There are many possible causes, but the most usual reason is deposits present in the water mains due to cast iron water mains. This can happen when there is a change in the direction or speed of flow of the water, for example, due to a burst on a water main or the inappropriate opening or closing of valves on the main. DWQR expects Scottish Water to operate and maintain its distribution system in a way that minimises the chance of supplying discoloured water to consumers.
Occasionally, discolouration may be caused by the condition of your service pipe connecting your house to the water main or the condition of the internal plumbing within your house. If the issue is only present at taps other than the mains-fed kitchen tap, a storage tank in the house may be the cause.
In some parts of Scotland, manganese is naturally present in the supply and if it is not removed by the treatment process it can cause black or dark brown discolouration to supplies. DWQR expects Scottish Water to work to ensure all supplies at risk from manganese have adequate treatment to remove it.
The usual cause of discoloured water supplies is iron or manganese, and both of these substances are more of an aesthetic concern than a health one. If you are persistently receiving discoloured water at a mains-fed tap in your property you should contact Scottish Water to give them the opportunity to investigate and resolve the problem.