated carbon is tiny carbon granules with large surface area; treated and activated by high temperature • Adsorb organic chemicals from waste water • Effective against many toxic organics • Carbon is reactivated by heating at high temperature  adsorbed matter is oxidized to CO2 and water

  16. Chlorine and ozone treatment

  17. E6.3 Fresh water from sea water • Distillation • Reverse osmosis

  18. Distillation • Heat sea water and pass into evacuated chamber where it boils • Steam passed through condenser, cooled by pipes containing more sea water • Condensed steam is pure water • Dissolved compounds left behind • Warm sea water from around condenser is then heated and distilled

  19. Distillation Advantages Disadvantages • Efficient (multi-stage) • Produces water on large scale • Higher quality / purer water • High energy cost • Environmental concerns due to production of carbon dioxide • Corrosion of distillation equipment by sea water and high temperature

  20. Reverse osmosis • Osmosis is movement of water from dilute to concentrated solution through a semi-permeable membrane • Pressure greater than osmotic pressure (70 atm) is applied to sea water • Pure water passes through the membrane • Dissolved salts left behind • Membrane must withstand high pressures

  21. Reverse osmosis Advantages Disadvantages • No phase change so requires less energy • Cheaper • Faster to build • Simpler to operate • Must be kept running to preserve membrane • Membranes require careful maintenance to prevent fouling • Pretreatment of sea water required (remove organisms, particles)

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