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I am looking for a statistical amounts which are allowed. Some students say it is 0 for all bacteria, which I think is false. I found this USA source.

I found there

  • Total Coliforms (including fecal coliform and E. Coli) mg/l. No more than 5.0% samples total coliform-positive (TC-positive) in a month.
  • Giardia lamblia: 99.9% removal/inactivation.

What is the right place to look for such information?

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  • $\begingroup$ According to a text I'll reference later, humans face larger risk with microbial contaminants that chemical ones. This is the reason why most water quality standards and guidelines (including who) recommend (or allow) 0 counts per 100 ml for bacterial indicators. $\endgroup$ – Chimango Chisuwo Nov 29 '16 at 14:49
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No, your colleages are right: There should be no coliforms in drinking water in Europe following the Council Directive 80/777/EEC. This PDF from the Northern Ireland Environment Agency (which follows the directice mentioned above) shows this a bit nicer. It makes sense, since coliforms are a sign of mixing sewage and fresh water. Nothing you want to have happening.

This is the relevant table from the linked PDF:

   enter image description here

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