Recovery of VFAs from anaerobic digestion of dephenolized Olive Mill Wastewaters by Electrodialysis

Electrodialysis (ED) is applied for the first time as a separation-concentration step of the Volatile Fatty Acids (VFAs) enriched effluent resulting from fermentation of olive mill wastewaters. The process would represent a key step of an integrated multi-purpose biorefinery scheme including the biotechnological production of polyhydroxyalkanoates from acidified bioresidues. A feasibility study is introduced by testing separation efficiency of membranes with synthetic model solutions of different compositions (sodium acetate and NaCl, or acetic-propionic-butyric acids with NaCl) and with actual OMW (a complex mixture of sodium salts of acetic, propionic, isobutyric, butyric, isovaleric, valeric, isocaproic, caproic, eptanoic acids, NaCl and other electrolytes). Experiments were performed at room temperature in a lab-scale 2-compartments ED stack, containing Neosepta AMX-CMX membranes, at constant current density (31 A/m2). Membrane resistance was tested by performing additional ED experiments according to a proper protocol developed in this work. Removal of VFAs from actual OMW is feasible; no evidence of remarkable damages nor losses of membrane performance were observed after dedicated trials. Under the operative conditions investigated, VFAs removals ranged from 30 to 35%, resulting in a concentration factor between 1.2 and 1.5 with respect to the initial mother solution. Chloride removal was remarkably high and it was greatly favoured at the beginning of operation; competition between chloride and acidic anions was remarkable as far as chloride concentration was high, whereas acidic anions transport across the membrane increased after NaCl removal overcame 50%. Removal and concomitant concentration of VFAs were accomplished by a slight fractionation of acids with respect to the initial solution; the main outcome of which was an enrichment of acetate in the concentrate. Transport of each acidic anion across the membranes was affected by the concomitant role of concentration and diffusivity, which can shift the natural order imposed by the steric hindrance of the species.