What is the oily film in my water when I make coffee?

From time to time, the Utility receives complaints from customers regarding their water and the appearance of an oily surface film that is particularly evident when preparing coffee or tea. This "film", although not appealing, is not harmful and is the result of a chemical phenomenon that occurs during the water heating process. A more detailed explanation of this phenomenon is as follows:

1. The Village of Grafton water supply is characterized as “very hard” water. Chemically speaking, this means it contains high levels of calcium carbonate. During the heating process, calcium carbonate will normally "precipitate out" from water and cause a scale build-up that adheres to surface. Grafton adds a polyphosphate chemical to its water supply that normally inhibits the precipitation or "scaling" process; however, as temperature rises (i.e., during the water heating process), the effectiveness of the polyphosphate chemical is reduced).

2. Coffee and/or tea contain a derivative called “tannin”. This is the "color" which leaches out during the preparation process. When the polyphosphate chemical becomes less effective during the heating process and fails to condition the water hardness (prevent scaling), the tannin from the coffee or tea reacts as a substitute conditioner and in effect, forms the "oily film" which becomes evident on the surface of your coffee or tea.

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4. There was a water main break near my house recently. What do I do if I'm experiencing unusual water system issues?
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7. How does one check for water leaks in the house?
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11. What are the ‘flat charges’ on my water bill?
12. What is the typical water bill for Village of Grafton residents?
13. Why is my drinking water cloudy at times?
14. What is the hardness level of Grafton’s water, and do I need a water softener?
15. How do I know my water is safe to drink?
16. How do my water & sewer rates compare with other Wisconsin communities?
17. Does Grafton have any summer sprinkling restrictions?
18. What is the oily film in my water when I make coffee?