OXIDATION PROCESSES IN DRINKING WATER TREATMENT (EXAMPLES)

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)

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)

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…

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)

THE APPLICATION OF AIR/OXYGEN AS OXIDIZING AGENT

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

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

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

THE APPLICATION OF OZONE

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

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

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)

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

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

THE APPLICATION OF CHLORINE

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 !!

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

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

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

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

THE APPLICATION OF CHLORINE-DIOXIDE

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

THE APPLICATION OF OZONE

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

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

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

Regrowth of microorganisms after disinfection 5 4 3 2 1 after treated by ozone logCFU/mL after treated by chlorine 0 1 2 3 4 5 6 7 8 9 10 11 time (day) (Miettinen et al.)

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

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

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

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

PROBLEMS WITH THE WATER QUALITY AFTER DISINFECTION

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.)

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