Heavy metals removal by sand filters inoculated with metal sorbing and precipitating bacteria

Large volumes of wastewater containing metals like Cd, Zn, Cu, Pb, Hg, Ni or Co are mainly treated by precipitation processes. However the metals concentration does not always reach the regulatory standards and in many cases ecotaxes must be paid on the heavy metals load in the discharged water. Therefore a second polishing treatment is often necessary. In order to be economically acceptable the technology must be cheap and adapted to the treatment of large volumes. The use of sandfilters inoculated with heavy metal biosorbing and bioprecipitating bacteria fulfils these objectives. The system is based on a moving bed sandfilter. A biofilm is formed on the sand grains after inoculation with heavy metal resistant bacteria able to biosorb or to bioprecipitate heavy metals. Passage of the wastewater over these biofilms leads to the binding of the metals to the biofilm and consequently the removal of the metals from the wastewater. The metal laden biofilm is removed from the sand grains in a sand washer created by an airlift for the continuous movement of the filterbed. The metal loaded biomass is separated from the sand in a labyrinth on the top of the sand washer. Nutrients and a carbon source are provided continuously in the system in order to promote the regrowth of the biofilm on the sand grains. The reactor can be used for the removal of heavy metals and some COD. The obtained biosludge contains heavy metals at concentrations of more than 10% of the dry weight.