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.
>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).