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
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
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
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
PCBs
 Electrical transformers and capacitors
 Plasticizers
 Adhesive industries
 Polychlorinated biphenyls
6.2 Waste water treatment
Before returning to environment
 Remove hazardous materials
 Reduce BOD
 Kill microorganisms
Primary methods
 Not sufficient to improve water quality to safe levels
 Can remove about 30-40% of BOD waste
 Steps:
1. Passed through screens and grids to filter large
insoluble solids and remove floating objects and grease
2. Passed into sedimentation tank to settle
3. 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 Methods
 Activated sludge process
 Removes about 90% of organic oxygen-demanding
wastes and suspended particles
 Primary & secondary do not remove inorganic
substances
Secondary methods
1. Waste water mixed with bacteria-laden sludge
2. 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 methods
 Precipitation
 Ion exchange
 Biological methods
 Activated carbon bed
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)
Ion exchange
 For soluble nitrates and cations
 Resins or zeolites exchange nitrate ions with hydroxide
ions
X-OH- + NO3- (aq) 
(ion exchange)
X-NO3- + OH- (aq)
(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 H

2O
 Very expensive for large volumes since resins and
zeolites need to be regenerated
 Ion exchange can also remove salt from sea water
Biological methods
 Anaerobic denitrifying bacteria turn nitrates into N2
 Algae ponds can also remove nitrate ions
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
Chlorine and ozone treatment
Chlorine
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
E6.3 Fresh water from sea water
 Distillation
 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 water
Dissolved compounds left behind
Warm sea water from around condenser is then heated
and distilled
Distillation
Advantages
Disadvantages
 Efficient (multi-stage)
 High energy cost
 Produces water on large scale
 Environmental concerns due
 Higher quality / purer water
to production of carbon
dioxide
 Corrosion of distillation
equipment by sea water and
high temperature
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
Reverse osmosis
Advantages
Disadvantages
 No phase change so requires
 Must be kept running to
less energy
 Cheaper
 Faster to build
 Simpler to operate
preserve membrane
 Membranes require careful
maintenance to prevent
fouling
 Pretreatment of sea water
required (remove organisms,
particles)