In the manual of my new dishwasher there is a table of values of water hardness. It goes until 50 dGH (degrees of general hardness) which corresponds to 9,0 mmol/l in the same table.
1 dGH is equivalent to 0.17832 mmol per litre of elemental calcium and/or magnesium ions
The interesting thing is that 6 out of the 10 options for water softener levels are for water hardness greater than 19 dGH and most websites consider 20 dGH as very hard water already.
I couldn't' find any world map or regional map from any place containing dGH values greater than 20. I was wondering whether water hardness levels between 20/30 and 50 are commonplace somewhere in the world.