Abstract: Method for treating a fluid containing anaerobically digestible nutrients and organic substrates wherein a plurality of individual anaerobic microorganisms are established upon rotatable discs mounted within an enclosed housing and a stream of treatable fluid is passed through the housing to wet more than fifty percent of the disc surfaces. The discs are rotated through the fluid whereby the microorganisms are able to feed upon the nutrients and substrates in the fluid and expel a process gas into the atmosphere maintained over the fluid. To further enhance both the feeding of the microorganisms and the expulsion of gas, the housing is divided into a number of individual compartments or stages and the pressure is reduced below atmospheric pressure in each compartment.