• sedimentation may be done at the household level or community level
  • sedimentation by itself is not a complete water treatment process
    • necessary preliminary step before
    • filtration and disinfection
    • and safe storage

turbidity

  • removes suspended particles in water
    • large particle due to gravity
    • small particles do not settle
  • organic suspensions such as algae

  • inorganic suspensions such as silt and clay

  • units of turbidity
    • nephlometric turbidity units (NTU)
    • formazin turbidity units (FTU)
    • jackson turbidity units (JTU)
  • no health-based target
    • impacts on subsequent treatment of filtration and disinfection
    • < 5-10 NTU typically,
    • aesthetically obvious above 5-10 NTU
    • people habituated to turbid water will not notice a difference

settling

  • gravity settling
    • depends on size of particle
      • limited settling for larger particles
    • simplest HWTS method
    • may improve visual appearance and can remove some turbidity
    • limited pathogen removal
      • large protozoans, helminth eggs
    • opportunities for secondary contamination
      • when stored for sedimentation if container is not hygienic
  • three-pot sedimentation
    • drinking water has been stored at-least for two days
    • gives time for some pathogens as schistosomes
      • cause of schistosomiasis, dies off
  • a three pot pipeline is used: pot 1 » pot 2 » pot 3
    • pot 1: un-sedimented water from source is let to sit
    • pot 2: partially sedimented water from pot 1 is put in this
    • pot 3: water that has undergone further sedimentation is put in this
    • transfer: flexible siphon pipe may be used to transfer water to the next pot without disturbing the sediment at the bottom
    • extraction: water is allowed to further sediment in pot 3 and then used for drinking
    • addition: water from source is added to pot 3

coagulants

  • they make the particles in water stick to each better
  • they also stick to each other
  • so they make the smaller particle lump and become heavier
    • this helps smaller particles form larger globules
  • many colloids have negative charge
  • two negatively charged particles repel each and don’t glob together
    • coagulants have positive charge and neutralize the negativity of the colloidal particles
    • larger coagulant molecules help bridge two negative colloidal particles
      • polymeric bridging
  • also remove dissolved compounds and improve color of water

plant based coagulants

  • moringa olefera (drumstick tree)
    • africa/asia
    • dry the seeds, grind them, put them in water
    • 200 mG/L
    • anti bacterial also
  • prickly pear cactus (opuntia ficus indica)
    • latina america
  • nirmali seeds (strychnos potatorum)
    • india
    • not a primary coagulant, coagulant aid
  • plant based material contains a chemical that is often a soluble protein
    • that has a positive charge
    • it can be poly-cationic
  • interacts with particles that have a negative charge
    • makes them stickier and settle down having formed globs
    • more effective at higher turbidity

drawbacks of plant based coagulants

  • organic carbon dissolved in water
  • can foster bacterial regrowth
  • eventually cause taste and odor problems

chemical coagulants

  • used more widely than plant based
  • common of these are metal salts using Al or Fe
  • aluminum sulphate (alum) is usually available locally
    • used to stop bleeding shaving cuts
  • alum:
    • has aluminum, sulfate and a number of water molecules
    • creates aluminum ions with three positive charges
    • very able to interact with those negatively charged particles
    • helps them precipitate
  • alum potash:
    • using potassium as well as aluminum, usually locally available
  • ferric salts:
    • less widely available
    • work well over a broader pH range that alum

plant based vs chemical

  • flocs produced by chemicals are larger
  • sedimentation is faster using chemical coagulants
  • so more sludge is created using chemical coagulants
  • HWTS product called PuR produced by P&G
  • this coagulant is added to water and is stirred
  • then there is a period of settling
  • this is filtered through some cloth
  • clean drinking water is available in 30 mins

factors affecting coagulation

  • temperature
  • pH and alkalinity
    • coagulation consumer alkalinity
    • may need to add alkalinity
    • lime (Ca(OH)_2) or sodium carbonate (Na_2CO_3)
  • mixing
    • rapid mixing necessary as soon as added
    • to ensure even distribution
  • coagulant dose
    • optimal dose - too much is less effective

residual alum in drinking water

  • there are some concerns about alum residues in drinking water causing alzhemier’s disease

  • WHO has not set a health-based guideline for drinking water
    • drinking water is a small portion of total intake
    • considering food is a larger factor of aluminum intake
  • aesthetic recommendations
    • 0.1 mg/L for large treatment plants
    • 0.2 mg/L for small ones
    • dose optimization, filtration