1. Water Treatment

2. Water treatment plants receive water from different sources – rivers, lakes & reservoirs
7 step process – screening, flocculation, settling, filtration, chlorination, fluoridation, pH adjustment

3. Treatment Plant

4. 1) Screening:
Water passes through large screens of wire mesh
This removes large suspended solids like branches, twigs or litter

5. Screening

6. 2) Flocculation:
A flocculating agent called aluminium sulfate added to water
Acts as a coagulant – makes small suspended particles stick together and form large ones

7. Flocculation

8. 3) Settling/Sedimentation:
Flocculated water pumped into bottom of large tanks
Water slowly rises to the top
Large suspended particles sink to bottom

9. Settling

10. 4) Filtration:
Water passes from tanks through sand and gravel beds
This removes any remaining suspended particles
Filter beds regularly cleaned

11. Filtration

12. 5) Chlorination:
Chlorine added to sterilise water – killing any micro-organisms
Added in very small forms – just enough to keep water sterile
Added in form of chlorine gas or sodium hypochlorite

13. Chlorination

14. 6) Fluoridation:
Fluorine added to help prevent tooth decay
Added in form of sodium fluorosilicate
Only added in some countries (incl. Ireland)

15. Fluoridation

16. 7) pH adjustment:
Tap water should be in range of pH 6 – 8
If pH too low (too acidic) lime is added
If pH is too high (too basic) sulfuric acid is added
If the pH is too low it can cause pipes to corrode

17. pH adjustment

18. Eutrophication
Eutrophication is the over-enrichment of water by nutrients like phosphates and nitrates
This is reason for tertiary stage of sewage treatment as it removes phosphates & nitrates

19. Occurs when agricultural waste like fertiliser and animal slurry are dumped in water
Fertiliser and animal slurry are full of these nutrients
Use of fertiliser and control of dumping are vital to prevent this pollution

20. The nutrients in the water act like fertilisers to plants and increase growth of plants and algae in lakes and rivers
Can see the algae bloom formed

21. Algae Bloom

22. Algae do not live long
As they decay they consume the oxygen dissolved in the water
By using up this dissolved oxygen they cause the death of animal life (animals living in the water and those that rely on that water supply)

23. Heavy metal pollution
Heavy metals = metals with high relative atomic masses
Examples = mercury (Hg), cadmium (Cd) and lead (Pb)
Sources = car batteries or dry batteries

24. Have started recycling batteries which will improve level of pollution
But if dumped in water can pass into drinking water
All these metals are poisonous and if build up in body can cause serious illness

25. But before being dumped in water they can be removed
Removed by precipitation
There are different treatment processes for each metal

26. Water quality Controlled by EU regulations
Limits have to be set because of toxic effects
Examples:
Chemical: Limit:
phosphates p.p.m
nitrates p.p.m
lead p.p.m
mercury p.p.m

27. Sewage Treatment
Domestic sewage dumped untreated in waterways for years
Huge source of pollution to water supply
Some areas now have sewage plants to treat the effluent
3 stage process – primary, secondary and tertiary treatment

28. Sewage Plant

29. Primary Treatment Physical process Screening and settling
Effluent passes through large wire mesh to remove large particles and break up sewage

30. Passes through long grit channels on way to settling tanks to remove more particles
Grit channels regularly cleaned
Enters settling tank at bottom and slowly rises to top

31. Sludge settles at bottom and is removed regularly
Machine skims surface of water to remove any floating particles
Effluent flows from top of tank on to secondary stage

32. Secondary Treatment Biological process
Bacterial breakdown of nutrients in effluent
Effluent oxidised aerobically (in air) by a trickling filter OR an activated sludge unit

33. Trickling filter – effluent passes through bed of stones containing micro-organisms that digest the nutrients

34. Trickling Filter

35. Trickling Filter

36. Activated sludge unit – effluent passes into tank containing micro-organisms that digest the nutrients. Tank is kept oxygenated by a mechanical rotar

37. Activated Sludge Unit

38. Aerobically digested effluent then passes into settlement tanks
It is now safe to pass into waterways where it will be diluted and less harmful to environment
But still contains nitrates and phosphates so moves on to tertiary stage

39. Tertiary Treatment Removal of nitrates and phosphates
Very expensive process so not all plants carry out this stage
Removed by precipitation

40. Phosphates react with aluminium sulfate (flocculating agent) and are removed by filtration
Nitrates removed by biological denitrification – effluent left in tanks with no air so uses the oxygen bonded to nitrogen in nitrates to digest effluent