Turbidity reduction

  • The device reduces the turbidity of untreated water to a level tolerated by local end users.
  • The effectiveness of reducing turbidity is a trade off with the life of the filter element and frequency of maintenance.

Key indicators

  • >0.1 nephelometric turbidity units (NTU) may reduce effectiveness of disinfectants.
  • Threshold acceptance of effluent turbidity value: ≤5 NTU.
  • Most desirable effluent turbidity value: <1 NTU.
  • Turbidity is apparent to the average human eye at ~5 NTU.
  • Variable source waters affect turbidity levels:
    • Clear mountain stream (~1 NTU).
    • Large river in dry season (~10 NTU).
    • Relatively clear appearing lake (<25 NTU).
    • Large river during wet season with runoff sediments (10s to 100s NTUs).
    • Completely opaque sample (2,000 NTU).

Notes and exceptions

  • Appearance is a primary water quality indicator for users.
  • Consumers generally accept water with turbidity less than 5 NTUs, though tolerance can vary significantly by population.
  • Median turbidity >0.1 NTU can reduce the effectiveness of disinfection.
  • Most people can detect colors above 15 true color units (TCU), so levels of color <15 TCU are generally acceptable.
  • Surface waters may be highly turbid (over 30 NTU) and require pre-filtration or settling before treatment.
  • Turbid water sources deemed unfit for drinking by individuals in Busia, Kenya, ranged from 10.5 to 87.0 NTU for 10 open water sources (CDC report to IPA, 2006).
  • A study of turbid water sources from over 25 countries deemed that 91.7% of turbid waters (10–100 NTU) demonstrated a higher oxidant demand and hence the Center for Disease Control (CDC) recommended a double dose (3.75 mg/L) of liquid hypochlorite solution for disinfection treatment (Lantagne, 2008).

Supporting evidence

  • World Health Organization (WHO). Guidelines for drinking-water quality: Incorporating 1st and 2nd addenda, Vol.1, Recommendations, 3rd ed. Geneva: WHO; 2008. Available at: http://www.who.int/water_sanitation_health/dwq/fulltext.pdf.
  • Lantagne D. Sodium hypochlorite dosage for household and emergency water treatment. Journal of American Water Works Association. 2008;100(8).
  • Davis J, Lambert R. Engineering in Emergencies: A Practical Guide for Relief Workers. Warwickshire, UK: RedR/ITDG Publishing; 2002. 2nd Ed.
  • Laurent P. Household Drinking Water Systems and their Impact on People with Weakened Immunity. MSF-Holland Public Health Department; 2005.
  • The Sphere Project. The Sphere Project: Humanitarian Charter and Minimum Standards in Disaster Relief. Oxford, UK: Oxfam Publishing; 2004. Available at: http://www.sphereproject.org/content/view/27/84/lang,english/.
  • Indian Standard for Drinking Water per Bureau of Indian Standards (BIS) (IS 10500:1991).
  • Full list of supporting evidence and additional materials.