Abstract: A method and plant are provided for sterilizing waste sewage sludge with methyl bromide vapor, to produce a natural fertilizing material. The moisture content of the sludge is reduced to a level where the vapor permeability of the sludge is sufficiently high to reduce the contact period of the methyl bromide vapor with the sludge to an economically viable level. The methyl bromide is provided at a dose of at least 50 g/m.sup.3 to the top of a bed of sewage sludge having a moisture content of less than 35% and about 1 meter thick and allowed to permeate the bed over a period of about 48 hours under the force of gravity, the bed of sewage sludge being covered during the entire treatment period with a cover which is impermeable to the methyl bromide vapor. Harmful pathogens and parasites such as Ascaris ova are effectively eliminated.