• ceramic filters fall in between sand and synthetic membrane in terms of pore size and physical exclusion

ceramic filteration

  • simple operation
  • built-in storage
  • no electricity or consumables
  • locally produceable

shapes

  • ceramic pot filters
  • candle filters

types of ceramic filters

  • clay
    • shouldn’t have sand or too much organic material in it
  • burn-out material
    • sawdust, ground rice husks
    • proportion and size
    • affects filter flow rate and efficacy
  • additives
    • silver
    • iron oxides
  • mixed, pressed, dried and fired
    • 700ºC - 980ºC
    • around 6-20 hours firing, 12-24 hours cooling

quality control

  • visual inspection
  • auditory testing: ‘ping test’
  • pressure testing
  • flow rate testing: 1.5 - 3 l/hr
  • microbiological testing
  • failure rate: 10% - 20%
    • glog: already fired clay ground up used in production of new clay filters

silver

  • metallic silver has anti-microbial properties
  • ceramic filters are coated with silver after firing
    • dipping in silver solution
    • brushing it onto the sides
  • amount is more critical than
    • if silver coating is on the inside or outside
  • low levels of silver leeching into water filtered
    • no health hazard and subsequently no WHO guideline for silver contamination
  • arsenic however can go into water
    • arsenic exists naturally in clay
    • and reaches safe levels
  • silver seems to have two effects
    • reduces bacteria
    • prevents bio film formation that clogs flow rates
  • two types of silver used on clay pots
    • colloidal silver (nano-silver) - 100 nm
    • ionic silver (silver-nitrate) - 5 microns
  • nano silver is known to enter bacterial cells and cause internal damage
  • locally produced nano-silver isn’t known to be as effective as commercial silver

disinfection effectiveness

  • protozoa
    • 2-5 log-reduction value (LRV)
    • mainly by size exclusion, physical filtration
  • bacteria
    • somewhat lower reduction 1-2 LRV
    • regrowth and recontamination occurs on the clean sides of the ceramics
  • viruses
    • 0-2 LRV
    • mainly electrostatic removal mechanism

electrostatic removal

  • viruses have negative surface charge
  • iron oxides have positive charge
    • laterite soil has high levels of goethite - an iron oxide with high positive charge
    • some clay filters have laterite soil added to them
  • possibility of saturation
  • in general ceramic filters have poor virus removal

examples

  • RDIC: Resource Development International Cambodia
  • IDE: International Enterprise Development
  • No subsidy
  • Direct sales, vendors, NGOs
    • sale price ~ USD8 - USD15
  • production ~2000 filters per month
  • 100s of thousands sold

advantages of ceramic filtration

  • social acceptability, high use
  • safe storage
  • one time capital cost
  • possible of local production
  • visual improvement in water quality

challenges

  • moderate effectiveness
  • low flow rate
  • supply chains for replacement
  • variable quality
  • lack of residual protection