• there is always some risk of health being harmed in any of the myriad ways water is used
  • the question is determining what level of risk is acceptable

  • how can problems be identified and managed before the risks go beyond acceptable levels

  • example: flying in a plane
    • pilots train a long time before they planes
    • routine and rigorous maintenance checks on the aircraft itself
    • periodic audits and inspections on application of safety measures
      • to keep risks within acceptable levels
    • the fate of being involved in an accident is less than 1 in a million

safe drinking water

  • is that which does not represent any significant risk to health over a lifetime of consumption
  • including different sensitivities that may occur between life stages

WHO guidelines

  • acceptable limits set for 90 different chemicals by WHO
    • for tolerable amounts of risk
    • 1 DALY/million person years:
      • if 1 million people drink water with chemical
      • at the prescribed limit for a year,
      • then it costs 1 DALY of disease burden
  • not zero risk, but tolerable levels
  • not national standards, each state implements its own model of risk

health bases targets

  • described in WHO guidelines
    • health based targets
    • water service provider’s safety plan
      • system assessment
      • operational monitoring
      • management and communication
    • verification
  • health outcomes in terms of DALYs/year

  • cancer cases per lifetime exposure

  • water quality guidelines (chemicals)
  • performance: specified removal of hazards
  • specified technologies: requiring that surface water be filtered

performance measures

  • measure of contaminant reduction through treatment
  • log reduction values: log_10(pre-treatment/post-treatment)
    • 3 log removal is 10*3 == 1000 times contaminant removal

multi-state treatment

  • calculate LRV for each stage
  • add these LRVs to obtain total reduction of all stages

water safety plan

  • water quality measurement ≠ water safety
step 1 - system assessment
  • comprehensive understanding
  • identify all hazards catchment to consumer
  • put in place control measures
step 2 - operational monitoring
  • check that control measures are implemented
  • the metric shouldn’t be the plane not crashing
    • it should be a conscious check of all the metrics in place
    • to keep risk within the tolerable range
step 3 - management and communication
  • what to do when things go wrong
  • good record-keeping
  • two-way communication with consumers

anatomy of a water safety framework

  • step 1: health-based targets
    • acceptable level of risk
  • step 2: water safety plan
    • risk identification and management
    • most of the work is in this step for
      • system assessment
        • contamination is variable in time and space
        • e.coli is not a perfect indicator
      • operational monitoring
  • step 3: verification
    • water quality measurement
  • so to call water safe, the contamination indicator should return false AND a monitoring system for water safety should be in place
  • main source of contaminant of water is fecal matter
    • put barriers as needed to stop entry of fecal matter into water supply
    • remove pathogens that do make it into water supply
      • different pathogens have different resistance to water treatment
      • so use multiple treatments as a single measure can miss the target pathogens

conventional treatment process

  • resource protection
    • zoning laws
      • restricted use of chemical in the catchment area of water sources
    • wellhead protection areas
  • treatment plant process
    • pretreatment
      • reservoirs
      • roughing filters
      • bank infiltration
      • reduce large sized and bulk suspended solids
      • improve by some degree the microbiological quality of water
      • prepare water for subsequent treatment steps
    • coagulation, flocculation, sedimentation
      • conventional clarification to remove turbidity, suspended solids
      • dissolved air flotation
      • lime softening
    • filtration
      • rapid filtration
      • slow sand filtration
      • membrane filtration
      • removes larger pathogens
    • primary disinfection
      • chlorine
      • chlorine dioxide
      • ozone
      • ultraviolet
  • distribution
    • continuous pressure
    • residual disinfectant

HWTS

  • source protection
    • get water from improved sources
  • sedimentation
    • simple settling
    • coagulation
  • filtration
    • ceramic filtration
    • biosand filtration
    • membrane filtration
  • disinfection
    • chlorine
    • ultraviolet
    • heat
  • safe storage
    • appropriate container
    • hygienic location

difference between conventional and HWTS

  • the processed applied are the same
  • but the points where the processes are applied are different
  • HWTS requires households to take charge of their own safety
    • challenging because householder are not trained engineers
    • probably don’t even have the concept of germ theory
  • implementing safety plan required fundamental changes in human behavior
    • we all know how that goes,
    • especially in a mob-like herd-mentality environment where water problems exist in the first place