• heat kills
    • enzymes denature and become non functional
    • affects protein unfolding and change shape
    • cell pressure increases and damages it

classes of heat disinfection

  • boiling
  • pasteurization

heat disinfection

  • pressure affects boiling point
    • higher pressure, higher the boiling temperature
    • example: autoclave steam sterilization mechanism
      • 121ºC at 1 bar for 15 minutes kills pathogens
    • lower pressure lower boiling point

thermal death kinetics

  • heat dose = heat * time
    • temperature of exposure
    • time of exposure
  • for a given temperature a fixed amount of time is required to kill pathogens

  • higher the temperature, shorter the time needed for disinfection

  • first-order kinetics assumed
    • exponential decay
    • D = time required at 121ºC for 1 Log-Reduction-Value (LRV)

canning

  • the food canning industry is concerned with thermal death kinetics a lot
  • as food is pasteurized before being canned
  • canned food can be stored for years before being consumed
    • same with bottled drinking water
  • 12 D reduction is the standard for Clostridium botulinum spore disinfection
  • 7 LRV is considered complete disinfection for practical purposes

boiling

  • oldest method of HWTS
  • 600 million people report usual practice
  • effective against all classes of pathogen
  • not affected by turbidity
  • prone to recontamination as it is usually transferred to another container for storage
  • causes flat taste as it removes all dissolved gases in it
  • if hard water is boiled, calcium and magnesium can precipitate out forming small white particles which can cause aesthetic problems

fuel and time

  • biomass fuel:
    • wood and crop residues being used a fuel
    • has substantial negative impacts
  • soot from biomass fuel burnt indoors can lead to lower respiratory infections
    • major cause of child mortality

  • exacerbates deforestation, released CO2 into the atmosphere

  • costly in terms of time and fuel that is needed
    • 0.5% - 1.5% of monthly income
    • time:
      • collecting fuel if not bought
      • extra treatment before boiling water
      • 2% - 5% monthly income

pasteurization

  • not necessary to reach 100ºC

  • protein damage

  • egg can be cooked at 72ºC
    • damages egg proteins enough to make the egg whites and yolk to become opaque
  • milk pasteurization
    • 72ºC for 15 seconds
    • 63ºC for 30 minutes
    • both 7 LRV of common pathogens

  • heat-up and cool-down time quicker than boiling

  • there is difficulty in knowing when the effective treatment has been reached
    • 70ºC for pasteurization
  • relatively less experience with using pasteurization for treating water at household level

disinfection effectiveness

protozoa

  • 6-9 LRV
  • crypto cysts inactivated after 1 min at 72ºC

bacteria

  • 6-9 LRV
  • B. anthrax spores can survive boiling for several minutes

viruses

  • 6-9 LRV
  • easily killed even at 60ºC

variation with altitude

  • boiling point variation in altitude
    • mean sea level: 100ºC
    • aspen, colorado: 91.3ºC
    • mt. fuji: 87.3ºC
    • mt. everest: 72ºC
    • dead sea: 106ºC
  • pathogen removal is effective whenever water boils, regardless of the temperature
  • there is a lot less oxygen with lesser atmospheric pressure

time of boiling exposure

  • leading causes of diarrheal diseases are all vulnerable to boiling
  • pasteurization alone should be enough to protect drinking water from these pathogens
  • pathogens die during heating up and boiling off as well
  • if the water boils at all, common pathogens are deactivated
  • WHO guidelines recommends full rolling boil

consideration for heat treatment

advantages

  • highly effective
  • not affected by turbidity
  • simple operation, no special equipment needed
  • widelty accepted, understood, promoted

challenges

  • takes long time including cooling
  • changes taste of water
  • possibility of recontamination
  • high energy and time costs