environmental chemistry waste water treatment(2)

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

-

environmental chemistry waste water treatment(2)

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