Which are the requirements for the water supply on board? Which are the requirements for the water supply on board of a commercial airplane? I know that there are minumum mandatory requirements, but I don't know where to find them and how they can change with the category of the aircraft. Surely there are different requirements from a small turboprop without a toilet to a A320.
The IATA standards as per the IATA Drinking Water Quality Pool are based on 


  ISO 17775, WHO requirements or EU/98/83/EC or those set by the civil aviation authorities of the participating airlines


Source

Airlines and operators will periodically test their water in aircraft, ground service vehicles, and filling stations for bacteria contamination (E-coli, coliform, etc).

Some aircraft may not actually have potable water tanks or may have removed them. Horizon Air operates their Q400 Dash 8 aircraft without potable water.