Why is my water coloured?

Cathal Walsh
Content Creator
Monday, 24th June 2019
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Why is my water coloured?

Table of Contents


Maximum amount allowed in water:

Whatever is acceptable to consumers without any abnormal change.

What is colour in water?

Colour in water is caused by the interaction of light with suspended particles within the water. As these particles are likely to have a colour of their own, the more of them in the water the greater the colour the water will appear to have.  There are two terms used to describe the colour of water, these are apparent colour and true colour.

Apparent colour is what we actually see and is the light reflected from both the suspended particles and the dissolved particles in water.

True colour is the light reflected from the dissolved particles only. Laboratories prefer using true colour when analysing water.

As a guide for true colour, tea has a true colour of about 2,500 HU (Hazen units). A true colour of 15 HU could be detected in a glass of water and a true colour of 5 HU could be detected in a white bathtub filled with water.

Few people could even detect anything at 3 HU, and a true colour of 25 HU would be accepted for drinking by most people.

What causes colour disturbances in water?

Colour disturbances in water can be caused in a variety ways. It can come in the source from organic matter and decaying vegetable matter. Red water in bore wells can be caused from iron.  Black discolouration can be caused by manganese bacteria. Some of these compounds form fine suspensions,

or are only partially dissolved and so contribute to apparent rather than true colour.

The dissolution of metal in pipes and fittings can also discolouration in water.


What does its presence tell us?

The colour of water acts as a useful warning for any underlying problem in the water.  It can reflect degradation in the source, corrosion in the distribution system or changes in the treatment processes.


What risk does it pose?

Colour itself does not pose a risk to health. It does however show that some micro-organisms have gotten through the treatment process.  The effectiveness of UV water treatment may also be diminished with a high apparent colour in water.

What to check for

Raw water sources should be checked at regular intervals to see what the normal colour levels are. Then it should be checked how events like heavy rainfall change these levels.  If the colour level is high at the time of disinfection, then water should be checked for disinfection by products.


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