What is the difference between colour and turbidity in water?

The colour of water (usually measured in Hazen units can be as a result of both dissolved organic matter and suspended solids. Therefore to obtain a true reading of the colour of water a sample should first be filtered, normally through a 0.45µm filter. This will remove the visible suspended solids and leave any dissolved matter thus giving a true reading of the colour of the water.

The colour in untreated water is as a result of precipitation (rain, snow etc.) infiltrating soil as it falls on to the land surface. As it infiltrates the soil layers it absorbs organic matter from the peat and soil humus (in the form of humic and fulvic acids) and it is this that gives the raw water its distinctive yellow/brown colour.

This can be particularly noticeable during heavy rainfall events although it should be noted that there will be a need to differentiate between what is colour in the water and what is suspended solids giving it turbidity.

Turbidity is a measure of the overall quality of water. Turbidity is caused by materials in suspension although it is difficult to be accurate on a consistent measurement of this as particle size and shape will affect any reading. Fewer larger suspended particles can give a similar reading to a higher quantity of smaller suspended particles

The unit of measure is generally the Nephelometric Turbidity Unit (NTU) although an equivalent measure (the Internationally recognised standard is the Formazin Nephelometric Unit (FNU). Worldwide the most generally used unit is the Formazin Turbidity Unit. 


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