A consortium of aerobic sulphur oxidizing microorganisms native to municipal sewage sludge has been developed that is capable of reducing the pH of sludge from 7.0 to 2.0 under mild-thermophilic (45 degrees C) temperatures. This mild-thermophilic, acidophilic acclimatized consortium has then been used to develop a thermophilic acidophilic culture capable of reducing the pH of municipal sewage sludge from 7.0 to 4.8 at 60 degrees C. Furthermore, these two cultures have been used to develop a novel two-stage aerobic process functioning under thermophilic and mild-thermophilic conditions capable of simultaneous sludge digestion, pathogen control and metal leaching. Studies with 1.5 L reactors indicate that at the end of the process, concentration of volatile suspended solids has stabilized, no coliforms are present, and 90-98% copper, 90-94% manganese and 95-100% zinc has been leached.