The removal of iron from groundwater is essential to avoid aesthetic issues of the produced drinking water and to reduce maintenance cost of the system. The most applied iron removal method of oxidation and filtration produces large volumes of aqueous iron sludge of little value and the method is more likely to fail at high iron concentrations. This research investigates the novel concept of removing iron(II) anaerobically from groundwater by precipitation as vivianite (Fe3(PO4)2 ¢ 8 H2O) by dosing phosphate to the water. Natural groundwater was used to investigate the proposed method. A better understanding of the ironphosphate chemistry was obtained in experiments with a synthetic iron solution. To find a possible alternative to the limited resource phosphate, the possibility of removing iron by forming a compact mineral was also tested by dosing sulphate and carbonate. The chemical equilibriums and carried out experiments were evaluated by a geochemical model and the reaction products were analysed by X-ray diffraction. Up to 93% of iron removal by vivianite precipitation was obtained by dosing phosphate to anaerobic groundwater spiked with 100 mg Fe/L. An additional aeration step increased the efficiency to 99.9%, a higher total removal efficiency compared to the conventional aeration-filtration technique. The geochemical model showed that the anaerobic removal stopped when the saturation index (SI) of vivianite drops below 4, which explains why the last 7% were not removed anaerobically. Increasing the pH increases the SI of vivianite and can enhance further removal. Theoretically iron can be removed by vivianite precipitation starting from a concentration of 1 mg/L at a pH of 8.5. A second order kinetics was found for the removal of iron by vivianite precipitation at pH 7 with a rate constant of 2.27 M/s. The corresponding half life of iron is 4 minutes, while the half life of iron oxidation is 16 minutes at the same pH. Vivianite was the only crystalline end product detected and this decreased the sludge volume by a third compared to the sludge currently produced with oxidation and filtration. The total iron removal by sulphate addition only reached 73%, probably caused by the formation of iron(III)- sulphate complexes, and is therefore not a proper alternative to phosphate. The addition of carbonate reached an anaerobic removal of 59% and was increased to 99.9% by aeration. The formed sludge contained of a mixture of several oxidised compounds and the volume was almost 6 times higher compared to the currently produced aqueous iron sludge, which is why carbonate is not considered as an interesting alternative. The possibility of removing iron(II) anaerobically from groundwater by forming a compact mineral is successfully demonstrated. This method can increase the efficiency of drinking water production: higher throughput rates can be reached and a valuable end product with interesting reuse opportunities is created, provided that the phosphate can effectively be recovered from the water. It can decrease the operational costs of groundwater production substantially. The proposed novel method is a promising alternative to the conventional treatment method of oxidation and filtration, especially at plants where large iron sludge volumes are currently produced caused by elevated iron concentrations.

Conventional groundwater treatment plants consist of aeration and rapid sand filtration steps, that are merely designed and optimized for iron (Fe), manganese (Mn) and ammonium (NH4+) removal. Understanding the various reduction-oxidation pathways, and interactions of manganese and iron, can play a major role in optimizing the performance of such filters. Interestingly, it is found that under certain conditions, mobilization of dissolved manganese can occur in such filters, which can be critical to the filter operation. Therefore, the main aim of this research is to dive deep into studying the possibilities of manganese reduction pathways occurring at the top layer of the filter media of a groundwater filter. Secondly, the research also focuses on knowing how the removal of manganese is related to the oxidation by MnO2+O2 systems, and also how these systems interact with each other under different pH conditions. To do so, manganese dioxide (MnO2) coated sand grains were obtained from the second filtration unit of Vitens groundwater treatment plant situated in Holten. Various batch scale experiments were done under aerobic as well as anoxic conditions, in the presence of Mn(II) or Fe(II). Additionally, the influence of pH on manganese removal efficiencies as well as the rates of both manganese and iron oxidation was investigated. It was found that the dissolved Mn was a reduction product of MnO2-Fe(II) system, where Mn(IV) got reduced to Mn(II), reaching an Fe(II) : Mn(II) molar ratio of 3.65:1 instead of 2:1, as there was a significant difference between the calculated theoretical values and the measured experimental values of both Mn(II) and Fe(II). There was mobilization of Mn(II) which took place from the MnO2 surface, when there was a presence of Fe(II) in the system, which simultaneously got partially oxidized to Fe(III). Also, it was observed that manganese could be removed by MnO2 under anoxic conditions, although under aerobic conditions the removal efficiency was high (93.32% vs 71.83%). Apart from oxidation, there is a possibility of adsorption over MnO2 due to its high sorption capacity towards cations like Mn2+, Mn3+ and Fe2+. This research also showed that a small fraction of Mn(II) reacts with Mn(IV) to form Mn(III) as a reaction product, enhancing the mobilization of Mn(II).