1. TVM 4145 Vannrenseprosesser / unit processes
Disinfection
Prof. TorOve Leiknes

2. Disinfection What do the directives / regulations state?
Demand: “Drinking water shall not have pathogenic organisms”
All drinking water requires disinfection
Principle of muliti-barriers

3. Disinfection process:
penetration of cell walls and destruction of internal structures
attacks cell membranes and changes properties
attacks microorganisms enzyme system
1. chemical - halogens
- oxidizing substances
- metals
- acids/bases
- surfactants
2. physical - heat
- UV-exposure
3. mechanical - sedimentation
- filtration / membranes
- adsorption
4. radiation - gamma and x-ray
Desinfeksjons metoder:

4. Disinfection kinetics:
Chick; neutralizing microbiological activity ca be compared to
(1908) a chemical reaction. A 1st order reaction.
Chick’s law:
Assumptions;
one culture media
constant temp., pH, water quality
constant disinfection dose 2 1
1. Chick’s law
2. Takes time for effect to start
3. “Easiest” are destroyed first
ln(N/No) 3
time

5. “destruction is a function of disinfection concentration”
Watson:
“destruction is a function of disinfection concentration”
Watson’s law:
(n is a function of disinfection agent)
n < 1, contact time t is more important than concentration C
n > 1, efficiency decreases with dilution
Combining Chick’s law and Watson’s law:

6. Parameters that affect the disinfection efficiency:
1. Temperature:
Effect follows Arrhenius relationship
disinfection temperature
2. pH:
pH < 3 and pH > 11
Die out or inactivation
3. Water quality:
Consumption of oxidation agent
inorganic material which is oxidized
organic material which is oxidized
- adsorption to the cell which hinders disinfection
- complex reactions which are less effective
- oxidation which removes disinfection properties

7. Halogens
Fluorine F
Chlorine Cl
Bromide Br
Iodine I
Of these chlorine is mostly used for disinfection systems
History:
1774, chlorine gas first prepared by Scheele
1825, first used in wastewater treatment, France
1831, used as prophylactic against cholera epidemics
1900s used as disinfection of drinking water
1917, chloramines taken into use, Canada og USA
Characteristic:
Yellow-green gas, very strong characteristic smell.
Very strong oxidizing agent, can be explosive
Poisonous, forms several undesired chlorinated substances

8. Chlorine reactions with water vann:
(dissociation pH dependent)
HOCl more effective than OCl-
defined as “free chlorine”
will react with other substances

9. Chloramines: Reactions - Mono-chloramine Di-chloramine Tri-chloramine
Formation of chloramines is pH dependent
Referred to as “bound chlorine”
Break-point chlorination
Disinfection efficiency:
HOCl > OCl- > NH2CL > NHCl2 > NCl3

10. Advantages/disadvantages with chlorination:
Well known technology
Works quickly
Easy to measure
Good residual effect on the distribution system
Overall al economics relatively low
Can cause taste and odour problems
Can cause harmful byproducts (THM)
Chlorine gas is poisonous, health / safety issues for operation etc.

11. Oxidizing substances
Ozone, O3
H2O2
Permanganate
Of these ozone (O3) is mostly used for disinfection
History:
1783, discovered by Van Marum
1857, first ozone generator constructed by Siemens
1893, first commercial use of the equipment
1893, used for disinfection of drinking water, Netherlands
1906, installed in a water treatment process, France
Characteristics:
Blue gas med strong characteristic smell.
Very strong oxidation agent.
Poisonous; max. conc. 0,2 mgO3/m3 air
Production:
Generated from dry air or oxygen in a high voltage
electric discharge.

12. Advantages/disadvantages with ozonation:
reduces odours, taste, colour
oxidizes organic pollutants
works very quickly
no residual chemicals
limited dangerous byproducts
no danger of “over dosing”
oxygenation of the water
effective against chlorine resistant pathogens
no residual effect
relatively high investment costs
must be generated on site
can be difficult to operate
difficult to control the effect
can require pretreatment
oxidizes other material
under specific conditions may form harmful byproducts

13. Thermal disinfection
History:
cooking is the oldest disinfection method known
Characteristics:
can reach near sterilization by boiling
Spores are thermal resistant, few water borne diseases are from spore forming bacteria
freezing will not disinfect water
Application:
Not used at treatment plants due practical and economic reasons, mainly a recommendation during emergencies.

14. History: Characteristics: UV - radiation
Exposure to direct sun light to improve water quality is well known in several historic documents.
1940s a proposal was made for guidelines on how to dimension and design UV-systems
Has not traditionally been used in larger plants but in recent years has had a lot of focus and interest as an alternative solution
Characteristics:
UV light is classified in three groups;
1. UV-A: nm, long wave lengths
2. UV-B: nm, medium wave lengths
3. UV-C: <280nm, short wave lengths

15. Disinfection effect between 200-300 nm. Highest effect at 256nm
Low-pressure lamps:
monochromatic radiation
quarts mercury lamps
wave length; 253,7 nm
high intensity
High-pressure lamps:
polychromatic radiation
several wave lengths in radiation
somewhat lower intensity

16. UV-disinfection process:
Photo chemical reaction:
UV wave length breaks down the structure in the DNA molecule
prevents the bacteria’s capacity to reproduce
NB!
All microorganisms have a certain capacity to repair their DNA structures by photo reactivation when exposed to light in the wave length area of nm. Some minor dark reactivation may also be possible.

17. Dimensioning of UV disinfection:
Efficiency is a function of radiation intensity and retention time
where: D = dose in mWatt-sek/cm2 (ultrad)
I = radiation in mWatt-sek
t = radiation time in sec
General demands:
- dimensioned for Qmax
- poorest raw water quality
- min ultrad
- should always be the final step in a treatment scheme
- alternative reserve disinfection system necessary

18. Advantages/disadvantages with UV disinfection:
works very quickly
no effect on odours and taste
no residual chemicals
minimal harmful byproducts
no danger of “over dosing”
no change in the waters natural composition
easy operation and maintenance
no residual effect on distribution network
traditionally viewed as system only suitable for
efficiency dependent on raw water quality

19. Mechanical
Adsorption: most microorganisms are not de independent or free suspended entities in the water phase and are commonly found in aggregates and on particles in the water. Unit processes that remove particles will to a certain degree therefore also remove microorganisms.
Processes which will remove microorganisms from water:
1. Chemical precipitation
2. Sedimentation / flotation
3. Filtration / membrane filtration
In general, mechanical removal of microorganisms with the exception of some membrane filtration processes will not be accredited as a hygienic barrier and recognized as a disinfection process.