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.