contaminated plastic waste material in the form of used petri dishes , so - called agar plates , is rendered harmless and cleaned by placing the agar plates in a bacterium - tight heating chamber maintained substantially at atmospheric pressure and contacting them with water at an elevated temperature , preferably 90 °- 98 ° c . for a predetermined period of time . after the predetermined period the agar plates are rinsed with water which is then drained off through an outlet in the bottom of the heating chamber . the agar plates to be treated are loaded into the heating chamber in batches contained in bags wholly or partly consisting of a plastic sheet material which is solid and substantially insoluble in water at room temperature but soluble in water at the elevated temperature . invention makes use of a property of agar plates , namely that the dishes and / or lids thereof warp due to internal stresses in response to the heated water to thereby open them up in the bacterium - tight heating chamber to enable internal cleaning and rinsing .