I don't live in Frankfurt but I live in Niersbach, Germany which is about 20 min from Bitburg (Bitburger pils hometown) I was recently looking for my water report which I found here. pwm
I don't have your answer for you but it took me some searching and google translating to find it (I'm a US citizen just stationed here). If you google zweckverband wasserversorgung with Frankfurt am Main hopefully that will get you started. What your looking for is something titled wasser analysen or something like that. Good luck!