Study of the elimination of Cr(VI) using an activated sludge after application

Several industries in sectors such as tanning, metallurgy, surface treatment, etc., produce important quantities of liquid waste containing chromium in the Cr(VI) form. This study is focused on the elimination of this very toxic element. Batch experiments were carried out with stirring and the material used was sludge from a biological waste treatment plant after it had been spread in fields. Several physicochemical factors affect the effectiveness of the elimination of the species studied on the chosen matrix. Indeed, based on observations of the influence on the process of factors such as the variation of the initial Cr(VI) concentration, the solid/solution ratio and how much oxygen is available in the system, an elimination mechanism has been proposed. The study has shown the necessity for the microbial flora present in the chosen matrix to undergo an adaptation to its environment. The importance of this adaptation process is reinforced by the fact that the species responsible for the elimination of Cr(VI) are more effective at concentrations lower than 30 mg/L, with 100% elimination rates reached after 48 hours of elapsed time. The greater effectiveness of closed systems and first-order kinetics for the process have been demonstrated. The benefits from the use of sludge in water treatment, particularly for water with Cr(VI) loads, would be twofold. This approach would lead not only to a reduction of waste products from biological waste treatment plants, but also to the total elimination of low levels of the toxic Cr(VI) species, thus leading to a cheaper operation of treatment plants required for the treatment of chromium-containing industrial effluents as encountered in the surface treatment sector.