1. Disinfection – Chapter 26
Class Objectives
Be able to define the term disinfectant
List different types of disinfectants
List factors that influence disinfection
Be able to write the disinfection inactivation equation and how it relates to ideal and non-ideal disinfection behavior
Be able to define a C ▪ t value
List the common disinfectants used in drinking water and their advantages and disadvantages

2. Disinfection
The destruction or prevention of growth of microorganisms capable of causing diseases
The final barrier against human exposure to pathogens
Disinfectants include:
heat – denatures proteins and nucleic acids
chemicals – uses a variety of mechanisms
filtration – physical removal of a pathogen
radiation – destroys nucleic acids
Some disinfectants also control taste and odor problems, organic matter, and metals such as iron and manganese

3. Factors Influencing Disinfection
Type of disinfectant
Type of microorganism
Disinfectant concentration and time of contact pH
Temperature
Chemical and physical interference, e.g., clumping of cells or adsorption to larger particles

4. Cell-Mediated Mechanisms of Resistance to Disinfectants
Modification of sensitive/disinfectant action sites – enzymes
Cell wall/cell membrane alterations - allow reduced permeability
Cellular aggregation - provides physical protection
Capsule production - limits diffusion of disinfectant into cell

5. Kinetics of Disinfection
Inactivation is a gradual process involving a series of physicochemical and biochemical steps. Inactivation is described by the equation:
Nt/N0 = e-kt
Where:
N0 = number of microorganisms at time = 0
Nt = number of microorganisms at time = t
k = a decay constant (1/time)
t = time
Ideally, inactivation follows first-order kinetics (blue line), but often non-ideal behaviors occur resulting from clumping of cells or multiple hits of critical sites before inactivation

6. Concentration and Contact Time
Effectiveness of chlorination depends primarily on the concentration used and the time of exposure
Disinfectant effectiveness can be expressed as a C ▪ t value where:
C = disinfectant concentration
t = time required to inactivate a 99% of the population under specific conditions
The lower the C ▪ t, the more effective the disinfectant
In general, resistance to disinfection is in the following order:
vegetative bacteria < enteric viruses < spore-forming bacteria < protozoan cysts

7. Common Disinfectants in Water Treatment
Chlorine
Chloramines
Chlorine dioxide
Ozone
Ultraviolet light

8. Chlorine
Most commonly used disinfectant
In water chlorine undergoes the following reaction:
Cl2 + H2O HOCl + HCl
HOCl H+ + OCl-
HOCl and OCl- is defined as free available chlorine
HOCl more effective than OCl- due to lack of charge
Presence of HOCL and OCl- is determined by pH
In drinking water 1 mg/L of chlorine for 30 min is generally sufficient to reduce bacterial numbers. In wastewater with interfering substances up to mg/L may be required

9. Interfering Substances
Turbidity can prevent adequate contact between chlorine and pathogens
Chlorine reacts with organic and inorganic nitrogenous compounds, iron, manganese, and hydrogen sulfide.
Dissolved organic compounds exert a chlorine demand
Knowing the concentrations of interfering substances is important in determining chlorine dose

10. Chlorine inactivation of microorganisms results from:
Altered permeability of the outer cellular membrane, resulting in leakage of critical cell components
Interference with cell-associated membrane functions (e.g., phosphorylation of high-energy compounds
Impairment of enzyme and protein function as a result of irreversible binding of the sulfhydryl groups
Nucleic acid denaturation

11. Chloramines
Chloramines are produced by combining chlorine and ammonia
NH HOCl NH2 + H2O monochloramine
NH2Cl + HOCl NH2Cl2 + H2O dichloramine
NH2Cl2 + HOCl NCl3 + H2O trichloramine
breakpoint reaction
Used mainly as secondary disinfectants, e.g., following ozone treatment, when a residual in the distribution system is needed

12. Chlorine Dioxide: ClO2
Extreme soluble in water
Does not form trihalomethanes
Must be generated on-site:
2NaClO2 + Cl ClO NaCl

13. Ozone: O3
Very strong oxidant (very low C▪t values) but has no residual disinfection power
Generated by passing high voltage through the air between two electrodes
More expensive than chlorination but does not produce trihalomethanes which are suspected carcinogens
Widely used in Europe, limited use in U.S.

14. Ox idant Advantages Disadvantages Chlorine Chloramines
Strong oxidant
Persistant residual
Chlorinated by -
products
Taste and odor problems
pH influences effectiveness
Chloramines
No trihalomethane
formation
Weak oxidant
Some organic halide
Taste, odor, and growth problems
Chlorine dioxide
Relatively persistant
residual
No trihalomethane prod.
No pH effect
Total organic halide formation
ClO3 and ClO2 by products On
site generation required
Hydrocarbon odors possib le
Ozone
No trihalomethane or
organic halide formed
No taste or odor prob.
Little pH effects
Coagulant aid
Some by products
biodegradable
Short half
life
Energy intensive
Some by products biodegradable
Complex
generation
Corrosive

15. UV Disinfection
Optimum ultraviolet light wavelength range for germicidal effect: 250 nm nm
Low pressure mercury lamps emit nm
Damages microbial/viral DNA and viral RNA by causing dimerization, blocking nucleic acid replication
Does not produce toxic by-products
Higher costs than chemical disinfection, no residual disinfection

16. Repair of UV Damage in Bacteria
Photoreactivation -enzymatic repair (dimers are split) occurs under visible light ( nm)
Dark repair - excision of dimers