Eline Anvik
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Recovering of resources is common practice in wastewater treatment, but little applied to drinking water production. The conventional method for iron removal from groundwater via oxidation and filtration produces a large waste stream of aqueous sludge with little value. Backwashing the filter and treating the concentrated stream is the most cost-intensive step and reducing or reusing the iron sludge is recognized as one of the most important steps in increasing the sustainability of the industry. Groundwater use is expected to increase in the future and membrane technologies such as nanofiltration (NF) and reverse osmosis (RO) are becoming a competitive option to the conventional treatment technologies. The disposal of the concentrate, which contains high concentrations of iron, is a downside of these techniques and a proper treatment method should be found to deal with the stringent European legislation regarding disposal.
A novel method is proposed in which iron(II) is removed anaerobically from the water by precipitation as vivianite (Fe3(PO4)2 • 8 H2O) by dosing phosphate anaerobically to the water. Vivianite has an economical value and is used for the production of lithium ion batteries or as a slow P-release fertilizer.
The concept was tested in groundwater with natural iron concentrations of around 3.8 mg/L and a spiked solution of 100 mg Fe/L. The removal efficiencies, sludge volume and kinetics were compared to the method of iron oxidation. The formed precipitate was analysed by X-ray diffraction (XRD) analysis and the experiments were evaluated with the program Spec8 of the Geochemist’s Workbench® (GWB®) model to determine the saturation index (SI) of vivianite formation.
In iron-spiked groundwater, 93.7% was anaerobically removed, which increased to 99.9% after oxidation. Vivianite was the only solid phase detected by XRD and the volume of the sludge produced was a third compared to iron oxidation. With natural iron concentrations 16% was removed, the sludge volume was too little to measure. For both solutions removal stopped when the SI dropped below 4. To enhance further removal, the SI can be increased by increasing the pH. The model showed that the minimal iron concentration at which the SI is higher than 4, is 1 mg/L at a pH of 9.
A second order removal rate was found for the spiked groundwater with a rate constant of 2.27 M/s at a pH of 7. A half-time of 4 minutes was found for anaerobic precipitation, which is 4 times faster compared to iron oxidation.
Iron was successfully removed from groundwater via vivianite precipitation. This novel method has the potential to reduce the costs of drinking water production and can be a solution for the treatment of groundwater and of the anaerobic concentrate from NF or RO.
Source: Goedhart, R. 2021. Vivianite precipitation for iron recovery from anaerobic groundwater. Resource recovery and wastewater treatment technologies, Clean Water & Ecosystem Restoration. AIWW 2021
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