Liquid sewage and waste, such as manure and/or sewage sludge, are treated to destroy pathogenic bacteria and remove offensive odors to make the material usable as a plant nutrient and soil conditioner. The liquid sewage is thoroughly aerated within a cone-shaped, pressurized vessel by injection of a jet of compressed air tangentially to the side walls of the vessel into contact with the liquid sewage. The vortex motion and agitation created by injection of the compressed air disintegrates the larger solids of the sewage or sludge so that the compressed air can permeate the material to a greater degree. A head of compressed air is built up above the liquid level in the pressurized vessel, which, on opening of a discharge valve in the lower end of the pressurized vessel, forces the contents of the vessel through an outlet pipe connected to the vessel through the discharge valve. The material may be used as is or discharged onto a pile of sawdust, ground bark or straw for composting. After an adequate composting period, the material can be packaged for sale or used directly as a soil amendment and fertilizer. To insure thorough sterilization of the material in the case of sewage waste, a metered amount of sulfuric acid is injected into the material being treated to lower the pH of the material to pH 3.0 or less. After an exposure of several minutes to this low pH environment to destroy pathogenic bacteria, the material is treated with a neutralizing agent to increase the pH to an optimum value for use as a plant food.