Process permitting the reduction of a substance S into a substance S reduced comprising: • bringing together a lamellar double hydroxide, as catalyst or precursor of said catalyst with the same crystalline structure as that of said catalyst, said LDH containing a divalent cation M 2+ partially or completely substituted by Fe II, and a trivalent cation T optionally substituted by Fe III, of the following general formula: [M 2+ (z) Fe II (1-y-z) T 3+ t Fe II (y-t) O 2 H 2] n+ [(y/n)A n-,mH 2 O] n- in which: 1/4 <y<1/3, z<1-y and t<y, A n- is an anion with charge n, n taking the values 1, 2 or 3, in particular 2, m is an integer varying from 1 to 10, in particular from 1 to 4, advantageously 3, and the ratio x = (y-t)/(1-z-t) varies from 0 to 1, said LDH being used in association with iron-reducing bacteria able to reduce Fe III to Fe II and in the presence of organic matter, • if x is greater than 0.66 at the initial moment, a start-up phase of the oxidation-reduction process corresponding to the reduction of Fe III to Fe II within said LDH by said iron-reducing bacteria, leading to a change in x to a value less than or equal to 0.66, in order to obtain said LDH in the form of a catalyst, without a substantial change in the crystalline structure of said LDH, • a phase of catalytic reduction of the substance S, added to the whole comprising the LDH, the bacteria and the organic matter, to a substance S reduced by oxidation of Fe II to Fe III within said LDH coupled to a step of catalytic oxidation of the organic matter by reduction of the Fe III to Fe II, the redox potential of the pair S reduced /S being greater than that of the pair Fe III /Fe II at the crystallographic sites of the Fe II, said LDH is in association with a metal chosen from cu, Ag, Cd, Ni, Hg, Pd and Mn, preferably Cu, in a proportion from about 2 to 20% (w/w) relative to the total Fe, said bringing together of an LDH with iron reducing bacteria susceptible to reduce Fe III to Fe II in the presence of organic matter taking place without solubilisation of the precursor followed by reprecipitation of the precursor; said Fe II being regenerated starting from Fe III et inversely, in a "cyclic continuous" manner, so long as there is no depletion of the substance S, the organic matter, and/or for as long as the iron-reducing bacteria remain active. • bringing together a lamellar double hydroxide, as catalyst or precursor of said catalyst with the same crystalline structure as that of said catalyst, said LDH containing a divalent cation M 2+ partially or completely substituted by Fe II, and a trivalent cation T optionally substituted by Fe III, of the following general formula: [M 2+ (z) Fe II (1-y-z) T 3+ t Fe II (y-t) O 2 H 2] n+ [(y/n)A n-,mH 2 O] n- in which: 1/4 <y<1/3, z<1-y and t<y, A n- is an anion with charge n, n taking the values 1, 2 or 3, in particular 2, m is an integer varying from 1 to 10, in particular from 1 to 4, advantageously 3, and the ratio x = (y-t)/(1-z-t) varies from 0 to 1, said LDH being used in association with iron-reducing bacteria able to reduce Fe III to Fe II and in the presence of organic matter, 1/4 <y<1/3, z<1-y and t<y, A n- is an anion with charge n, n taking the values 1, 2 or 3, in particular 2, m is an integer varying from 1 to 10, in particular from 1 to 4, advantageously 3, and the ratio x = (y-t)/(1-z-t) varies from 0 to 1, said LDH being used in association with iron-reducing bacteria able to reduce Fe III to Fe II and in the presence of organic matter, • if x is greater than 0.66 at the initial moment, a start-up phase of the oxidation-reduction process corresponding to the reduction of Fe III to Fe II within said LDH by said iron-reducing bacteria, leading to a change in x to a value less than or equal to 0.66, in order to obtain said LDH in the form of a catalyst, without a substantial change in the crystalline structure of said LDH, • a phase of catalytic reduction of the substance S, added to the whole comprising the LDH, the bacteria and the organic matter, to a substance S reduced by oxidation of Fe II to Fe III within said LDH coupled to a step of catalytic oxidation of the organic matter by reduction of the Fe III to Fe II, the redox potential of the pair S reduced /S being greater than that of the pair Fe III /Fe II at the crystallographic sites of the Fe II,