Transcript Drinking Water Treatment and Disinfection

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
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
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
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
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
0
Shoulder
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
Ideal, first order
Rapid, initial
inac tivation
o
Log (N t/N )
Where:
N0 = number of microorganisms at time = 0
Nt = number of microorganisms at time = t
k = a decay constant (1/time)
t = time
Ta iling o ff
-x
Time
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
Common Disinfectants in Water Treatment
• Chlorine
• Chloramines
• Chlorine dioxide
• Ozone
• Ultraviolet light
Chlorine
• Most commonly used disinfectant
• In water chlorine undergoes the following reaction:
Cl2 + H2O
HOCl
HOCl + HCl
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 20-40 mg/L may be required
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
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
Chloramines
Chloramines are produced by combining chlorine and ammonia
NH3
+ 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
Chlorine Dioxide: ClO2
• Extreme soluble in water
• Does not form trihalomethanes
• Must be generated on-site:
2NaClO2 + Cl2
2ClO2 + 2NaCl
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.
Oxidant
Advantages
Disadvantages
Chlorine
Strong oxidant
Persistant residual
Chlorinated by-products
Taste and odor problems
pH influences effectiveness
Chloramines
No trihalomethane
formation
Persistant residual
Weak oxidant
Some organic halide formation
Taste, odor, and growth problems
Chlorine dioxide
Strong oxidant
Relatively persistant
residual
No trihalomethane prod.
No pH effect
Total organic halide formation
ClO3 and ClO2 by products
On-site generation required
Hydrocarbon odors possible
Ozone
Strong oxidant
No trihalomethane or
organic halide formed
No taste or odor prob.
Little pH effects
Coagulant aid
Some by products
biodegradable
Short half-life
On-site generation required
Energy intensive
Some by products biodegradable
Complex generation
Corrosive
UV Disinfection
• Optimum ultraviolet light wavelength range for germicidal
effect: 250 nm - 270 nm
• Low pressure mercury lamps emit 253.7 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
Repair of UV Damage in Bacteria
• Photoreactivation -enzymatic repair (dimers are split) occurs
under visible light (300-500nm)
• Dark repair - excision of dimers