A method and apparatus are disclosed for mixing sewage sludge with spent water-softener lime in a ratio of approximately 10:1 on a wet basis. The mixture is introduced to a rotary pre-dryer kiln and a rotary combuster kiln to kill pathogens. An ash by-product and gases are thereby produced. The ash by-product can be disposed of at much less cost or can be incorporated in a variety of applications such as an aggregate substitute.