UV Disinfection Physical process (inducing photobiochemical changes within microorganisms) Two conditions must be met: ◦ Radiation of sufficent energy to alter chemical bonds must be available ◦ Such radiation must be absorbed by the target molecule or microorganism E ʎ = (h*Cv*AN)/ ʎ E ʎ = Radiant energy associated with given wl(kcal/einstein) h = Planck’s constant, 1.583 * 10 -37 kcal.s Cv = Speed of electromagnetic radiation in a vacuum, 3 * 10 17 nm/s ʎ = Wl of electromagnetic radiation, nm AN = Avogadro’s number, 6.023*10 23 photons/einstein
E ʎ = 112.8 kcal/einstein ʎ = 253.7 nm BondBond Dissociation Energy (kcal/mole) O – H110 – 111 C – H96 – 99 N – H93 C – N69 – 75 C – C83 – 85
Physical process (inducing photobiochemical changes within microorganisms) Two conditions must be met: ◦ Radiation of sufficent energy to alter chemical bonds must be available ◦ Such radiation must be absorbed by the target molecule or microorganism DNA and RNA are the two most common forms of nucleid acid, that consists of single or double stranded polymers comprising building blocks called nucleotides. Purines: Adenine, Guanine Pyrimidines: Thymine, Cytosine (DNA), Uracil, Cytosine (RNA) Strong absorbers of UV light.
Source of UV Radiation Low-Pressure Low-intensity UV Lamps : Produce essentially monochromatic UV light at 253.7 nm. UV light is produced by mercury at low vapor pressure. Low-Pressure High-Intensity UV Lamps : Mercury-indium amalgam is used. Allows 2-4 times greater UV-C output. 25% greater lamp life. Medium-Pressure High-Intensity UV Lamps : Mercury vapor emission is carried out at higher lamp pressures and temperatures. Produce polychromatic UV light.
UV Reactor Configuration Open – Channel Disinfection Systems Lamp placement can be ◦ Horizontal ◦ Vertical Closed – Channel Disinfection Systems ◦ In most design configurations, the direction of flow is perpendicular to the placement of the lamps
UV Intensity and UV Dose UV Intensity is a measure of radiative power per unit of exposed area. The total UV intensity at a point in space is the sum of the intensity of UV light from all directions. UV Dose is the integral of UV intensity during exposure period.
UV Dose D = I x t D = UV dose, mWs / cm 2 I = UV intensity, mW / cm 2 t = Exposure time, s
UV Disinfection Kinetics The measured concentration of microorganisms before and after exposure provides the response, or log reduction of microorganisms from exposure to UV light. Log Reduction = log (N 0 / N), where, ◦ N 0 is the concentration of infectious microorganisms before exposure to UV light ◦ N is the concentration of infectious microorganisms after exposure to UV light. UV dose – response relationships can be expressed as either the proportion of microorganisms inactivated (log reduction) or the proportion of microorganisms remaining (log survival) as a function of UV dose.
Factors Affecting UV Disinfection Flow Rate UVT Suspended Solids Water Quality ◦ Iron ◦ Hardness D = I x t
UVT UVT is the percantage of light passing through material over a specified distance.
Suspended Solids Shadowing Microbes within particles Potential for microbes to pass through system without seeing UV light
Water Quality Iron and Hardness Deposition of minerals on the sleeve
Water Quality Iron is a strong absorber of UV light
Advantages of UV Disinfection Effective disinfectant More effective than chlorine in inactivating most viruses, spores, cysts No chemical addition required No formation of disinfection byproducts Water retains its natural flavour and smell Microorganism inactivation achieved within seconds Max operational safety Minimal operating costs