## 1 Introduction  
Tanneries can transform raw skin into leather, by using physico-chemical and mechanical techniques. The main chemical reagents, used to produce leather with desired commercial properties, are based on Cr(III) salts. However, due to the low effectiveness of the aforementioned tanning processes, tanneries produce large quantities of hazardous wastewaters [1]. In order to treat this waste, several physico-chemical and biological methods can be applied, resulting in sludge production with high chromium and organic matter content. Aiming at the minimization of environmental impact, the disposal options of tannery sludge has to be considered carefully (hazardous limit is 70 mg Cr\({}_{\text{total}}\)/Kg), and more sustainable management options should be promoted [2]. Following the suggestions of international environmental organizations, many researchers have studied and proposed alternative methods for the environmental sound management of tannery sludge. A specific case, generating great interest and showing certain prospects, is the application of thermal treatment, whether it concerns energy recovery [3], or as an intermediate stage of a more complex process [4]. The main feature that makes thermal treatment attractive is the great reduction in sludge volume, due to the combustion of organic matter content [5]. However, the application of increased temperatures can cause the oxidation of trivalent chromium to its highly toxic, carcinogenic and mutagenic hexavalent form (LD\({}_{50}\) = 50-150 mg/Kg) [6]. Simultaneously, Cr leaching can be achieved under milder conditions, due to the much higher solubility of hexavalent chromium. While the Cr(VI) oxyanions are very mobile (and toxic), Cr(III) cations are much less so and, instead, they can form the insoluble precipitate Cr(OH)\({}_{3}\), which is easily separated [7]. Therefore, by reducing Cr(VI) to Cr(III) the final removal (and recovery for reuse in the tanning process) of chromium from the effluent/leachate can be achieved easily by precipitation [8; 9]. Several methods have been reported for the reduction of hexavalent chromium to its trivalent form in industrial wastewaters, including some originating from tanneries [10]. Chemical reduction by using SO\({}_{2}\), NaHSO\({}_{3}\) and FeSO\({}_{4}\) as reducing agents [11; 12] is a typical treatment method, which is not very satisfactory, because of the respective use of chemicals, possibly creating secondary pollution problems and large amounts of produced sludge. Biological methods, involving e.g., bacteria [13] or fungi [14], for the reduction of Cr(VI) to Cr(III) are alternatives to conventional methods; however, their efficiency is sometimes limited, since the toxic metals (including Cr) can attack and destroy the membranes and the tissues of the used microbes. The drawbacks of the previously mentioned methods can be avoided by the application of electrochemical treatment, which appears to be greener and more effective in comparison with traditional technologies. They can be applied either indirectly, such as electro-coagulation [15], which involves the dissolution of a sacrificial anode made usually of iron or aluminum [16], or directly, such as electro-reduction, in which the free electrons, supplied by a power source through an appropriate electrode, can act as the reducing agent. When the techniques of chemical reduction, electrochemical reduction (with electro-generated Fe(II)) and direct electro-reduction of Cr(VI) have been compared [17], it was found that the consumption of energy was higher for the case of electro-reduction. Despite this fact, direct electro-reduction has turned out to be the cleanest method, because it does not result in the contamination of sludge with iron. In the electro-reduction technique, the cathode material is very important. Carbon-based materials such as graphite felt, fibrous carbon and reticulated vitreous carbon (RVC) [18; 19; 20] have successfully been used. Stainless steel and titanium electrodes [17; 21] have also been reported.