Abstract The treatment of acid mine drainage (AMD) through the utilization of alkali generating microbes has potential as an alternate approach to conventional lime treatment. Organic matter, a source of fixed carbon for the alkali generating microbial ecosystem, has been tested in 6 different types of AMD. The AMD characteristics range in acidities from 2 mg/l to 900 mg/l (CaCO 3 equivalent), while sulphate concentrations range from 75 to 7300 mg/l. Alkali generating populations identified include iron reducers, sulphate reducers and ammonifiers. In coal AMD amended with organic matter, the microbial alkali generation is dominated by ammonifiers. Concentrations of Al, Fe and Zn in the AMD water decreased with concurrent increases in pH (3.2 to 6.5) in localized areas in the test cells.