Rethinking aesthetic guidelines for manganese and iron in drinking water

Manganese and iron are both internationally known causes of aesthetic issues in drinking water, however, there are limited data supporting their specific aesthetic guidelines, of which typical values are 0.05 mg/L Mn and 0.3 mg/L Fe. This study aims to clarify the concentrations at which off-flavors and off-colors caused by manganese and iron may be detected by consumers. Triangle tests of Mn(II) determined a best estimate taste threshold of 165 mg/L Mn(II), which is much higher than the reported range of 0.03–0.17 mg/L for Fe(II). Unlike Fe(II), Mn(II) taste tests showed there is no relationship between individual taste threshold and subject age. Mn(II) and Fe(II) oxidation in artificial saliva showed that Mn(II) had a non-detectable amount of oxidation and Fe(II) had up to 80% oxidation within 5 minutes at both 22 and 37 °C. Neither Mn(IV) nor Fe(III) exhibited detectable tastes. Visual testing of Mn(IV) and Fe(III), using the one-in-five forced choice method, showed that oxidized metals are visually detectable at concentrations below their typical aesthetic guidelines. Reduced Mn(II) and Fe(II) are colorless at concentrations much greater than established standards. This study demonstrates that current manganese and iron aesthetic standards may not be protective of off-flavors and off-colors.