New hybrid electrocoagulation membrane process for removing selenium from industrial wastewater

High concentrations of heavy metal cations such as Cd, Cu, Zn, Pb and Fe as well as heavy metal anions such as As and Se are contained in the wastewater generated by industrial Cu production. The conventional technology for treating this kind of wastewater, which consists of lime neutralisation/FeCl3 coagulation and flocculation, does not satisfy the requirements for direct wastewater discharge which specify that Se concentration should be less than 30 μg/l. The residual selenium concentration, the large sludge quantities generated as well as the expense of reagents are just some of the shortcomings of this technology. To combat these problems, a new hybrid process combining electrocoagulation and microfiltration was drawn up and verified in experiments. Very good adsorption/coprecipitation properties for selenium removal were observed for iron hydroxide, generated by electrocoagulation although precipitation was difficult, due to the fines of sludge. Iron hydroxide could be completely removed from the water by applying an integrated filtration process involving submerged microfiltration flat sheet ceramic membranes (mean pore size of 0.3 μm). Furthermore, the cake, formed on the microfiltration membranes, could be used as a barrier adsorbent for reducing the residual Se. A new bench-scale electrocoagulation/membrane filtration cell was developed and optimised in experiments to test the new hybrid process. Firstly, very extensive optimisation experiments using model wastewater were performed followed by bench-scale experiments with real industrial wastewater from Cu production. Results showed that, at a continuous regime with a treatment time of 20 min and a current density of the anode of 4.8 mA/cm2, Se reduction was 98.7%, As 99.9%, Cu and Pb more than 98.0%, Zn and Cd more than 99.9%.