Conditions and rates of biotic and abiotic iron precipitation in selected Danish freshwater plants and microscopic analysis of precipitate morphology

This study compares the biotic precipitation of iron in the sand filters of a new freshwater plant, Astrup, with the abiotic precipitation of iron in the sand filters of a traditional freshwater plant, Forum, in the same area of Denmark. We have observed that a third freshwater plant, Grindsted, which was planned to precipitate iron in the traditional abiotic way is in fact precipitating iron biotically because of poor aeration and very low oxygen content of the raw water. The dominant iron-precipitating bacteria was Gallionella ferruginea in both Astrup and Grindsted. The morphology of the iron precipitates were investigated using light, X-ray, scanning electron and transmission electron microscopy. The physicochemical conditions governing precipitation and the precipitated iron sludge were also investigated. The biotically precipitated iron was shown to be oxidised and precipitated with a rate about 60 times faster than the traditional abiotic process in spite of the much poorer physicochemical conditions for the process. The faster kinetics indicate a catalytic activity due to the presence of exopolymers from Gallionella ferruginea. A model is proposed for the relationship between the differences in characteristics of the iron precipitate and the kinetics of the precipitation.