1. OXIDATION PROCESSES IN DRINKING WATER TREATMENT
(EXAMPLES)

2. What processes can we use?
1. oxidation-reduction
2. pH and buffering capacity adjustment (pH optimum of processes)
3. chemical precipitation (dissolved  particulate)
4. adsorption (bind to surface)
5. phase separation (removal of different phases: solid/liquid and gas/liquid phase separation)
6. other (e.g. reverse osmosis)

3. What processes can we use?
1. oxidation-reduction
3. chemical precipitation (dissolved  particulate)
4. adsorption (bind to surface)
5. phase separation (removal of different phases: solid/liquid and gas/liquid phase separation)
6. other (e.g. reverse osmosis)
2. pH and buffering capacity adjustment (pH optimum of processes)

4. Oxidation and reduction
The processes are parallel with each other, they take place at the same time
The oxidation agent will be reduced (it gets electron), while the reduction agent will be oxidized (it loses electron)
In drinking water treatment chemicals are oxidized in order to make them non-soluble, in order to make them less toxic or in order to kill bacteria (disinfection)
Oxidizing agents are: oxygen, ozone, chlorine, potassium permanganate, chlorine, chlorine-dioxide, chloramines…

5. To convert the soluble compounds into non-soluble compounds:
Oxidation is also used to oxidize some compounds to make them non-soluble
(oxidation of iron, oxidation of manganese)
To convert the soluble compounds into non-soluble compounds:
Fe(II)  Fe(III)
Mn(II)  Mn(IV)

6. THE APPLICATION OF AIR/OXYGEN AS OXIDIZING AGENT

7. Reaction of oxygen with dissolved iron:
Oxidation by air/oxygen
Reaction of oxygen with dissolved iron:
Fe2+ + 2H2O  Fe(OH)2 + 2H+
Fe(II)
Soluble form
4Fe(OH)2 + 2 H2O + O2  4Fe(OH)3
Fe(II)  Fe(III)
Non-soluble form

8. Reaction of oxygen with dissolved manganese:
Oxidation by air/oxygen
Reaction of oxygen with dissolved manganese:
Mn2+ + 2H2O  Mn(OH)2 + 2H+
Mn(II)
Soluble form
2Mn(OH)2 + O2  2MnO(OH)2
Non-soluble form Mn(II)  Mn(IV)
MnO(OH)2  MnO2 + H2O

9. Oxidation of hydrogen-sulfide:
Oxidation by air/oxygen
Oxidation of hydrogen-sulfide:
2H2S + O2  2S + 2H2O
H2S: rotten-egg odour

10. THE APPLICATION OF OZONE

11. manganese Mn(II)  Mn(IV) arsenic As(III)  As(V) organic compounds
Oxidation by ozone
Oxidation of:
iron Fe(II)  Fe(III)
manganese Mn(II)  Mn(IV)
arsenic As(III)  As(V)
organic compounds
compouds causing colour
compounds causing taste
certain micropollutants (cyanide pollution in 2000)
microorganisms

12. Generation: O3 3 O2 + energy  2 O3 air from oxygen
the preparation of feed-gas is needed before ozonation

13. Another purpose of oxidation...
the disinfection
water is a common medium for microorganisms  the spread of infectious diseases (pollution from waste water)
The most important aspect of water treatment is the removal of pathogenic microorganisms!
This process is called is disinfection and it is usually
an oxidation process (expect the UV radiation)

14. Disinfectants:
Chlorine Cl2
Chloramines NH2-Cl
Chlorine dioxide ClO2
Ozone O3
Silver Ag
UV radiation

15. Disinfection Aim of disinfection
to decrease the number of pathogenic microorganisms in water: bacteria, viruses, protozoa
from the 1910s: the application of chlorine
(cheap, efficient)
1970s: harmful by-products of chlorine were discovered
evaluation of other disinfectants

16. THE APPLICATION OF CHLORINE

17. Cl2 Cl2 + H2O HOCl + H+ + Cl- HOCl H+ + OCl- Chlorine
Oxidation by chlorine
Chlorine
Cl2
it reacts with the water:
hypochlorous acid
Cl2 + H2O HOCl + H+ + Cl-
hypochlorite ion
HOCl H+ + OCl-
HOCl is more efficient than OCl-
the best pH values: between 2 and 6,
but this is too low (corrosive water)!!
pH is between 7 and 7.5 !!

