In an improved method of treating foul water, the solids that are produced during the process of treatment of foul water such as wastewater or liquid sludges are treated with a heat treatment apparatus and the resulting thermal energy and/or gas is utilized in the treatment of said foul water. The foul water may be subjected, directly and/or after a treatment, to a filtration step using a filter packed with nonflammable particulate matter such as sand, and the trapped solids are fed into a fluidized-bed heat treatment apparatus such as a fluidized-bed incinerator or gasification furnace and, thereafter, the nonflammable particulate matter in the heat treatment apparatus is returned to the filtration step. The foul water can be treated with effective utilization of the energy of the organic matter in the foul water or the solids that are produced during the process of its treatment.