1. Waste water treatment
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.

2. 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

3. Water pollutants (1) pollutants sources pesticides agriculture dioxins
formed as part of large-scale processes like combustion of waste (waste incineration), manufacture of herbicides and paper pulp bleaching with chlorine; extremely toxic.
PCB’s: polychlorinated biphenyls;
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 symptoms are loss of appetite, nausea, vomiting and weakness
most PCB’s have low toxicity for humans

4. Water pollutants (2) pollutants sources organic matter
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

5. Water pollutants (3) heavy metals
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

6. Primary treatment: physical
methods:
filtration
flocculation
sedimentation
removes:
insoluble solids and liquids
most suspended particles
some oxygen-demanding wastes (organic matter) (

7. Secondary treatment: activated sludge http://en. wikipedia

8. Secondary treatment:trickler filter http://en. wikipedia

9. Secondary treatment removes 90% organic waste reducing BOD methods:
activated sludge
trickle bed filter

10. Secondary treatment
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.

11. Tertiary treatment:
removal of soluble particles, metal ions, nitrates, ammonium ions, phosphates, …
methods:
activated carbon-bed: organic waste is oxidized into CO2 and H2O by the carbon activated by heat, also removes dioxins
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)
reverse osmosis and ion exchange: nitrates.

12. 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
to precipitate phosphates, aluminium or calcium ions are added which form insoluble phosphates; these phosphates then precipitate out
3Ca2+ (aq) PO43- (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) H+ (aq)