The WHO guideline for copper is 2mg/l which is the same as in the German guidelines "Trinkwasserverordnung". The article linked by batter says right in the abst

The WHO guideline for copper is 2mg/l which is the same as in the German guidelines "Trinkwasserverordnung". The article linked by batter says right in the abstract: We found that the disease can even be caused by copper concentrations below the allowed concentration given by the German Guidelines for Drinking Water (Trinkwasserverordnung).