Abstract: Removal of nitrogenous and carbonaceous contaminants is carried out in a single sludge two-tank system by a symbiotic two-step process whereby the second tank mainly oxidizes ammonia to nitrate and the first tank mainly reduces nitrate to nitrogen gas. The biological sludge is the same in both tanks but different reactions occur as the mixed microbial population moves through various environmental conditions in the tanks. Most of the carbonaceous material is removed in the first tank by a combination of biological uptake with cell growth and use of the carbon as a final electron acceptor in respiratory denitrification.