Manganese removal mechanisms in a stirred UASB reactor

The mechanisms of the removal from anaerobically treated wastewaters of those metal ions which do not form precipitates with sulphide or carbonate ions, or which form highly soluble precipitates, has been little studied to date. Therefore, the removal of manganese as a representative of this group was specifically investigated in the course of a wider project studying metal removal in laboratory-scale UASB reactors. Mn removal, in the absence of sulphate in the reactor feed, was low at 7.2%, but up to 40% Mn removal could be achieved in the presence of sulphate. Mn was removed by adsorption onto the existing solids in the sludge and onto newly grown biomass, and also by co-precipitation processes, which can be strongly enhanced by other heavy metal ions in solution. In the presence of cadmium in the reactor feed, manganese removal could be increased to 56%.