Removal of iron is an important step in groundwater treatment for drinking water production. It is performed to prevent organoleptic issues and clogging in water supply systems. Iron can be eliminated with a purely physico-chemical (abiotic) method or biotically with the help of iron-oxidizing bacteria (FeOB). Each of the purification methods requires different operating conditions and results in formation of iron oxyhydroxide (FeOOH) precipitates. Knowledge about the differences in composition and properties of the biotic and abiotic precipitates is desirable from a technical, but also scientific point of view. In this study, a broad spectrum of analytical methods was employed to describe in detail the differences between FeOOH produced on a number of Danish waterworks using either the purely physico-chemical or the biological variation of the iron removal method. According to the results from X-ray diffraction (XRD), small angle X-ray spectroscopy, and visible light microscopy, the precipitate formed with the help of FeOB was more microcrystalline and consisted of agglomerated particles. The purely physico-chemical precipitate was more amorphous and, typically, was composed of smaller particles.