1. CE 370
Disinfection

2. Disinfection
Definition: is the process of destruction of living pathogenic microorganisms
Disinfection depends on:
The physico-chemistry of the disinfectant
The cyto-chemical nature and physical state of the pathogens
The interaction of the above
Temperature pH
Electrolytes
Interfering substances

3. Classification of Disinfectants
Oxidizing agents (ozone, halogens)
Cations of heavy metals (silver, gold, mercury)
Organic compounds
Gaseous agents
Physical agents (heat, UV, pH)

4. Disinfection Rate
Disinfection rate by a chemical agent obeys Chick’s Law:
dN/dt = rate of cell destruction (number / time)
k = rate constant
N = number of living cells remaining at time t
k depends on:
Microorganisms species
Disinfectant nature
Disinfectant concentration
Environmental factors (pH and temperature)

5. Disinfection Rate Another equation is:
C = concentration of disinfectant at time t = 0
tc = time of contact required to kill a given percentage of the microbes
K, n = exponential constants
n depends on the nature of disinfectant
If n > 1, then disinfection greatly depends on the concentration of the disinfectant
If n < 1, then disinfection greatly depends on the time of contact
K depends on type of microorganism and environmental factors such as pH and temperature

8. Disinfection Methods Chlorination Chlorine dioxide Ozonation
UV irradiation
High pH
Other halogens (iodine and bromine)

9. Chlorination Chlorine is widely used Chlorine is applied as:
Effective at low concentration
Cheap
Forms residual if applied in sufficient dosages
Chlorine is applied as:
Gas (most common)
hypochlorite
Chlorine is a strong oxidizing agent
It oxidizes enzymes necessary for metabolism

10. Chlorination Reaction
Chlorine gas reacts with water to form
Hypochlorous acid (HOCl)
Hydrochloric acid (HCl)
Hypochlorous acid dissociates to hypochlorite ion
The dissociation of the HOCl is a function of pH

11. Chlorination Reaction
Hypochlorite salts
Are available in dry form
They should be dissolved in water
OCl-1 seeks equilibrium with H+ ion
Therefore, acid may be needed to be added
Both hypochlorous acid and hypochlorite ions are effective disinfectants

13. Chlorination Reaction
Reaction with ammonia
At pH > 6.0, the monochloramine predominates
At pH about 5.0, the dichloramine predominates
Chloramines are effective against bacteria but not viruses

15. Chlorination Reaction
Reaction with organics
Reaction with phenol produces chlorophenols
Reaction with humic substances produoces trihalomethanes
Chloroform (CHCl3)
Bromodichloromethane (CHCl2Br)
Dibromochloromethane (CHClBr2)

16. Dosages, Demand and Residuals
Dosage: the amount of chlorine added
Demand: the amount of chlorine needed to oxidize materials
Residual: the amount of chlorine remaining after oxidation

19. Dosages, Demand and Residuals
Contact time
When increased, more microorganisms are killed
When increased, the demand increases
When increased, the amount of chlorinated by-products increase (if precursors are available)
Free chlorine residuals
Chlorine gas + hypochlorous acid + hypochlorite ion
Combined chlorine residuals
Chloramines + other chlorine reactive forms

20. Dosages, Demand and Residuals
Free chlorine residuals are faster acting than combined residuals
Free chlorine residuals have greater disinfecting capacity than combined residuals (especially for viruses)

21. Dechlorination
Chlorinated effluents can have negative impacts on receiving environment
Chemicals used for dechlorination include:
Sulfur dioxide (most common)
Sodium sulfite
Sodium bisulfite
Sodium thiosulfate
Hydrogen peroxide
ammonia

22. Dechlorination Activated carbon is:
Well established in the dechlorination process
It has limited use due to its high cost
Requires long contact times
It can be used for dechlorination and removal of organic compounds at the same time

23. Sulfur Dioxide (SO2) General characteristics Gas Colorless
Has strong pungent odor
Neither flammable nor explosive
With sufficient moisture, it is corrosive to most metals
Has solubility in water of 18.6% at 32 F (0 C)
When reacts with water, it forms a weak solution of sulfurous acid (H2SO3)
Sulfurous acid dissociates as follows:

