Yes, water can go off. Not in the same way that, for example, cheese goes off - there aren’t any sugars or proteins in it that can go rotten in the same way. However, water will undergo a number of chemical changes that affect the quality and the taste.

When it is exposed to the air - for example, when you leave a glass of water by your bed at night - water absorbs some CO2, a tiny proportion of which converts into carbonic acid which makes the water more acidic. This alters the taste, but your glass of water is still safe to drink the next day.

The problem could come later from the bacteria in the atmosphere. To make bottled water and tap water suitable for drinking, chemicals such as chlorine are added to keep us safe from bacteria. But the chemicals will evaporate after a day or two, which makes the water less safe.

Another contaminant you need to worry about is dust, which will settle into that glass of water and spoil the taste.

Nearby substances can affect your water

If you have an uncovered glass of water sitting next to a chemical for a while, the water will absorb elements of that chemical. So leaving a glass of water next to a cup of coffee overnight will result in the water smelling of coffee. For this reason, it is advisable to store water well away from cleaning products and other dangerous chemicals.

Is alcohol safer from bacteria than water?

We are fortunate in that we have access to the knowledge, processes and chemicals we need in order to purify our drinking water. It was a different matter entirely in the past. It is popularly believed that people used to drink beer and spirits because they were safer to drink than water. Sailors used to be given a daily tot of rum as a way of keeping them hydrated with sterile liquid - a tradition that lasted until the 1970s for British sailors!

While it is true that people used to drink a lot of beer in the same way that we now drink a lot of water, some believe they did it primarily for the calories rather than the water safety - beer does, after all, go off. Where the quality of the beer was better than water from the local pump was because it was boiled as part of the manufacturing process which killed a lot of the bacteria, as did the fermentation process. However, it wasn’t until the second half of the 19th century that Louis Pasteur discovered the link between bacteria and disease, so our ancestors would not have understood that boiling the water or the fermentation process was what made beer and spirits safer to drink than water.

So how can you stop water from going off?

There are a number of things you can do to protect the quality of your water:

Keep the tap water running

Water systems need to be flushed regularly in order to prevent stagnation. Anyone responsible for commercial premises will be familiar with the need to control the risk from the legionella bacteria by not allowing water to stagnate. If conditions are right for legionella, they will be right for other harmful bacteria too. Therefore, if there are areas of your premises that are used less frequently, it is a good precaution to ensure toilets are flushed and taps are run once a week to prevent stagnation and keep the water fresh.

Safe storage of water

As long as water is stored correctly, it will not ‘go off’. If your business has large containers of water stored for emergency use, replace the water every six months, making sure the containers are clean and sanitised. Always use a tight-fitting lid on each container to reduce the risk of contaminants.

We’re used to drinking water from plastic bottles, but not many people realise that after a couple of years, the plastic begins to break down and will release chemicals into the water. This is why plastic bottles of water always have a use-by date - because you need to use the water before the bottle begins to degrade. In this respect, it is safer to store water in glass or aluminium bottles.

Storing water in warm environments also increases the risk of bacterial contamination. So store water in cool conditions (10oC to 21oC).

How businesses can reuse water

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