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
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

12. Tertiary methods Precipitation Ion exchange Biological methods
Activated carbon bed

13. 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)

14. Ion exchange For soluble nitrates and cations
Resins or zeolites exchange nitrate ions with hydroxide ions
X-OH- + NO3- (aq)  X-NO 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

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

16. 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

17. Chlorine and ozone treatment
effective against bacteria but not viruses
effective against bacteria AND viruses
cheaper to produce
more expensive
longer retention time
shorter retention time
easily liquefied & shipped
must be produced on-site due to high reactivity
can form toxic chloro-organic compounds
oxidized products much less toxic
leaves ‘chemical taste’
leaves no taste
functions as strong oxidizing agent

18. E6.3 Fresh water from sea water
Distillation
Reverse osmosis

19. 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

20. 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

21. 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

22. 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)