## 1 Introduction  
The presence of chromium in natural and engineered aquatic systems is due to natural sources as well as to anthropogenic activities. Natural sources of chromium are mainly rock disintegration [1, 2], whereas anthropogenic activities that contribute to elevated chromium concentrations in water sources are mainly industrial activities, such as for example the electroplating industry, stainless steel production, leather tanning, and textile manufacturing [3]. In water, chromium is present in its two most stable oxidation states, namely, Cr(III), and Cr(VI). In chromium-containing natural water, the prevailing form is Cr(VI), because it is very soluble, whereas Cr(III) is quite insoluble, easily forming complexes with iron oxides or other minerals and, therefore, precipitating out of water [4]. Several studies have reported increased concentrations of chromium in natural waters, especially in groundwaters across the world such as in the USA, Mexico, India, Canada, and Italy as well as in Greece [1; 5; 6]. For example, [PERSON] et al. (2015) [1] investigated the occurrence of Cr(VI) in drinking waters in Greece. In approximately 600 samples taken throughout Greece, and focusing on areas with geological substrate predominated by ultramafic minerals, it was found that about 20% of samples contained Cr(VI) concentration of more than 10 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L, 14% of the samples had Cr(VI) concentrations of 10-30 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L and 6% 30-50 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L. Concentrations exceeding the limit of 50 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L were found in only 3% of the samples. In most of the cases, Cr(VI) presence was of natural origin of Cr(VI) in groundwater. In addition, it was found that in all cases, Cr(VI) was the species accounting for more than 95% of the total concentration of chromium. Concerning the effect of chromium species on human health, Cr(III) is relatively non-toxic, and is an essential trace element for the human body [7]. In contrast, water-soluble hexavalent chromium compounds are particularly irritating, corrosive and toxic to the tissues of the human body. Furthermore, Cr(VI) is considered a human carcinogen [8; 9]. It is for these reasons that Cr(VI) is classified as a priority pollutant as by the World Health Organization (WHO), the European Commission and the United States Environmental Protection Agency (US-EPA). To date, the maximum concentration limit for total chromium in water, for the European Commission and the U.S. EPA, is 50 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L and 100 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L, respectively. However, in July 2014, the state of California first set a limit for the Cr (VI) concentration at 10 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L. This is at the moment withdrawn [10], but the debate has started also within European countries of setting a limit for Cr(VI) at the lower ppb level. The WHO report recommends maintaining the current parametric value of 50 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L for total chromium, however it remarks that the value for chromium remains under review, suggesting the introduction of a lower value for chromium, and in particular for the more toxic Cr(VI). The European Commission is considering, therefore, proposing to reduce the value by 50% to 25 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L and offer a transition period of 10 years after the entry into force of the new Directive for water quality intended for human consumption. Therefore, in this study the primary aim is firstly to examine the removal of Cr(VI) by ferrous iron by this innovative treatment unit and secondly to examine the conditions under which residual concentrations of Cr(VI) below 25 and 10 \(\mathrm{\SIUnitSymbolMicro\text{g}}\)/L can be achieved [11].