TETRACHLOROETHENE RECOVERY AND TOXICITY REDUCTION OF SPENT POWDERS FROM DRY CLEANING PROCESS

Dry cleaning facilities using tetrachloroethene, also known as perchloroethylene (PCE) produce a solid waste consisting of spent filtering powder, mainly calcined diatomaceous earth, benthonites or other absorbent materials. These powders are normally used during dry washing for the organic solvent filtration and discoloration prior to its recovery by distillation and condensation with refrigerating fluid. Therefore spent filtering powder contains high concentrations of PCE, together with dyes and non-volatile residues. PCE is a compound belonging to the class of halogenated aliphatic hydrocarbons suspected of being carcinogenic and mutagenic to humans. For this reason spent filtering powders from dry cleaning are considered as hazardous waste, whose disposal in landfill is not allowed and for whom incineration represents the only viable alternative. Owing to the great concern that has been raised about potential emission of dioxins in the combustion of organic material in the presence of chlorine, alternative treatments should be considered. The objectives of this research were the characterization of a solid waste from dry cleaning activities, the evaluation of the organic solvent removal and recovery by means of different heat treatments with the aim of a reduction of the harmfulness of this waste for a possible landfill disposal as an alternative to combustion. The results obtained show that the spent filtering powders from dry cleaning contain about 25%wt of PCE. This large amount of solvent has confirmed the need for a recovery treatment for its possible reuse. Tests of distillation and of stripping with air and steam were carried out, evaluating the effectiveness of the treatments by quantifying the residual PCE in the treated samples. Leaching tests were performed on the treated samples, to verify the eligibility for disposal in landfill. The maximum PCE recovery was obtained by steam stripping. This treatment is of easy implementation in dry cleaning activities, where steam is widely produced for other purposes. However, steam stripping produces a mixture of PCE and water that has to be treated or re-used for steam generation, and a solid waste still not eligible for disposal in landfill due to the large DOC released into water during the leaching tests. This large DOC, detected in all the treated solid samples, is due mainly to the presence of dyes and other non-volatile organic matter and can be easily lowered by means of a treatment with boiling water, below the landfills eligibility criteria.