Transcript Environmental Chemistry part 3

Waste water treatment

List the primary pollutants found in waste water
and identify their sources.
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Outline the primary, secondary and tertiary stages
of waste water treatment, and state the substance
that is removed during each stage.
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Evaluate the process to obtain fresh water from sea
water using multi- stage distillation and reverse
osmosis.
Waste water treatment: why?
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water is very good solvent so any water
sample will always contain solutes e.g.
nitrates, phosphates, heavy metals,
carcinogenics, ...
pathogens and other micro-organisms
remove suspended particles
remove unpleasant odours and smells
reduce BOD
Water pollutants (1)
pollutants
sources
Pesticides
Agriculture
Dioxins
Formed as part of large-scale processes like
combustion of organochlorine compounds (waste
incineration), manufacture of herbicides;
extremely toxic.
PCB’s:
polychlorinated
biphenyls;
 Electrical insulators, capacitors, transformers,
coolants, …
 Overexposure result in a disease called
chloracne which is a skin condition which
produces cysts containing a straw coloured
liquid; other symptoms are loss of appetite,
nausea, vomiting and weakness.
 Most PCB’s have low toxicity for humans.
Water pollutants (2)
pollutants
Organic
matter
sources
Sewage, agricultural run off e.g. cleaning
out of stables, cow sheds, food industry.
Nitrates
Over-use of fertilizers results in leaching of
nitrates into rivers/run-off of fertilizers,
animal and human waste.
Phosphates
Use of phosphate-containing detergents.
Water pollutants (3) heavy metals
pollutants
sources
Mercury
•mercury cell e.g. used in the electrolysis of brine - greatest
dischargers;
•batteries;
•mercury salts used as fungicides to reduce formation of
•moulds on seeds (seed dressing)
Lead
•lead paints
•lead based solder in water pipes and fittings;
•car exhausts (leaded petrol);
•car batteries
Cadmiu
m
•by-product of the extraction of zinc and lead; cadmium is
always found near zinc
•used in pigments in ceramics;
•zinc-plating always contains some cadmium;
•rechargeable batteries
Primary treatment: physical
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Methods:
 filtration
 flocculation
 sedimentation
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Removes:
 insoluble solids and liquids
 most suspended particles
 some oxygen-demanding wastes
(organic matter)
(http://en.wikipedia.org/wiki/Sewage_treatment#Filter_beds_.28oxidising_beds.29
Flocculation (speeds up sedimentation)
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Involves the addition of alum (Al2(SO4)3 which
contains aluminium ions (flocculating agent) which
causes coagulation (= coming together) of the nonsettleable (do not settle at the bottom) suspended
solids.
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Al3+ ions (also Ca2+ can be used) react with alkali
substances to produce an Al(OH)3 precipitate which
traps/attracts the suspended solid particles and
causes them to settle at the bottom
(=sedimentation).
Secondary treatment
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Activated sludge: oxidation of organic waste
by aerobic bacteria encouraged by aeration
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Trickling filter beds: rotating pipes sprinkle
waste water over stones which
have bacteria and algae
growing on them which
consume the organic waste and
and some nitrates.
Secondary treatment: activated sludge
http://en.wikipedia.org/wiki/Activated_sludge
Secondary treatment:trickler filter
http://en.wikipedia.org/wiki/Trickling_filter
Secondary treatment
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removes 90% organic waste reducing
BOD
methods:
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activated sludge
trickle bed filter
Tertiary treatment: methods
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Activated carbon-bed: organic waste is oxidized
into CO2 and H2O by the carbon activated by heat,
also removes dioxins and PCBs.
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Denitrifying bacteria: nitrates into N2; under
anaerobic conditions .
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Chemical precipitation: heavy metal ions are
precipitated out by adding anions which form
insoluble salts with them (see next slide); also
removes phosphates
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Reverse osmosis and ion exchange: nitrates.
Tertiary treatment: precipitation
Ions like cadmium, lead, mercury and phosphate ions can be
precipitated by adding ions which form insoluble compounds
with the heavy metal or phosphate ions.
1. To precipitate phosphates, aluminium or calcium ions are
added which form insoluble phosphates:
3Ca2+ (aq) + 2PO43- (aq)  Ca3(PO4)2 (s)
Al3+ (aq) + PO43- (aq)  AlPO4 (s)
2. To precipitate heavy metal ions, hydrogen sulphide gas is added; the
heavy metal ions form their sulphide salts which have very low
solubilities:
Pb2+ (aq) + H2S (g)  PbS (s) + 2 H+ (aq)
Ion exchange
X = resin beads
Equation of exchange:
X – Cl- + NO3-  X – NO3- + Cland
Y – H+ + M+  Y – M+ + H+
also (X – OH- + NO3-  X – NO3
+ OH-)
Fresh water from seawater
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Reverse osmosis
A high pressure (greater than
osmotic pressure i.e. greater than 70
atm) is applied to sea water (low
concentration of water) side of a
partially permeable membrane (only
allows water through and no ions or
solute particles). The pure water
from the sea water is forced out of
the solution through the membrane
leaving the salt behind (salt ions
cannot pas through) and into pure
water (high concentration of water).
Fresh water from seawater
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Sea water is heated in a series of coiled pipes and then
introduced into a chamber at lower pressure. Under this
lower pressure some of the sea water boils immediately.
The water vapour is condensed by contact with cold water
pipes which carry the sea water. The condensed water now
is free of impurities. In this way the heat released during
the condensation is used to preheat some of the water.
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The remaining sea water that has not boiled yet is directed
to another set of chambers each with a lower pressure than
the preceding chamber. This method maximises the use of
energy.
evaluation
Multi- stage distillation
Reverse osmosis
advantages
 Efficient
 Requires less energy
 Can produce water on a
 Cheaper
large scale
 Removes more impurities
disadvantages
 High energy costs
 Produces large amounts
of CO2
 Must be kept running
 Pre-treatment of
water is needed e.g.
debris, living
organisms)