Abstract: Organic waste and/or sewage sludge is digested by passing it downwardly through a closed vessel while air is passed upwardly through it. The air is finely divided or distributed into the vessel so that it flows through the entire cross sectional area of the material being digested. The flow of air through the vessel is regulated by measuring the content of CO.sub.2 or O.sub.2 in the exhaust air from the vessel and comparing the measured value with a desired value. Similarly, the moisture content of the material being digested is measured in at least two spaced locations and the moisture content is compared with a desired value for determining the amount of water to be supplied into the air flowing into the vessel. In addition, the air supplied to the vessel is heated based on the temperature conditions to be maintained within the vessel. After exhaust air leaves the vessel, its heat and water contents are withdrawn and it is filtered to remove objectionable odors and other noxious matters.