Transcript 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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Presence of nitrates and phosphates in polluted water is harmful
to humans.
 Excessive nitrate level affects oxygen transport by
interacting with hemoglobin in young infants.
 Blue Baby Syndrome
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Heavy Metals are toxic.
 e.g) Mercury, Lead, Cadmium
 They reacts with the necessary ions in the body like
Calcium, Magnesium or Zinc.
 They affect ecological issues such as biomagnification in
food chain, declination of fish population etc.
Waste water treatment: why?
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Purpose of waste water treatment:
 Remove toxic materials and other microorganisms
 Toxic materials and microorganisms harms human and
other living organisms in the water.
 Reduce the BOD
 Aquatic region with high level of BOD is unfavorable
condition for the fish.
Water pollutants (1)
pollutants
sources
pesticides
agriculture
dioxins
Dioxins are from:
* The incineration of waste materials that contain
Organochloro compounds.
* Consists in some weed killers (herbicides)
10,000 times more poisonous than the cyanide ion
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
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
cadmium
•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:
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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
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.
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-