Tannin Removal

A tannin (a.k.a. vegetable tannin, i.e. a type of biomolecule, as opposed to modern synthetic tannin) is an astrigent, bitter plant polyphenolic compound that binds to and precipitates proteins and various other organic compounds including amino acids and alkaloids.  It seeps into the ground and into well water.

If ingested in excessive quantities, tannins inhibit the absorption of minerals such as iron, which may, if prolonged, lead to amemia.

This is because tannins are metal ion chelators and tannin-chelated metal ions are not bioavailable (can't be absorbed into the blood)

Tannins interfere with iron absorption through a complex formation with iron when it is in the gastrointestinal lumen, which decreases the bioavailability of iron.  In order to prevent these problems, the removal of tannins in drinking water is strongly recommended.