Groundwater is one of the major sources for drinking water supply worldwide. Conventional iron removal via aeration-filtration produces about 72,802 t of iron sludge annually in the Netherlands alone. Iron sludge comprises low-density flocs of little to no commercial value. The current study explored a novel concept for iron removal, namely anoxic iron sulfides formation in a fixed bed continuous flow reactor. Iron sulfides usually form dense structures and offer a wider range of re-use applications. A packed bed up-flow column reactor filled with pyrite granules was fed iron and sulfide containing solutions. Produced solids were analyzed applying X-ray diffraction analysis, Raman spectroscopy, digital microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Rapid iron sulfides formation was observed after < 10 min. The formed minerals were partially retained by the pyrite granules. The molar ratio of removed Fe(II) to removed S(-II) equaled up to 0.76 ± 0.16 mol Fe(II)_rem/(mol S(-II)_rem). Our results show that iron sulfides formation can present an interesting alternative to iron removal via aeration-filtration due to its compact particle sizes and fast formation rates.

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.

Cape Town is a city with over four million people and a growing population. Due to three consecutive dry summers as a result of climate change, a growing population and an increased per capita water demand, the city’s main water supply was nearly depleted. Cape Town depends for 98% on surface water stored in dammed reservoirs, which is replenished by rainfall. There is a temporal mismatch between water availability and peak demand, and thus harvesting rainfall can be potentially become an additional source of water. Cape Town’s urban drainage system has 737 detention ponds, which are used to attenuate flooding in case of heavy rain events. These ponds can be used to harvest the stormwater and store it in the Cape Flats aquifer using managed aquifer recharge for seasonal availability. The complexity of retrofitting stormwater ponds into infiltration ponds calls for a systematic approach. This research offers a retrofitting framework for the context of Cape Town. The framework can be used to determine suitable detention ponds to allow managed aquifer recharge via infiltrating stormwater, and to retrofit these detention ponds into infiltration ponds. The framework consists of three phases; spatial assessment, physical assessment and conceptual design. It is highly flexible in usage due to the fact that every phase can be used separately. Additionally, the framework can be extended to include important socio-economic aspects. Following the framework standardizes the procedure of obtaining data on individual ponds, which allows for objective comparison in assessing their suitability for infiltration.