Implementing Decision 2018/1147 provides for a new, more restrictive discharge limit for chromium, set at 0.01-0.3 mg L-1; this requirement, for existing plants, must be met within four years of its notification. The objective of this study was to evaluate, by carrying out Jar Tests to test the current efficiency of the clariflocculation stage, the upgrading of a wastewater and industrial waste treatment plant, in order to understand whether such an intervention would be sufficient to comply with the future regulatory limit. The tests carried out suggest that an intervention is essential to correct the timings adopted to date in the chemical-physical treatment, from 20 to 3 min for coagulation and from 7 to 30 min for flocculation. Using the reagents normally employed in the plant, an optimal pH for metal removal was found to be in the range of 8.0-8.5, and it was shown that increasing the dosages by 50% did not result in significant increases in removal efficiencies. The use of milk of lime (currently not dosed), on the other hand, led to better results at higher pH. The results of the Jar Tests also showed that the residual chromium is predominantly in dissolved form: the effect of the biological treatment was therefore evaluated on this, whose removal efficiency averaged 20%, but showed an important range of variability. Finally, the adoption of a refining treatment, using the reagents employed in the plant, showed a further removal of dissolved chromium of around 16-20%. All the solutions identified, however, are not sufficient to guarantee an adequate safety margin with respect to the new limit; it will therefore be essential to evaluate the implementation of specific technologies for advanced metal removal.
