Concentration of soluble and nonsoluble zinc-based impurities by unidirectional freezing: basis for a method of sludges treatment.

Heavy metals are known to be significant pollutants in sludges produced by wastewater treatment. A newtechnique to make the metallic contaminants migrate is to submit such sludges to a slow freezing in order to purify the frozen region and to concentrate impurities in the non-frozen zone. In this paper, we apply this method to model contaminated sludges consisting of clayey suspensions charged with a heavy metal (zinc). The effect of some physicochemical parameters (zinc and clay concentrations, pH) on the effectiveness of the method is experimentally investigated. The pH is the most relevant parameter and the highest purification rate (approximately 80%) is obtained for pH approximately 9.

Wastewater treatment by radial freezing with stirring effects.

Radial freezing experiments on wastewater models were conducted in the presence of imposed stirring in order to remove impurities. The studied samples (dilute Na-montmorillonite suspensions charged with nitrates and with zinc or lead) were placed inside a cylindrical annulus, cooled at a controlled temperature around -7 degrees C at its inner wall which rotated around a vertical axis. The freezing front propagated toward the still outer wall which was maintained at a constant temperature around +1 degrees C. Thanks to stirring, considerable purification rates up to 99.97% were attained. It was also demonstrated that combining radial freezing and stirring ended in residual concentrations which agreed with drinking water standards.