Disinfection – Chapter 26 Class Objectives

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 20-40 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 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

12. Chlorine Dioxide: ClO2 • Extreme soluble in water • Does not form trihalomethanes • Must be generated on-site: • 2NaClO2 + Cl2 2ClO2 + 2NaCl

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

15. 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

16. Repair of UV Damage in Bacteria • Photoreactivation -enzymatic repair (dimers are split) occurs under visible light (300-500nm) • Dark repair - excision of dimers