If you are involved with drinking water quality, turbidity one of the key indicators that is measured and monitored.
What is Turbidity?
Turbidity is a measure of sample clarity. The term is used to describe the cloudy or milky appearance of a liquid.
Turbidity is due to particles of varying sizes scattering or absorbing light, giving the medium in question a cloudy appearance. A high value is caused by particles such as silt, clay and micro-organisms. It is not a direct measure of these particles but rather a measure of how these particles scatter light.
Turbidity is measured in Nephelometric Turbidity Units (NTU) with an instrument called a turbidimeter.
You will be able to see high turbidity in water as cloudy / milky, but turbidity below 20 NTU is not easily seen.
Orange River – Klippunt – Aug 2009 by Heinz Punt
Why monitor turbidity?
Measurement of this parameter is important in many manufacturing operations such as beverage production, food processing and potable water treatment plants.
For drinking water, this value may give an indication of bacteria, pathogens or particles that can shelter harmful organisms from the disinfection process.
In industrial processes, it can be part of quality control to verify the efficiency in treatment or production processes.
Turbidity as an indicator of microbial quality:
Research on the "Interplay of factors involving chlorine dose, turbidity flow capacity and pH on microbial quality of drinking water in small treatment plants"* indicates that there is a correlation between turbidity and microbial quality.
A high turbidity range may indicate a high faecal coliform count, although further analysis can be performed, in order to confirm.
|Turbidity Range||Guideline for Microbial Quality|
|0 – 1||0 – insignificant chance of infection|
|1 – 10||clinical infections unlikely in healthy adults, but may occur in some sensitive groups|
|10 – 100||clinical infection common, even with once-off consumption|
|> 100||serious health effects common in all users
possibility of infection when bathing or doing laundry
*by CL Obi1>, JO Igumbor2*, MNB Momba3 and A Samie4, available on the South African Water Research Commission (WRC) website
South African Drinking Water Quality Standards (Blue Drop Limits):
SANS 241:2015 Drinking Water Standards for Turbidity:
Operational value: ≤ 1
Aesthetic value: ≤ 5
How to select an instrument to measure turbidity:
1. White light or infra-red?
ISO 7027 requires an infra-red LED light source.
EPA regulations now recommend using infra-red light, which is more modern and accurate than white light because white light may produce colour interference (false negative readings) as well as flourescence interference (false positive readings).
This choice to use white light is sometimes based on historical records, for the purpose of comparison. In South Africa, this is often the case when regulations have been using white light measurements.
Lovibond® white light, hand-held Turbidity Meter
2. Where will you be taking the measurements?
If you will be measuring turbidity in the field, a portable instrument is best.
For a laboratory, a bench-top instrument is ideal.
A drinking water or industrial plant will find that an on-line system will allow them to monitor turbidity in real time.
Lovibond® bench-top Turbidity Meter
Different instruments that feature a varying level of technological innovation and accuracy are available.
A higher level of accuracy is often required by drinking water plants.
Some new innovations include bluetooth capability, links to apps that make calculations, a bubble-free method for calibration and low water flow rates that require less water.
Operator using a Lovibond® on-line Turbidimeter in a drinking water plant
8 Top Tips for Better Turbidity Tests:
1. Most important: DO NOT SHAKE sample vials (or calibration standards) before testing, this can cause bubbles.
2. Rather invert (turn upside down) twice before reading the results, don’t let the contents settle.
3. Samples should be measured immediately to prevent changes due to temperature or settling.
4. Clean with hot, soapy water, rinse with distilled water and cap the cells to prevent dust and contamination from entering the cells
5. The vials must be very clean. Don’t use silicone / oil, use a tissue / contact lens tissue to remove fingerprints or dust.
6. If there are any scratches or stains on the vials, have them replaced.
7. Calibrations should be preformed on a quarterly basis, sometimes more often in older or more heavily used instruments.
8. Daily verification will also help ensure accurate results.
Updated June 2019.
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