Transcript Edwin, Sam – E.6-Environmental-Chemistry-waste-water

Waste water treatment
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List the primary pollutants found in waste water
and identify their sources.
Outline the primary, secondary and tertiary stages
of waste water treatment, and state the substance
that is removed during each stage.
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
suspended particles
unpleasant odours and smells
reduce BOD (biochemical oxygen demand)
Water pollutants (1)
pollutants
pesticides
dioxins
sources
Agriculture, insecticides, fungicides, and herbicides.
Dichloro-diphenyl-trichloroethane DDT is an example
of pesticides.
formed as part of large-scale processes like
combustion of waste (waste incineration), manufacture
of herbicides and paper pulp bleaching with chlorine;
extremely toxic. Weed killers (herbicides)
PCB’s:
 electrical insulators, molecules added to plastics
polychlorinated  overexposure result in a disease called chloracne
biphenyls;
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
Effects of Water pollutants (1)
pollutants
pesticides
dioxins
effects
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Carcinogenic
Birth Defects
Neurological Disorders
Leads to biological magnification – toxic levels build
up over a period of time in animals at the top of
food chains
• Chloracne, a server form of skin disease
• Reproductive and developmental effects
• Liver damage and cancer
PCB’s:
• Acne-like breakouts
polychlorinated • Hearing and vision problems
biphenyls;
• Irritation of gastrointestinal treat
• Affect reproductive efficiency
• Liver damage and cancer
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
Human sewage
Agricultural run-off from crops
Pulp and paper industry
Chemical and fertilizer manufacturing
Detergent
Water pollutants (2)
pollutants
organic
matter
nitrates
effects
• Retards the photosynthesis process in
plants
• Babies can develop
methemoglobiemia or ‘blue baby
syndrome’ caused by a lack of oxygen
in the blood. (oxidation of Fe2+ to Fe3+)
• Carcinogenic
phosphates
• Eutrophication accelerates plant and
algae growth
• Kidney damage
• Osteoporosis
Water pollutants (3) heavy metals
Mercury
Source
•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)
• agriculture
Health
hazard
• The most dangerous of the metal pollutants; causes serious
damage o the nerves and the brain
• Symptoms of mercury poisoning result from damage to the
nervous system: depression, irritability, blindness and insanity
Environ- • Reproductive system failure in fish
mental
• Inhibits growth and kills fish
hazard
• Biological magnification in the food chain
Water pollutants (3) heavy metals
Lead
Source
•lead paints
•lead based solder in water pipes and fittings;
•car exhausts (leaded petrol);
•car batteries
Health
hazard
•Burning pains in the mouth and digestive system followed by
constipation or diarrhoea
•In severe cases there is a failure of the kidneys, liver and heart
which can lead to coma and death
•Can cause breain damage particularly in young children
Environme- • Toxic to plants and domestic animals
ntal hazard • Biological magnification in the food chain
Water pollutants (3) heavy metals
Cadmium
Source
•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
•Metal plating
Health
hazard
•Replaces zinc in enzymes making them ineffective
•Itai-itati disease makes bones brittles and easily broken
•Kidney and lung cancer in humans
Environme
ntal hazard
•Toxic to fish
•Produces birth defects in mice
Waste-water Treatment
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If water is discharged untreated in to rivers/
sea, it is eventually decomposed by
microorganisms.
Septic tanks or cesspits are used in some
areas. The waste is broken down by
bacteria.
Waste-water Treatment
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Classified as primary, secondary or tertiary methods,
including filtration, sedimentation, flocculation and
precipitation processes.
Includes both physical and chemical treatment.
Primary treatment: physical
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methods:
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filtration
flocculation
sedimentation
removes:
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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
Primary Treatment
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First passed through screens and grids which filter out
large insoluble solid objects, and remove floating objects
and grease from the surface.
Then is passed into a sedimentation tank (settle). The
resulting sludge is removed from the bottom of the tank.
Flocculation: speed up the sedimentation process by
adding chemicals which allow suspended particles to join
together to form large clumps.
Large flocs are formed by the addition of chemicals into
water: Al2(SO4)3(aq) + 3Ca(OH)2(aq) -> 2Al(OH)3(s) + 3CaSO4(s)
(Primary treatment is generally not sufficient to improve
the quality of water to safe levels) A typical primary
treatment domestic sewage plant can remove about 3040% of the BOD waste.
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
Secondary treatment
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Activated sludge: oxidation of organic waste
by aerobic bacteria encouraged by aeration
Trickling filter beds: rotating pipes sprinkle
waste water over stones which
have bacteria and algae
growing on them which
consume the waste and
some nitrates.
Secondary treatment
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Activated Sludge Process: The waste water
enters aeration tanks where oxygen is mixed with
the water. This creates ideal conditions for
aerobic bacteria to grow and flourish. The
bacteria then digest the organic matter and any
untreated organic matter again settles out as
sludge in a second sedimentation or clarifier tank.
The water is returned to the aeration tank and the
aeration process is repeated until all of the
organic matter has been removed.
Removes Dissolved organic matter: Most
(about 90%) of organic oxygen-demanding
wastes and suspended particles.
Tertiary treatment
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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
denitrifying bacteria: nitrates into N2
chemical precipitation: heavy metal ions are
precipitated out by adding anions which form
insoluble salts with them (see next slide); also
removes phosphates
reverse osmosis and ion exchange: nitrates.
Tertiary treatment: precipitation
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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
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to precipitate phosphates, aluminium or calcium ions are added
which form insoluble phosphates; these phosphates then
precipitate out
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3Ca2+ (aq) + 2PO43- (aq)  Ca3(PO4)2 (s)
2Al3+ (aq) + PO43- (aq)  AlPO4 (s)
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
+ Cl-
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Distillation
Can be used to obtain fresh water from sea water.
Sea water is heated and then passed into an evacuated
chamber where it boils, leaving dissolved compounds in
solution.
The steam is then passed through a condenser which is
cooled by pipes containing more sea water.
The warm sea water is then heated
and distilled in turn.
Reverse Osmosis
Osmosis: movement of water from a dilute
to a concentrated solution by passing
through a semi-permeable membrane.
 This process can be reversed if a pressure
of 70atm (the osmotic pressure) is applied
to the more concentrated salt solution.
 The water passes through the semipermeable membrane and leaves the
dissolved salts behind.
(the semi-permeable membrane must be able
to withstand high pressures)
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