N3-UV Disinfection

I get many questions asking me how well UV (Ultra Violet) light disinfection works. My answer is that it has been successfully applied to numerous water and wastewater treatment applications. The following will outline the theory, advantages, and disdvantages of UV light disinfection.

Theory

  • UV light is a non-chemical disinfection alternative to more traditional chemical disinfection techniques.
  • UV light disinfection uses electromagnetic radiation to distort DNA and prevent DNA replication.
  • The germicidal wavelength is in the range of 250 nm to 270 nm. This corresponds to the maximum absorption wavelength for nucleic acids.
  • When the DNAin the cell nuclei absorb UV light they form pyrimidine dimers that distort the DNA and prevents DNA replication. This can eventually kill the pathogen.
  • UV disinfection can achieve close to 100% deactivation of many pathogens.
  • The key parameter of disinfection efficacy is the product of the light intensity, I, and the contact time, t, where the UV dose required is D=I.t.
  • The contact time, t, required to deactivate a microorganism is a function of :
    • dose energy per square meter,
    • size of microorganism, and
    • resistance of cell wall to UV light penetration.
  • The general rate equation is:
    • dN/dt = k.N2.I  where
      • N = #MPN/100 mL at time t (where t = exposure or contact time)
      • k = rate constant
      • I = average UV intensity in chamber
    • Therefore,  1/N – 1/N0 = k.I.t
  • Turbidity and total suspended solids (TSS) can affect UV disinfection efficiency. As turbidity and TSS concentrations increase, UV disinfection efficiency decreases. For UV applications the TSS concentration must not exceed 20 mg/L. In order to achieve this, many sites must reduce the upstream TSS concentration (and turbidity) by such means as settling, flotation, and/or filtration stages.

Advantages:

  • From an environmental standpoint, UV light does not produce trihalomethanes, as well as, other harmful by products that usually result from chemical disinfection techniques (such as chlorination).
  • UV light has been found to be capable of destroying many taste and odour causing substances.
  • UV disinfection can achieve close to 100% deactivation of many pathogens.
  • Ideal disinfection technique for point source applications, such as:
    • final disinfection treatment at waste water treatment plants that have primary, secondary, and tertiary treatment stages.
    • direct to customer drinking water supply plant that consists of sedimentation and filtration stages upstream of the UV contact chamber. This may be applicable to cotttage environments and to those areas of the world where infrastructure and environmental concerns prevent consistant chemical supplies for disinfection.

Disadvantages:

  • Visible light wavelengths in the range of 310 nm to 500 nm can photoreactivate dimerized DNA by reversing the dimerization reaction. Therefore, care must be taken to ensure adequate contact time, energy, and germicidal wavelength is supplied to water requiring disinfection. In addition, any visible light should be prevented from entering the UV light lamp/contact chamber.
  • Limited to no effectiveness against viruses, protozoa, and larger microorganisms (helminths).
  • Disinfection efficiency decreases as turbidity and total suspended solids increase.
  • May cause mutations in some surviving viruses (indicated by aberations on the chromosomes).
  • Limited to point source disinfection since UV light does not produce any lasting downstream disinfectant residual for pipe distribution systems (which many chemical disinfectants can do).
  • More costly than many of the chemical disinfection options due primarily to energy and maintenance costs.