CHROMIUM REMOVAL FROM WASTE-WATER: EVALUATION OF AN INDUSTRIAL PLANT UPGRADING

Abstract

The European Decision 2018/1147 establishes a new discharge emission level for chromium, called Best Available Techniques-Associated Emission Level (BAT-AEL), which will be placed equal to 0,01-0,3 mg L^-1. Therefore, the present work evaluates, by Jar Tests implementation, the upgrading of the Ecologica Naviglio S.p.A. wastewater treatment plant (WWTP) in order to fulfil this new requirement. We focused on the clariflocculation phase, currently operated at the WWTP.

In order to achieve a higher chromium removal, the results obtained by carrying out the Jar Tests showed that it is recommended to change the duration of the clariflocculation process by switching it from 20 min to 3 min for the coagulation phase and from 7 min to 30 min for the flocculation one. The best pH value was observed in the range 8,0-8,5 by dosing the chemicals currently used at the plant. No evident benefits were obtained by increasing the chemicals dosage up to 50%. On the contrary, the introduction of lime milk (not yet used at the plant) allowed to achieve better removal efficiencies at higher pH values. Moreover, the Jar Test results pointed out that the residual chromium is mainly present in the dissolved form. For this reason, we also evaluated the biological treatment effect on this form, which resulted in an average removal efficiency of 20%. Finally, a potential refining treatment, carried out by dosing the chemicals currently used at the plant, could increase the removal of dissolved chromium of about 16-20%.

All the interventions, nonetheless, are not enough to guarantee the achievement of the new emission level. The assessment of other feasible solutions and the implementation of specific and advanced technologies for chromium removal are therefore required.