E.6 Water Treatment
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Transcript of E.6 Water Treatment
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E.6 Water Treatment
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E6.1 Primary pollutantsHeavy metals (i.e. mercury, lead, cadmium)PesticidesDioxinsPolychlorinated biphenyls (PCBs)Organic matterNitratesPhosphates
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Sources of pollutantsMercury – paints, batteries, fungicides/seed
dressings, dental almalgamsLead – lead pipes, lead paint or glaze,
tetraethyl lead in gasolineCadmium – metal plating, rechargeable
batteries, pigments, by-product of zinc refining
Nitrates – fertilizersPhosphates - detergents
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PesticidesInclude 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
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DioxinsFrom incineration of organochloro-
compoundsSome weed killers (herbicides)
Two benzene rings connected via 1-2 oxygensEach ring can have up to 4 Cl atoms10 000 times more poisonous than cyanide
ion
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PCBsElectrical transformers and capacitorsPlasticizersAdhesive industries
Polychlorinated biphenyls
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6.2 Waste water treatmentBefore returning to environmentRemove hazardous materialsReduce BODKill microorganisms
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Primary methods Not sufficient to improve water quality to safe levelsCan remove about 30-40% of BOD wasteSteps:
1. Passed through screens and grids to filter large insoluble solids and remove floating objects and grease
2. Passed into sedimentation tank to settle3. 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
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Secondary MethodsActivated sludge processRemoves about 90% of organic oxygen-
demanding wastes and suspended particlesPrimary & secondary do not remove
inorganic substances
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Secondary methods1.Waste water mixed with bacteria-laden
sludge2.Large blowers bubble high-oxygen air
through mixture (bacteria oxidize organic matter)
3.Water with decomposed suspended particles passes through sedimentation tank large quantity of biologically active sludge collects
4.Part of active sludge is recycled
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Tertiary methodsPrecipitationIon exchangeBiological methodsActivated carbon bed
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PrecipitationHeavy metal ions (Cd, Pb, Hg) removed as
sulfide saltsH2S bubbled throughSolids removed by filtrationCd2+(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)
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Ion exchangeFor soluble nitrates and cationsResins 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 H2OVery expensive for large volumes since resins and
zeolites need to be regeneratedIon exchange can also remove salt from sea water
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Biological methodsAnaerobic denitrifying bacteria turn nitrates
into N2
Algae ponds can also remove nitrate ions
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Activated carbon bedActivated carbon is tiny carbon granules with
large surface area; treated and activated by high temperature
Adsorb organic chemicals from waste waterEffective against many toxic organicsCarbon is reactivated by heating at high
temperature adsorbed matter is oxidized to CO2 and water
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Chlorine and ozone treatmentChlorine Ozone
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 functions as strong oxidizing agent
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E6.3 Fresh water from sea waterDistillation Reverse osmosis
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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 waterDissolved compounds left behindWarm sea water from around condenser is
then heated and distilled
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Distillation
Advantages DisadvantagesEfficient (multi-stage)Produces water on large
scaleHigher quality / purer
water
High energy costEnvironmental concerns
due to production of carbon dioxide
Corrosion of distillation equipment by sea water and high temperature
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Reverse osmosisOsmosis 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 membraneDissolved salts left behindMembrane must withstand high pressures
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Reverse osmosis
Advantages DisadvantagesNo phase change so
requires less energyCheaper Faster to buildSimpler to operate
Must be kept running to preserve membrane
Membranes require careful maintenance to prevent fouling
Pretreatment of sea water required (remove organisms, particles)