18. Ca(OCl)2 + 2H2O  2HOCl + Ca(OH)2
Oxidation by chlorine
Cl2 + H2O  HOCl + H+ + Cl-
Reaction of chlorine with water
Reaction of calcium-hypochlorite with water
Ca(OCl)2 + 2H2O  2HOCl + Ca(OH)2
Reaction of sodium-hypochlorite with water
NaOCl + H2O  HOCl + NaOH
formation of hypochlorous-acid (HOCl) !!
Addition of Cl2, Ca(OCl)2, NaOCl

19. NH3 + HOCl NH2Cl + H2O NH2Cl + HOCl NHCl2 + H2O
Oxidation by chlorine
Breakpoint chlorination
Chlorine reacts with ammonia to form chloramines:
monochloramine
NH3 + HOCl NH2Cl + H2O
dichloramine
NH2Cl + HOCl NHCl2 + H2O
trichloramine
NHCl2 + HOCl NHCl3 + H2O

20. Disadvantage of chlorination
Oxidation by chlorine
Disadvantage of chlorination
Formation of trihalo-methane compounds and
halogenated organic compounds
Examples:
chloroform
chlorinated phenol

21. Summary of advantages & disadvantages of chlorination
Oxidation by chlorine
Summary of advantages & disadvantages of chlorination
Advantages
strong oxidant
easy to prepare (from NaCl)
cheap
safe
sufficient residual can be maintained
Disadvantages
reaction with ammonium
formation of harmful by-products

22. THE APPLICATION OF CHLORINE-DIOXIDE

23. ClO2 Chlorine-dioxide Advantages no reaction with ammonium
Oxidation by chlorine-dioxide
Chlorine-dioxide
ClO2
Advantages
no reaction with ammonium
no THM formation
Disadvantages
By-products:
formation of aromatic chlorinated
compounds
formation of chlorithe
ClO2-
formation of chlorathe (harmful)
ClO3-
it has to be produced on-site
because of the risk of explosure

24. THE APPLICATION OF OZONE

25. concentration of ozone (mg/l)
T = 20 ˚C
pH = 6-7
Decomposition of ozone –
ion-free water
time (min)
concentration of ozone (mg/l)
T = 10 ˚C
pH = 8,6
Decomposition of ozone – tap water

26. Secondary disinfectant
Oxidation by ozonation
Secondary disinfectant
o o o O3
Advantages
Disadvantages
It decompoeses easily;
There is no residual disinfectant
Efficient disinfectant
THMs are not formed
By-product formation;
easily biodegradable
organic compounds

27. Secondary disinfectant
Oxidation by ozonation
Secondary disinfectant
o o o O3
BAC
Advantages
Disadvantages
It decompoeses easily;
There is no residual disinfectant
Efficient disinfectant
THMs are not formed
By-product formation;
easily biodegradable
organic compounds
bromate formation
Expensive

28. Regrowth of microorganisms after disinfection5 4 3 2 1
after treated by ozone
logCFU/mL
after treated by
chlorine
time (day)
(Miettinen et al.)

29. APPLICATION OF UV LIGHT FOR DISINFECTION
(THIS IS NOT AN OXIDATION PROCESS)

30. It inactivates the microorganisms by physical way
Disinfection by UV radiation
UV light –
It inactivates the microorganisms by physical way UV
253,7 nm T G A C
DNA
Formation of double bond inhibits replication

31. Incomplete penetration Region of limited cellular damage
Disinfection by UV radiation
Incomplete penetration
UV light scatter
Particle shading
Region of limited cellular damage
Complete penetration UV
lamp

32. UV light – advantages and disadvantages
Disinfection by UV radiation
UV light – advantages and disadvantages
Advantages
It inactivates the microorganisms by physical way
Harmful by-products are not formed
Short contact time
Disadvantages
There is no residual disinfectant
The impact of water quality on the efficiency of disinfection
Buildup of Ca, Mg, Fe scales on the sleeve
Biofilm formation on the sleeve
Absorption of UV in water; particle interactions

33. PROBLEMS WITH THE WATER QUALITY AFTER DISINFECTION

34. Biofilm formation Treated by ozone Without any disinfection
g/m2 inner pipe surface
Treated by
ozone
Without any
disinfection
Treated by
UV radiation
Treated by chlorine
(Lund et al.)

35. Summary, conclusions The importance of the evaluation of water
quality parameters
The efficiency of disinfection
The evaluation of other oxidation processes
Formation of by-products
Problems with the water quality after
disinfection