Biotic Iron Precipitation in Sand Filtration Systems by Gallionella ferruginea: Morphology and content of Exopolymers

Erik Gydesen Søgaard, Charlotte Simonsen

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Biotic Iron Precipitation in Sand Filtration System by Gallionella ferriuginea: Morphology and Content of Exopolymers

Erik G. Søgaard and Charlotte Simonsen

University of Aalborg, Institute of Chemistry and Applied Engineering Science, Niels Bohrs Vej 8, 6700 Esbjerg, Denmark;
Keywords: Iron filtration, ferrihydrate, Gallionella ferruginea,, exopolymers

The normal physico-chemical way to decrease iron content in raw water used for drinkable water is by aeration to saturation of air components. In this way iron will start being oxidised from iron(II) to iron(III). Due to rather high pH the aeration process will result in precipitation of iron(III)oxides in sand filters built for the purpose. An alternative method for iron precipitation is biological where bacteria are actively involved in the process (Mouchet 1992, Søgaard et al. 2000). The boundary between best conditions for principally biotic or abiotic iron precipitation is not well defined. An rH2 greater than 14 e.g. corresponding to an Eh a little above zero at slightly acidic pH is stated to be the best condition for biotic iron precipitation (Degremont, 1991). Abiotic iron precipitation is performed at a pH of about 8 and an Eh of about 2-500 mV, e.g. rH2 greater than 22. The iron precipitating bacteria Gallionella ferruginea and Leptothrix ochracea are often found in water supply systems and especially in sand filters of fresh water treatment plants. In recent years some fresh water treatment plants are built with the purpose of biotic iron precipitation in order to reduce frequency of backwashing filtration systems. This is possibly due to the fact that biologically precipitated iron has a much denser structure than the corresponding abiotic precipitates (Søgaard et al. 2000). Both kinds of iron(III)-precipitates consist of poorly crystallised ferrihydrite. Normally the particle size of biotic precipitated ferrihydrite is biggest. However, the most important difference between biotic and abiotic iron precipitates is the existence of exopolymers produced by iron precipitating bacteria. Amorphous iron oxides are present in twisted structured stalks from Gallionella ferruginea or in sheaths from Leptothrix ochracea. The exopolymers are heavily encrusted with iron oxides probably as a result of a metabolic oxidation / precipitation process performed by iron precipitating bacteria. By help of these exopolymers Gallionella ferruginea can increase rate of oxidation / precipitation process by several orders of magnitudes (Søgaard et al. 2001).
In order to obtain more information about the function of exopolymers in the iron precipitation process we investigated the content of exopolymers of Gallionella ferruginea produced in a laboratory sand filtration reactor. Compared to a work of Emerson and Ghiorse in 1993 about bioorganic content of the exopolymeric sheats of Leptothrix discophora we found higher but comparable carbohydrate / protein ratios.

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Mouchet, P. (1992) From Conventional to Biological Removal of Iron and Manganese in
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Søgaard E.G., Medenwaldt R. and Abraham-Peskir J. (2000) Conditions and rates of biotic
and abiotic iron precipitation in selected Danish freshwater plants and microscopic
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Søgaard E.G., Aruna R., Abraham-Peskir, J. and Bender Koch, C. (2001) Conditions for
iron precipitation by Gallionella ferruginea in a slightly polluted groundwater. Appl. Geochem., 16 1129-1137
Original languageEnglish
Publication date28 Jan 2004
Number of pages1
Publication statusPublished - 28 Jan 2004
EventInternational Workshop on Geomicrobiology : Morphology and Content of Exopolymers - University of Aarhus, Denmark, Denmark
Duration: 28 Jan 200431 Jan 2004


ConferenceInternational Workshop on Geomicrobiology : Morphology and Content of Exopolymers
CityUniversity of Aarhus, Denmark

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