slow sand filtration

  • been in use for centuries to purify water
  • has been adapted for household use
    • commonly known as bio-sand filtration
  • though superficially similar to rapid sand filtration
    • mechanism is quite different

process in biological filtration

  • size exclusion: large particles get trapped in sand
  • electrostatic effects: adsorption
  • biological activity: predation, pathogen die-off
    • bio-film develops on the surface of the filtration sand
    • mostly invisible to the naked eye, thin-biological layer
  • much slower than rapid sand filtration
  • bio-sand filters have a long pathogen residence time
    • pathogens die off due to lack of food or oxygen
    • or bad temperature match

biological filtration design

  • sand bed ~100 cm

  • i.e. fairly uniform sand grains needed
    • uniformity coefficient: 2 - 3
    • effective size D_10: 0.15 - 0.35 mm
  • water column above it ~100 cm
    • continuous flow

  • filtration rate: 10-30 cm/h as high as 50

  • biological active layers should not dry out
    • it would die
    • a hydraulic control system for this exists
    • water outlet is higher than the level of the sand bed
  • when a new filter is out online, it takes some time for the bio film to develop
    • the ‘ripening’ period - about a month
    • the layer that forms is called the schmutzdecke i.e. the dirty layer
    • a lot of the filtration takes place in this layer

  • biosand filtration is slow sand filtration scaled down to the household level

  • biosand filtration:
    • water layer: 5 cm
    • sand bed: 55 cm
      • grain size: < 0.7 mm
    • intermittent flow
    • designed for 40 cm/hr
    • 600 cm^2 area
    • 400 mL/min, 24 L/hr
    • concrete housing
    • plastic housing
  • slow sand filtration:
    • water layer: ~100 cm
    • sand bed: 55 cm
      • grain size: D_10 (0.15 - 0.35 mm)
    • continuous flow
    • filtration: 10 - 30 cm/hr (as high as 50)
    • flow control
    • initial ‘ripening’ period
      • schmutzdecke

filter cleaning and ripening

  • new filter biological layer takes time to develop
  • new filter has a high flux rate
    • slowly declines over time
  • backwashing does not work for bio-sand filtration
    • top layer is scraped off and stored for later use
  • clean filter when the flow is low
    • every 20-60 days
  • swirl and dump method is used
    • swirling water removes the top layer of clogging
    • and the dirty water is dumped
    • removes only a part of biofilm layer, much of the dirty layer stays back in the sand
    • takes a couple of days to regrow fully

operation of bio-sand filters

  • consistent water source
    • rain water vs. surface water vs. rain water
    • the bio-film microbes get habituated to a certain kind of water source
  • also good if the water turbidity is limited to 50 NTU
    • low turbidity water preferred
  • use filter everyday
    • rest period of 1 hour
    • 3-4 times
    • dissolved oxygen in the water can be consumed, hurts the microbes in the film
  • make sure the biologic layer is always wet
    • the schmutzdecke will die if it dries
    • many setups so have a hydraulic control to keep the outlet above the sand layer
    • check for leaks
  • check flow rate
    • too fast: filtration is not effective
    • too slow: filtration needs cleaning
  • biosand filtration doesn’t given residual disinfectant
    • so storage after removal must be safe
    • must be managed hygienically after treatment

removal efficiency

pathogens

  • helminths: > 2 LRV
  • protozoa: > 2 LRV
  • bacteria: > 1-2 LRV
  • viruses: < 1 LRV

inorganic pollutants

  • turbidity: 85% - 95% removal
  • iron: 90% - 95% removal
  • arsenic: adapted version with iron source, i.e. iron nails
    • competing ions such as phosphorous and silicon can very highly affect performance
  • nitrate, nitrate: may increase

considerations

advantages

  • simple operation
  • no change to taste of water
  • reduces turbidity
  • can be manufactured on site
  • possible long term use ~ 8 years

disadvantages

  • need for cleaning, ripening period
  • high risk of recontamination
  • little effect on chemicals, may nitrify
  • heavy
  • limited removal of pathogens