E.6 Water Treatment

E6.1 Primary pollutantsHeavy metals (i.e. mercury, lead, cadmium)PesticidesDioxinsPolychlorinated biphenyls (PCBs)Organic matterNitratesPhosphates

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

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

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

PCBsElectrical transformers and capacitorsPlasticizersAdhesive industries

Polychlorinated biphenyls

6.2 Waste water treatmentBefore returning to environmentRemove hazardous materialsReduce BODKill microorganisms

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

Secondary MethodsActivated sludge processRemoves about 90% of organic oxygen-

demanding wastes and suspended particlesPrimary & secondary do not remove

inorganic substances

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

Tertiary methodsPrecipitationIon exchangeBiological methodsActivated carbon bed

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)

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

Biological methodsAnaerobic denitrifying bacteria turn nitrates

into N2

Algae ponds can also remove nitrate ions

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

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 & shippedmust 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

E6.3 Fresh water from sea waterDistillation Reverse osmosis

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

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

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

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)