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Accueil du site → Doctorat → Pays-Bas → 2003 → Subsurface aeration of anaerobic groundwater : iron colloid formation and the nitrification process

Wageningen Universiteit (2003)

Subsurface aeration of anaerobic groundwater : iron colloid formation and the nitrification process

Wolthoorn, A.

Titre : Subsurface aeration of anaerobic groundwater : iron colloid formation and the nitrification process

Auteur : Wolthoorn, A.

Université de soutenance : Wageningen Universiteit

Grade : PhD thesis 2003

Résumé
In anaerobic groundwater iron and ammonium can be found in relatively high concentrations. These substances need to be removed when groundwater is used for the production of drinking water. Subsurface aeration can be applied to remove iron before the groundwater reaches the purification plant. The primary goal of subsurface aeration is to oxidise iron in-situ . As a side effect subsurface aeration can strongly enhance the microbiological removal of ammonium (i.e. nitrification) in sand filters. It is recognized that subsurface aeration could be a practical tool to enhance the nitrification process. Until now, subsurface aeration and the nitrification process were not specifically considered as related processes. It is hypothesized that mobile iron colloids may be the link between subsurface aeration and the positive effect on the nitrification process. To gain insight into the processes that can explain the effects of subsurface aeration the fate of iron after the application of subsurface aeration was studied. The potentially mobile iron colloids are of particular interest. A method is developed that could be used to study the effects of subsurface aeration of an anaerobic groundwater well under well-defined laboratory conditions. The first issue was whether mobile iron colloids could be formed as a result of subsurface aeration. At a pH > 7 the oxidation of Fe 2+is a heterogeneous oxidation process. The heterogeneous oxidation was described using a model with a homogeneous and an autocatalytic oxidation rate constant.

The results of this study showed that the application of subsurface aeration of a groundwater system with a pH higher than 7 leads to the formation of iron colloids. A field experiment was performed to assess whether mobile iron colloids could be detected in an aerated groundwater well. From this field experiment it followed that a subsurface aerated well contained more iron colloids than a groundwater well that was not aerated. The iron colloids from the field were analysed using both chemical analysis and electron microscopy. The characteristics of the iron colloids from the field were used to prepare a synthetic analogue. The effect of the synthetic iron colloids on the nitrification process was studied by building a purification set up on a laboratory scale. In conclusion the results of this study strongly support the hypothesis that mobile iron colloids may be the link between subsurface aeration and the positive effect on the nitrification process.

Mots clés : groundwater - aeration - anaerobic conditions - nitrification - iron - colloids - water supply - groundwater extraction

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