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Is it OK to drink cloudy tap water?

There’s something offputting about tap water that’s cloudy or milky – it doesn’t invite you to drink it. But is cloudy water actually bad for you?

Researchers at Drexel University in the US found a link between cloudy tap water and outbreaks of gastrointestinal illness, even when the cloudiness was within the limits allowed by some cities. The link, however, is not straightforward.

The technical term for the level of cloudiness of drinking water is “turbidity”. It is a measure of the amount of light that is bounced off material in a sample of water. The instrument used to measure turbidity is a nephelometer. It measure turbidity in nephelometric turbidity units (NTU). The World Health Organisation says that the turbidity of drinking water should not exceed five NTU and should ideally be below one NTU.

Turbidity standards of 5, 50 and 500 NTU. US Geological Survey/Wikimedia Commons

The numerical value of the turbidity in a crystal-clear water sample is typically less than one NTU. But even at higher turbidity levels, we may not be able to notice cloudiness.

The turbidity of purified water at a water treatment works is typically kept below one. Often, the cloudy, raw (unprocessed) water that is purified in a water treatment works contains micro-organisms that cause stomach upsets. The process of water purification makes water look crystal clear and gives some degree of assurance to the engineers that most micro-organisms have been removed.

But crystal-clear water does not always mean bug-free water – it’s not a perfect proxy indicator. The cloudiness is not caused by the bugs but by many other constituents, such as silt, clay and organic matter, which may or may not be carrying bugs.

A water-treatment plant. Dmitri Ma/Shutterstock

Finding a link

The researchers at Drexel University analysed existing studies from North America and Europe that investigated the link between drinking-water turbidity and acute gastrointestinal illness, caused by bugs such as norovirus, Giardia and Cryptosporidium.

Of the 14 studies included in the review, ten found an association between water turbidity (as measured at the water treatment plant) and the incidence of acute gastrointestinal illness.

The remaining studies show discrepancies, suggesting that the usefulness of turbidity as a proxy may depend on other things. For example, in the raw water, before purification, high turbidity means high levels of micro-organisms, but there may be no correlation between the levels of turbidity and micro-organisms in the purified water that is supplied to our homes. Also, other factors, such as seasonal changes in water bodies, may influence the levels of turbidity and micro-organisms in water.

Overall, the authors of the study concluded that cloudiness alone can’t be used as an indicator for predicting endemic gastrointestinal illnesses.

It makes sense for the water utilities and process engineers to treat water cloudiness as an indirect indicator of potential bug presence in the water at the water treatment works because, in the raw water, both micro-organisms and fine particles are present and assumed to be represented together by the numerical value of the turbidity.

Picking up cloudiness along the way

Water from the tap comes after travelling through kilometres of water supply pipe networks – and it might pick up the cloudiness on the way through broken pipes with or without picking up bugs. There are also local sources of harmless cloudiness in the tap water, for example, scaling in domestic water pipes.

If the tap water looks cloudy, it may still be free from bugs responsible for gastrointestinal illnesses. Unfortunately, without carrying out laboratory testing for tap water samples in your home, it is impossible to confirm the presence of bugs in the water, irrespective of the level of cloudiness.

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