Environmental Chemistry Waste Water Treatment(2)
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Transcript of Environmental Chemistry Waste Water Treatment(2)
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8/2/2019 Environmental Chemistry Waste Water Treatment(2)
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water treatment List the primary pollutants found in waste water
and identify their sources.
Outline the primary, secondary and tertiary stagesof waste water treatment, and state the substance
that is removed during each stage.
Evaluate the process to obtain fresh water fromsea water using multi- stage distillation andreverse osmosis.
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Waste water treatment: why? 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
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Water pollutants (1)pollutants sources
pesticides agriculturedioxins formed as part of large-scale processes like
combustion of waste (waste incineration), manufacture
of herbicides and paper pulp bleaching with chlorine;extremely toxic.
PCBs:
polychlorinatedbiphenyls;
electrical insulators, molecules added to plastics overexposure result in a disease called chloracne
which is a skin condition which produces cysts
containing a straw coloured liquid; other symptomsare loss of appetite, nausea, vomiting andweakness
most PCBs have low toxicity for humans
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Water pollutants (2)pollutants sources
organicmatter
Sewage, agricultural run off e.g. cleaningout of stables, cow sheds, food industry
nitrates Over-use of fertilizers results in leachingof nitrates into rivers/run-off of fertilizers,animal and human waste.
phosphates Use of phosphate-containing detergents
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Water pollutants (3) heavy metalspollutants sourcesmercury 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
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Primary treatment: physical methods:
filtration
flocculation
sedimentation
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
http://en.wikipedia.org/wiki/Image:Primary_sedimentation_tank1_w.JPGhttp://en.wikipedia.org/wiki/Image:Primary_sedimentation_tank1_w.JPGhttp://en.wikipedia.org/wiki/Image:Primary_sedimentation_tank1_w.JPG -
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Secondary treatment: activated sludgehttp://en.wikipedia.org/wiki/Activated_sludge
http://en.wikipedia.org/wiki/Image:Activated_Sludge_1.png -
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Secondary treatment:trickler filterhttp://en.wikipedia.org/wiki/Trickling_filter
http://en.wikipedia.org/wiki/Image:Trickle_Filter.png -
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Secondary treatment removes 90% organic waste reducing BOD
methods:
activated sludge
trickle bed filter
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Secondary treatment
Activated sludge: oxidation of organic waste
by aerobic bacteria encouraged by aeration
Trickling filter beds: rotating pipes sprinklewaste water over stones which
have bacteria and algae
growing on them whichconsume the waste and
some nitrates.
http://en.wikipedia.org/wiki/Image:Trickling_filter_bed_2_w.JPG -
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Tertiary treatment methods:
activated carbon-bed: organic waste is oxidizedinto CO2 and H2O by the carbon activated by heat,
also removes dioxins and PCBs denitrifying bacteria: nitrates into N2
chemical precipitation: heavy metal ions areprecipitated out by adding anions which form
insoluble salts with them (see next slide); alsoremoves phosphates
reverse osmosis and ion exchange: nitrates.
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Tertiary treatment: precipitation ions like cadmium, lead, mercury and phosphate ions can be
precipitated by adding ions which form insoluble compounds withthe heavy metal or phosphate ions
to precipitate phosphates, aluminium or calcium ions are addedwhich form insoluble phosphates; these phosphates thenprecipitate out
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 lowsolubilities:
Pb2+ (aq) + H2S (g) PbS (s) + 2 H+ (aq)
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Ion exchange
X = resin beads
Equation of exchange:XCl- + NO3
- XNO3 + Cl
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