The Dechromatation Step in Wastewater Treatment Plants: Fundamental Role and Optimization
Résumé
Chromium compounds represent an important class of chemicals and are widely used in many industrial processes. Their release in the aquatic environment constitutes a concerning source of toxicity for flora and fauna. Chromium exists mainly in the trivalent and hexavalent forms, with the latter being much more soluble and toxic and requiring specific treatment before discharge. A review of the literature data reveals that there are few treatment methods applicable on an industrial scale, mainly because of efficiency and maintenance issues. Among them, the reduction at acidic pH by NaHSO(3) and the further precipitation of the more insoluble Cr(3+) species formed at neutral pH is the most employed technique, given the opportunity to automatically monitor the reaction by means of simple pH and ORP sensors. On this account, we optimized the setting of pH and ORP parameters, applying the method to synthetic solutions and real industrial wastewater from surface-treatment industries. The experimental results showed that optimization of the parameters (i.e., pH = 2.5 and ORP = 280 mV) leads to a reduction of reducing agent employment and fewer chemical compounds in the solution and final sludge.