In the event of a longer power cut, the water purification system may not function properly. Electricity is needed to produce drinking water and then pump it to reservoirs and water towers.
If in doubt, boil it before using. Boiling water for one minute is the best way to kill bacteria and parasites.
Waste water safety
In many buildings, waste water or drainage water is drained into the sewer system by means of pumps. If a power cut is accompanied by heavy rain (extreme weather), drain water can no longer be drained into the sewer system. Basements are therefore at risk of flooding.