A combined chemico-biological process has been developed to remove sulphate and heavy metal ions from waste waters after microbial depyritization of coal. The waters are initially neutralized to a pH of about 3.5, diluted with pure water and then treated in an anaerobic bioreactor for microbial sulphate reduction in which nutrients are added to support the growth of bacteria. In the bioreactor the sulphate ions are reduced to hydrogen sulphide and the heavy metals are precipitated as the relevant insoluble sulphides. The process then involves H2S stripping from the anaerobic bioreactor using nitrogen as a carrier gas, sulphur production from H2S oxidation by bacterially produced ferric ions, aerobic solution polishing by microbial degradation of the organics, and secondary anaerobic processing. The product solution is rich in carbonate and may be recycled to the sulphate reduction stage or to the ferric ions production stage or may be discharged from the system.