24. Sulfur Dioxide (SO2)
Free and combined chlorine forms react readily with sulfite ion (SO3-2) as follows:
Required mass ratio of sulfur dioxide to chlorine is 1.1:1

25. Ozonation General characteristics of ozone Ozone production O3
Powerful oxidant
More powerful than hypochlorous acid
Unstable in aqueous solutions
Has a half-life of 20 to 30 minutes in distilled water
Widely used in drinking water treatment
Is produced on-site and can not be stored
Ozone production
Air is refrigerated to remove moisture (-40 to -60 C)
Air is dried through desiccants (silica gel and activated alumina)
Air is passed between oppositely charged plates

30. Ozonation Performance
Some investigators reported total destruction of polioviruses in distilled water at a dose of 0.3 mg/l and a contact time of 3 minutes
Presence of SS may increase the dose and contact time
For good results, SS should not be very high
Ozone is never used as a terminal treatment (no residual ozone)

31. Ozonation Advantages When compared to chlorination:
Ozone degrades to O2, so no toxic residues
O2 level in effluent is usually at saturation
Does no increase TDS in effluent
Less cost with pure oxygen activated sludge process
Ozone has been used to remove phenols, cyanides and heavy metals
Ozone can render some refractory organics biodegradable

32. Current Research Trends
Identification of byproducts of the reaction of ozone with organic materials
Ozone reaction with pesticides may produce a more toxic material

33. Chlorine Dioxide (ClO2)
ClO2 was originally used to remove taste and odor from water
General characteristics of ClO2
More powerful oxidant than chlorine
Does not react with water
Can be easily removed from water by aeration
Readily decomposed by exposure to UV radiation
Does not react with ammonia
Maintains a stable residual

34. Preparation of ClO2 Acid and sodium chlorite
Chlorine gas and sodium chlorite (excess chlorine)
Sodium hypochlorite and sodium chlorite

35. Applications of ClO2 Disinfection
ClO2 has longer lasting residual than HOCl
In wastewater, ClO2 use is limited to phenolic wastes and the control of sulfide in wastewater collection systems
ClO2 does not produce measurable amounts of THMs (trihalomethanes) or TOXs (total organic halogens)
Cost of equipment and sodium chlorite are high

36. Ultraviolet (UV) Irradiation
Was used in drinking water disinfection
In wastewater treatment, and when compared to chlorination and Ozonation, UV was found
More effective
More economical
UV irradiation is gaining prominence

37. Advantages and Disadvantages of UV
Advantages (in wastewater treatment)
Effective in pathogen inactivation
Ability to achieve disinfection goals
Viable applications to wide range of wastewater qualities
Cost effective
Relative simplicity
Absence of residuals
Disadvantages (in water treatment)
Lack of residuals

38. Types of UV Installations
There are two types:
Overhead bulb radiating downward through a shallow depth of water (2.5 to 5.1 cm)
Submerged UV lamb encased in a quartz tube
The submerged type was found more effective
The efficiency of the process is not significantly affected by turbidity and suspended solids
UV performs well with viruses

39. Performance of UV Irradiation
Dose-response for three organisms showed similar trends:
y = log reduction
x = log UV dose
Dose (D) = radiation intensity  detention time

40. Performance of UV Irradiation
For total and fecal coliforms:
D = 24,800 x – 60,000 (fecal coliforms)
D = 20,000x – 48,200 (total coliforms)
Where x = log reduction

42. High pH Treatment Lime can destruct bacteria at high pH values
In this case, no residual will remain after neutralization
Studies showed that
higher removal of viruses was obtained with higher pH values
Optimum pH in the range of 11.2 to 11.3
Optimum contact time in the range of 1.56 to 2.40 hours
Complete destruction of viruses was obtained at pH of 11.0 and contact time of 5.0 hours and 10 minutes

43. High pH Treatment
The process requires an additional residual disinfectant (minimum amount)
High pH treatment can also remove ammonia and phosphorous
Recarbonation of the treated water might be needed before discharge
Lime stabilization of sludge is implemented at pH of 12 for 30 minutes (higher destruction of bacteria at higher pH values)
No information about viruses
Little effect on parasites