• two sources of UV radiation at household level
    • natural
    • artificial

UV radiation

  • is one of the waves of the electromagnetic spectrum
    • UV wavelength: 100-400 nm
    • visible light wavelength: 400-700 nm

UV-bandwidth

fig: UV Bandwidth

efficacy against pathogens

  • dose = intensity * time
    • 1 mJ/cm^2 = 10 J/m^2
  • DNA absorbs in 200-300 nm range
    • peak at 260 nm
    • most commercial UV treatment solutions target this wavelength
  • effective against (in order of most effectiveness)
    • bacteria, then
    • protozoan cysts, then
    • viruses, bacterial spores
  • typical dose 400 J/m^2 (40 mJ/cm^2)
    • for good removal

specific pathogens

  • to achieve ~3 LRV
    • very effective against e.coli
      • ~ 7 mJ/cm^2 UV dose
    • rotavirus takes a bit longer
      • ~ 25 mJ/cm^2 UV dose
    • B. subtilis spores take the longest time
      • ~ 60 mJ/cm^2

artificial UV-C generation

  • generated by lamps
  • low pressure mercury lamps: 254 nm
  • medium pressure lamps: 200-440 nm
    • broader spectrum
  • LED lamps developing rapidly
    • focussed at a particular wavelength

effective in water

  • no impact of pH
  • turbidity can shield pathogens from radiation
  • modest temperature effect, more dose needed at lower temperatures

mesita azul

  • UV-C water disinfection
  • 1200 J/m^2

  • flow rate: 5 liters per minute, storage in jugs

  • evaluation:
    • water quality better than pre-intervention,
    • than control group households
    • than alternative disinfection methods like commercial filters
    • exclusive use is low: 40% households

solar disinfection (SODIS)

method

  • fill clean PET bottle with water
  • lay it on a flat surface, maybe on a roof top
  • to expose it to natural sunlight for at least 6 hours
  • if cloudy for two days
  • after the specified amount of solar exposure, the water is safe to drink

SODIS

  • used in more than 20 countries
  • by 5 millions people
  • doesn’t make use of UV-C
    • UV-C is absorbed by ozone and water vapor in the atmosphere
  • the UV radiation that reaches the earth is mostly UV-A
    • small component of UV-B
SODIS mechanisms
  • since a different portion of the UV wavelength reaches the earth’s surface
    • the disinfection mechanism is different
  • direct inactivation
    • by UV-B component
    • a lot of this removed by PET
  • indirect inactivation
    • by UV-A component
    • generation of reactive oxygen species (ROS)
    • internal: chromophores in cell
    • external: organic matter, iron
  • heat

factors affecting SODIS

UV radiation
  • sun height
    • summer vs winter
  • latitude
    • northern vs southern hemisphere
  • cloud cover
  • altitude
    • UV increases 10% every 11km
  • ozone
    • ozone absorbs UV-C and some UV-B
  • ground reflection
  • water column
    • a longer column attenuates more UV
water temp
  • for water at 50ºC, one hour exposure is sufficient
  • due to synergy between heat and UV, accelerating the disinfection process
turbidity
  • must be less that 30 NTU
  • higher turbidity negatively impacts UV disinfection

disinfection effectiveness

protozoa

  • 2-4 LRV
  • including Giardia cysts, Cryptosporidium spores

bacteria

  • 3-5 LRV
  • Salmonella > E.Coli > Shigella > V.Cholera

viruses

  • 1-5 LRV
  • Adenovirus, phiX174 > echovirus, MS2
  • testing methods are difficult

SODIS and plastic toxicity

  • PET = polyethylene terephthalate
    • antimony is catalyst in production of PET plastic
  • concerns exist for antimony leeching into water

antimony

  • antimony leeches into water when in contact for weeks of longer

  • or the temperature goes beyond 70ºC

  • SODIS is treating water in PET bottle
    • only for a few days
    • only upto 50ºC temperature, rarely goes beyond that

adipates and phthalates

  • are used as softeners in production of certain types of plastic
    • PVC: used in plastic pipes
  • they are not particularly toxic
    • but are toxic with prolonged exposure to large quantities
  • not needed in PET production

aldehydes

  • formed when plastic is heated in the manufacturing process of PET

  • effect of exposure to sun with water in PET bottles:
    • concentration of acetaldehydes does not increase
    • concentration of formaldehyde increases with increase in exposure time
  • however, it usually stays within the state regulated limit for drinking water
  • this is actually more of a risk factor for carbonated water
    • low pH
    • high pressure

bisphenol A

  • expected to cause breast cancer
  • present in some plastic bottle
    • not in PET
    • but in polycarbonate

genotoxicity

  • assess chemical safety of water by checking if exposure to water results in mutations in certain plants or protozoa, or bacterial cells
    • instead of checking for certain chemicals
  • water stored in plastic bottles for long periods of time, in the order of several months, show cell mutations in the organisms in the water
    • this happens with or without UV exposure
    • similar to studies of leeching of antimony
  • SODIS does not involve long periods or high temperatures
  • so SODIS is said to cause no chemical contamination in water during treatment

considerations for UV treatment

advantages

  • highly effective (UV-C > SODIS)
  • simple (SODIS)
  • inexpensive (SODIS)
  • low risk of recontamination (SODIS)

challenges

  • limit to low turbidity waters (UV-C and SODIS)
    • UV-C requires electricity (UV-C generation)
  • depends on climate (SODIS)
  • takes significant time (UV-C and SODIS)
  • small volumes (UV-C and SODIS)

references