PURPOSE: To make a large amt. of industrial waste unharmful efficiently by treating the industrial waste contg. harmful materials with an aq. soln. contg. iron chloride or iron sulfate and sulfur compds.

CONSTITUTION: Industrial waste contg. harmful materials is treated with an aq. soln. contg. iron chloride or iron sulfate and sulfur compds., and if necessary, calcium compds. Ferrous chloride is preferable as the iron chloride and ferrous sulfate is preferable as the iron sulfate, both of which are used respectively at about 2W25wt% based on the weight of the waste in terms of the weight of the 20% aq. soln. There are sodium sulfide, thiourea, etc. as the sulfur compds., which are used at about 0.1W5wt%. There are calcium chloride, calcium oxide, etc. as the calcium compds. which are used at about 0.1W5wt%. It is preferable to execute the detoxicating method by controlling the pH of the waste in 4 days after the treatment at about 6.5W9.5.