The company which runs Kenley Water Treatment Works says there would be no immediate risk to water supplies if it floods.
Sutton and East Surrey Water has made preparations to re-route supplies from other sources if the plant has to be shut down.
And it has also treated water storage reservoirs in the area which will maintain supplies for several days and will mean there is no loss of supply even if treated water cannot be produced from the site in Godstone Road, Kenley.
Its employees are working 24 hours a day with the emergency services and Croydon Council to pump out the flood water.
There are also sandbags and other defences around key operating buildings and plant to protect them from the flood water.
- GLA elections: The candidates vying for votes in Croydon and Sutton
- More strikes planned as talks between Southern and the RMT collapse over future of conductor roles
- Lidl recalls tins of herring because label fails to warn it contains fish
- Croydon Central MP Gavin Barwell said to be under investigation by police after claims he broke election spending laws
- Super-sized SLUGS coming to gardens thanks to mild winter weather