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)

2. pH and buffering capacity adjustment (pH optimum of processes)

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)

Oxidation and Oxidation and reductionreduction

The processes are parallel with each other, The processes are parallel with each other, they take place atthey take place at the same time the same time

The oxidation agent will be reducedThe oxidation agent will be reduced (it gets (it gets electron)electron), while the reduction agent will be , while the reduction agent will be oxidized oxidized (it loses electron)(it loses electron)

In drinking water treatment chemicals are In drinking water treatment chemicals are oxidized in order to make them non-soluble, oxidized in order to make them non-soluble, in order to make them less toxic or in order in order to make them less toxic or in order to kill bacteria (disinfection)to kill bacteria (disinfection)

Oxidizing agents are: oxygen, ozone, chlorineOxidizing agents are: oxygen, ozone, chlorine, , potassium permanganate, chlorine, chlorine-potassium permanganate, chlorine, chlorine-dioxide, chloraminesdioxide, chloramines……

To convert the soluble compounds into non-soluble compounds:

Fe(II) Fe(III)Mn(II) Mn(IV)

Oxidation is also used to oxidize some compounds to make them non-soluble

(oxidation of iron, oxidation of manganese)

THE APPLICATION OF AIR/OXYGEN AS OXIDIZING

AGENT

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

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 by air/oxygen

Oxidation of hydrogen-sulfide:

2H2S + O2 2S + 2H2O

H2S: rotten-egg odour

Oxidation by air/oxygen

THE APPLICATION OF OZONE

Oxidation of:

iron Fe(II) Fe(III)manganese Mn(II) Mn(IV)arsenic As(III) As(V)organic compoundscompouds causing colourcompounds causing tastecertain micropollutants (cyanide pollution in 2000)microorganisms

Oxidation by ozone

O3

Generation:

from

3 O2 + energy 2 O3

the preparation of feed-gas is needed before ozonation

airoxygen

water water isis a common medium for a common medium for microorganisms microorganisms the spread of the spread of infectious diseaseinfectious diseases (pollution from waste s (pollution from waste water)water)

The most important aspect of water The most important aspect of water treatment is treatment is the removal of pathogenic the removal of pathogenic microorganisms!microorganisms!

This process is called is disinfection and it is usuallyThis process is called is disinfection and it is usuallyan oxidation process (expect the UV radiation)an oxidation process (expect the UV radiation)

Another purpose of oxidation...the disinfection

DisinfectantsDisinfectants::

ChlorineChlorine ClCl22

ChloraminesChloramines NHNH22-Cl-Cl

Chlorine dioxideChlorine dioxide ClOClO22

OzoneOzone OO33

SilverSilver AgAg

UV radiationUV radiation

Disinfection

to decrease the number of pathogenic microorganisms in water: bacteria, viruses, protozoa

evaluation of other disinfectants

from the 1910s: the application of chlorine (cheap, efficient)

1970s: harmful by-products of chlorine were discovered

Aim of disinfection

THE APPLICATION OF CHLORINE

Chlorine Cl2it 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 !!

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

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

Oxidation by chlorine

Disadvantage of chlorination

Formation of trihalo-methane compounds andhalogenated organic compounds

chloroform

chlorinated phenol

Examples:

Oxidation by chlorine

Summary of advantages & disadvantages of chlorination

Advantages

easy to prepare (from NaCl)

strong oxidant

cheap

safe

sufficient residual can be maintained

Disadvantages

reaction with ammonium

formation of harmful by-products

Oxidation by chlorine

THE APPLICATION OF CHLORINE-DIOXIDE

Chlorine-dioxide

no reaction with ammonium

no THM formation

By-products:

formation of aromatic chlorinated compounds

it has to be produced on-site because of the risk of explosure

Advantages

Disadvantages

ClO2

formation of chloritheClO2-

formation of chlorathe (harmful)ClO3-

Oxidation by chlorine-dioxide

THE APPLICATION OF OZONE

Decomposition of ozone - Milli-Q water

0

0,05

0,1

0,15

0,2

0,25

0 50 100 150 200

time (min)

ozone c

oncentr

ati

on (

mg/l) T = 20 ˚C

pH = 6-7

Decomposition of ozone – ion-free water

time (min)

con

cen

trati

on

of

ozo

ne

(mg

/l)

Decomposition of ozone - Tap water

0

0,05

0,1

0,15

0,2

0,25

0 20 40 60 80 100 120 140time (min)

ozo

ne c

once

ntr

ati

on (

mg/l) T = 10 ˚C

pH = 8,6

Decomposition of ozone – tap water

time (min)

con

cen

trati

on

of

ozo

ne

(mg

/l)

Advantages Disadvantages

Efficient disinfectant

THMs are not formed

It decompoeses easily; There is no residual disinfectant

o o oo o oo o o

O3

Secondary disinfectant

By-product formation;

easily biodegradable organic compounds

Oxidation by ozonation

o o oo o oo o o

O3

Secondary disinfectantBAC

Advantages

Efficient disinfectant

THMs are not formed

Disadvantages

It decompoeses easily; There is no residual disinfectant

By-product formation;

easily biodegradable organic compounds

Expensive

bromate formation

Oxidation by ozonation

0 1 2 3 4 5 6 7 8 9 10 11

5

4

3

2

1

0

time (day)

logC

FU/m

L

Regrowth of microorganisms after disinfection

after treated by chlorine

after treated by ozone

(Miettinen et al.)

APPLICATION OF UV LIGHT FOR DISINFECTION

(THIS IS NOT AN OXIDATION PROCESS)

UV light – It inactivates the microorganisms by physical way

TG

A A C

T T G T

A

G

C

C

A

TG

C

G

CA

AC

GT

UV253,7 nm

Formation of double bond

inhibits replication

DNA

Disinfection by UV radiation

Incomplete penetration

UV light scatterParticle

shading

Region of limited cellular

damage

Complete penetration

UV light scatter

Region of limited cellular

damage

UVlamp

Particle shading

Incomplete penetration

Complete penetration

UVlamp

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

Disinfection by UV radiation

PROBLEMS WITH THE WATER QUALITY AFTER DISINFECTION

g/m

2 in

ner

pip

e s

urf

ace

Biofilm formation

(Lund et al.)

Without any disinfection

Treated by chlorine

Treated by UV radiation

Treated by ozone

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