Turbidity is the amount of cloudiness in the water.
This can vary from a river full of mud and silt where it would be impossible to see through the water (high turbidity), to a spring water which appears to be completely clear (low turbidity). Turbidity can be caused by : • silt, sand and mud ; • bacteria and other germs ; • chemical precipitates.
It is very important to measure the turbidity of domestic water supplies, as these supplies often undergo some type of water treatment which can be affected by turbidity. For example, during the rainy season when mud and silt are washed into rivers and streams, high turbidity can quickly block filters and stop them from working effectively. High turbidity will also fill tanks and pipes with mud and silt, and can damage valves and taps. Where chlorination of water is practised, even quite low turbidity will prevent the chlorine killing the germs in the water efficiently.
Some treatment systems, such as sedimentors, coagulators and gravel prefilters are designed to remove turbidity. It is important for operators of both large and small treatment systems to know how well these systems are working. Measuring the turbidity of the water before and after each part of the system can tell the operator where maintenance or cleaning is needed.
For drinking water supplies, the following guidelines should be taken into consideration :
• Drinking water should have a turbidity of 5 NTU or less. Turbidity of more than 5 NTU would be noticed by users and may cause rejection of the supply.
• Where water is chlorinated, turbidity should be less than 5 NTU and preferably less than 1 NTU for chlorination to be effective.
The units of turbidity from a calibrated nephelometer are called Nephelometric Turbidity Units (NTU).