Arsenic removal from groundwaters containing iron, ammonium, manganese and phosphate:

The city of Malgara in the municipality of Aksios, in northern Greece, relies on local groundwater for the municipal water supply. The groundwater has pH 7.9 and contains elevated concentrations of arsenic (20 µg/L), phosphate (550 µg/L), manganese (235 µg/L) and ammonium (1.2 mg/L), whereas the iron concentration (165 µg/L) is relatively low. Arsenic, manganese and ammonium exceed the parametric values, according to the EC directive 98/83. This directive has been adopted as national law since the beginning of 2003 and a groundwater treatment plant is in operation since the beginning of 2005. The treatment plant consists of aeration, up-flow filtration for the biological oxidation of ammonium, manganese and arsenic, followed by coagulation with FeClSO4 at a concentration of 2.3 mg Fe/L, and final down-flow filtration for the removal of arsenic and the additional iron. In a final stage, the water is disinfected with NaOCl before the distribution to the consumers. During aeration, Fe(II) is oxidized and some phosphate is sorbed on the formed iron oxides but remains in suspension until it is removed during the subsequent biological filtration stage. Mn(II) is oxidized by biological oxidation and the produced insoluble + manganese oxides are removed by filtration. NH4 is biologically oxidized and removed from the water via nitrification and formation of nitrates. As(III) is oxidized but not removed during the biological filtration stage. Arsenic is removed to below 10 µg/L during the subsequent coagulation and filtration treatment stage. Similarly, the final concentrations of Fe(tot), Mn(tot) and NH4+ are below the EC parametric values of 200, 50 and 500 µg/L respectively.

The decision on a remedial action for the removal of arsenic from groundwater depends strongly on the local groundwater composition. In the present case study, the following factors were important for the design of the treatment method: (a) As(III) constituted roughly 70% of the total arsenic content (b) manganese and ammonium concentrations were higher than the EU parametric values and (c) the phosphate concentration was sufficient to out-compete arsenic sorption on the bacteriogenic iron and manganese oxides. The biological oxidation of manganese and ammonium promoted As(III) oxidation as well, which was essential to achieve efficient arsenic removal. However, due to the presence of phosphate and low natural Fe(II) concentration, an additional treatment stage was necessary to produce drinking water with arsenic concentrations below 10 µg/L. In an added coagulation step with addition of FeClSO4, HFO colloids with sorbed arsenic were formed and subsequently removed by filtration. The final treated water complies with the EC directive 98/83 of water intended for human consumption. This method presents several advantages over conventional treatment methods, such as low operational costs due to the circumvention of added oxidants and high reaction rates due to biological oxidation. It is a combined groundwater treatment approach, i.e, bioremediation combined with physicochemical treatment and can find broader application in other cases, such as in arsenic affected regions in southeastern Europe (Romania, Hungary, and Croatia). Acknowledgments Funding of Dr. Ioannis A. Katsoyiannis from the Marie Curie Intra European Individual Fellowship (SASURU, 23930) is greatly appreciated. We would like to thank Mr. T. Ruettimann for laboratory assistance. Thanks are extended to the Mayor of the city of Aksios, Dr. N. Giotikas, for support during the project.